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Template-free synthesis and catalytic behaviour of aluminium-rich MFI-type zeolites
Applied Catalysis A: General 181 (1999) 29±38
Template-free synthesis and catalytic behaviour of aluminium-rich MFI-type zeolites Francisco J. Machadoa,*, Carmen M. LoÂpeza, MarõÂa A. Centenob, Caribay Urbinab a
Centro de CataÂlisis PetroÂleo y PetroquõÂmica, Escuela de QuõÂmica, Universidad Central de Venezuela, Apartado 47102, Caracas 1020-A, Venezuela b Centro de MicroscopõÂa ElectroÂnica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47102, Caracas 1020-A, Venezuela Received 12 August 1998; received in revised form 9 November 1998; accepted 9 November 1998
Abstract A systematic study of MFI-like zeolites synthesis starting from template-free reaction gels was carried out. It was possible to prepare either MFI- or MOR-like zeolites from the same synthesis gel by only changing either temperature or crystallisation time. An upper limit for the incorporation of silica in the MFI framework was observed regardless of the silica-to-alumina ratio of the starting gel, the resulting solids having Si/Al ratios ranging from 12 to 29. A good linear correlation between the corrected rate for n-hexane cracking and the Al loading con®rmed the composition and purity of the zeolitic MFI-phase obtained. Hydrothermal treatments applied to two MFI zeolites with different Si/Al ratio caused a higher percentage of dealumination for the one containing the highest Al loading, but the number of Al atoms per unit cell remaining after severe (1023 K) steaming was the same (1.40.35) in both cases. Enhancement of the catalytic activity for n-hexane cracking was observed for the samples steamed under mild (873 K) conditions with this effect being more pronounced for the aluminiumrich zeolite. Treatments of the sample showing the highest activity enhancement with HCl solutions of increasing concentration caused a progressive removal of extraframework Al species (EFAL) with a concomitant decrease in the catalytic activity. However, only a partial extraction of EFAL was observed, even with the more concentrated acid solution, suggesting a strong interaction of these species with the zeolite framework. # 1999 Elsevier Science B.V. All rights reserved. Keywords: MFI-zeolites; Template-free synthesis; Steaming; Cracking activity; Dealumination
1. Introduction MFI-type zeolites have been successfully used as commercial catalysts in several hydrocarbon transformations proceeding via carbenium ions [1,2]. Particularly, this structure has been used as gasoline octane-booster additive [3]. Silicon-rich steamed *Corresponding authors. Fax: +58-2-6052220; e-mail: [email protected]
ZSM-5 zeolites have been recently studied to rationalise the low gasoline loss per octane gain observed commercially for high-silica ZSM-5 FCC additives [4]. Normally, the synthesis of these zeolites comprises the use of expensive and corrosive organic alkylammonium ion as a structure directing template [5]. Therefore, special reactor materials and additional separation operations are required. Moreover, for the use of these materials as catalysts, organic com-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00383-4
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F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
pounds inside pores and channels of the zeolite must be removed. It is then desirable to prepare MFIzeolites without the use of organic templates. Several authors [6±12] have reported template-free synthesis of ZSM-5. In most cases the range of composition of the starting gel is very narrow, the crystallisation times are high (up to several days) and seeding procedures are frequently used. The resulting zeolites have high silica-to-alumina ratio, typically higher than 25. It has been shown that the catalytic activity for nhexane cracking is proportional to the framework aluminium (FAL). This reaction has been used to evaluate the crystallinity of ZSM-5 zeolite [13] and should also be good to estimate the tetrahedral Al loading. Low silica to alumina ratio should provide a high acid site density increasing the catalytic activity. Regeneration of catalysts usually implies high temperature in the presence of wet-air. Under these conditions, framework aluminium atoms can be removed. The coexistence of intra and extraframework aluminium can enhance the catalytic activity as has been previously shown [14±17]. The optimal FAL to EFAL ratio to reach the highest n-hexane cracking activity was shown to be the unity for a ZSM-5 with a starting Si/Al ratio of 30 [14], supporting the superacid site model of Mirodatos and Barthomeuf [18]. However, this situation might change with the initial framework composition of the zeolite. In fact, it was shown that ZSM-5 with high aluminium content is less resistant to dealumination [19]. Additional proposals on the possible nature of sites responsible for the observed enhancement of cracking activity of mild steamed ZSM-5 zeolites have been advanced [15±17]. Thus, the group from Mobil [15] proposed that paired Al framework atoms are required for the formation of enhanced sites. Zholobenko et al. [16] concluded that Lewis sites associated with EFAL species were responsible for such a behaviour. More recently, Masuda et al. [17] observed a dependence between this phenomenon and the amount of distorted octahedral Al atoms on the outer surface of the MFI crystals suggesting that accessibility to acid sites instead of acid strength is the major in¯uencing factor on the catalytic activity. Based on the previous review, it should be noted that the nature of the enhancement of catalytic sites after mild hydrotreatment is, by no means, a settled issue.
This work is intended to study the effect of mild steaming over the n-hexane cracking activity as a function of the Si/Al ratio of the starting MFI-zeolite in an attempt to contribute to the understanding of the true nature of the enhanced catalytic sites. A detailed synthesis procedure for preparing aluminium-rich template-free MFI-type zeolites at a relatively low crystallisation time and conventional temperature is disclosed. Based on the previous characteristic study, the unexpected catalytic properties shown by the octane-booster additive based on this material, designated ST5 in a previous patent [20], is rationalised. 2. Experimental The following starting materials were used for the synthesis: 56 wt% Al2O3 sodium aluminate (from Aldrich), 40 wt% SiO2 colloidal silica (Ludox AS40 from Dupont), 97 wt% sodium hydroxide (from Aldrich) and demineralised water. To prepare synthesis gels, an aqueous sodium aluminate solution was ®rstly prepared by adding the sodium aluminate to an aqueous sodium hydroxide solution. Then this aluminate solution was added to the colloidal silica with continuous stirring until a homogeneous gel was obtained. The composition of the resulting gels, expressed as mole ratios of components, as well as some other synthesis conditions are given in Table 1. Gels were then transferred to stainless-steel autoclaves provided with rocking-like agitation systems and heated at temperatures ranging from 403 to 473 K for crystallisation times between 24 and 72 h. The resulting solids recovered by ®ltration were copiously washed with demineralised water and dried at 393 K overnight. Hydrothermal treatment was carried out in a tubular reactor made up of quartz using samples of about 2 g of zeolite. First, the zeolite was heated at the steamtreatment temperature by increasing the temperature at a heating rate of 5 K/min under a dry nitrogen ¯ow of 60 ml/min. Then, a nitrogen stream saturated with water at about 323 K was passed trough the hydrogen form of the zeolite sample at a ¯ow rate of 30 ml/min at either 873 K (mild treatment) or 1023 K (severe treatment) for times ranging from 0 to 120 min. To study the in¯uence of the zeolite composition on the properties of the steamed product, two zeolite samples
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
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Table 1 Synthesis conditions used during the preparation of template-free MFI-like zeolites Exp.
Tc (K)
tc (h)
Starting gel composition SiO2/Al2O3
S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 S-10 S-11 S-12 S-13 S-14
403 438 473 438 438 438 438 438 438 438 463 463 463 463
48 48 48 48 48 72 24 48 48 48 24 24 24 24
18 18 18 18 18 18 18 40 10 18 40 60 80 120
prepared with the lowest and the highest gel silica-toalumina ratio (S-2 and S-14 in Table 1) were chosen for this experience. Chemical analyses for Al and Si were performed using atomic absorption spectrometry in a Varian Techtron model AA6. Samples were previously fused with lithium metaborate and dissolved in diluted nitric acid before the analyses. X-ray diffractograms were obtained with a Philips diffractometer PW 1730 using Co K radiation Ê ) operated at 35 kV and 20 mA and (1.790255 A scanning speed of 28 2/min. Diffraction lines between 208 and 408 2 were taken to determine the fractional crystallinity (Cryst.) in the usual way. The sample resulting from the preparation of S-12 in Table 1 (MFI-12 in Table 2) with the highest intensity summation of the chosen lines was used as the reference (100% crystalline).
Result
Na2O/SiO2
H2O/SiO2
0.10 0.10 0.10 0.08 0.19 0.10 0.10 0.10 0.10 0.10 0.16 0.16 0.16 0.16
18 18 18 18 18 18 18 18 18 18 43 43 43 43
AMOR MFI MOR AMOR MOR MORMFI AMORMFI AMOR MOR MFI MFI MFI MFI MFI
N2-speci®c surface areas (SSA), expressed as m2/g of catalyst, were measured on a Micromeritics 2200 sorptometer at a liquid nitrogen temperature. All the samples were pre-treated at 623 K under vacuum overnight. Mid-IR spectra where structural T-O vibration occurs (below 1200 cmÿ1) were acquired with the aid of a Perkin Elmer 1760-X FTIR spectrometer using the KBr-sample pellet technique for specimen preparation. Spectra were recorded at room conditions. Scanning electron micrographs were taken on a Hitachi S-500 microscope operated at 20 keV and 50 mA. Samples were Au-coated for 15 min on an Eiko Engineering instrument. Average particle size (APS) was evaluated with the aid of a Ladd Microcomputer Graphic Data Analyser system from Ladd Research Industries. Local crystal composition was
Table 2 Some characteristics of the MFI zeolites obtained from experiences in Table 1 Sample
SSA (m2/g)
Cryst.
(Al/CU)a
(ÿrH) (mmol/g h)
(ÿrH)* (mmol/m2 h)
APS (mm)
MFI-2 MFI-11 MFI-12 MFI-13 MFI-14
373 364 310 352 294
0.55 0.74 1.00 0.93 0.82
7.3 4.2 4.0 3.2 3.7
67.2 52.3 59.8 39.8 39.8
0.33 0.19 0.19 0.12 0.17
1 4 ± ± 4
See text for more details about definition, units and labels of the heading parameters. As determined from bulk chemical analysis.
a
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F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
determined by electron probe microanalysis on an EDX detector Kevex 7000 System. The transformation of n-hexane was carried out at 653 K in a continuous ¯ow tubular reactor made up of glass under atmospheric pressure. Dry nitrogen was used as a carrier gas at a molar ratio N2/n-hexane of 2.0 and n-hexane partial pressure of 0.34 atm. The space velocity (WHSV) calculated at 298 K and 1 atm was 22.0 hÿ1 (grams of n-hexane per hour per gram of catalyst). Reaction products taken over a period of 60 min were analysed in a line by FID-GC using an noctane/Porasil C packed-column of 2.5 m length and 1/8 in. diameter. Fractional conversion of n-hexane (X) was evaluated from the corrected chromatographic areas for each product (Ai) as follows: X X (1) X Ai ÿ An-hexane = Ai : The rate of n-hexane transformation (ÿrH) expressed in mmol of n-hexane converted per hour per gram of catalyst can be obtained from the following expression: ÿrH 103 WHSV X=Mn-hexane ;
(2)
where Mn-hexane stands for the molecular mass of nhexane. A modi®ed transformation rate (ÿrH)* to take into account differences in both speci®c surface area and crystallinity of the catalysts studied, expressed in mmol of n-hexane converted per hour per m2, was de®ned as follows: ÿrH ÿrH = Cryst: SSA:
(3)
The speci®c transformation rate of n-hexane (STR), expressed as molecules of n-hexane transformed per hour per framework aluminium, was calculated as follows: ÿ3
STR 10 ÿrH Mz = Cryst: FAL;
(4)
where Mz represents the formula weight of the zeolite unit cell (5760 g/mol), and FAL stands for the number of framework Al atoms per unit cell.
3. Results and discussion The in¯uence of the synthesis conditions on the resulting solid phase can be observed from the experi-
ences (exp.) summarised in Table 1. By comparing exp. S-1 to S-3, it can be observed that by increasing the temperature of crystallisation from 403 to 473 K, the solid product switches from an amorphous phase (AMOR) to the mordenite phase (MOR) leading, at the intermediate temperature of 438 K, to the MFI phase. Successive phase transformation during the synthesis of molecular sieves has been largely observed [21,22]. Such a behaviour, associated to solid-phase metastability, has been discussed in terms of free energy relations between the different solid phases as a function of temperature [21]. Similarly, increasing the alkalinity of the reaction medium (Na2O/SiO2 from 0.08 to 0.19), the same transformation sequence was observed as illustrated by the exp. S-4, S-2 and S-5. The effect of the crystallisation time can be observed by comparing exp. S-7, S-2 and S-6. As shown, the solid product changes from quasiamorphous to MFI by increasing the crystallisation time from 24 to 48 h. At the highest crystallisation time studied of 72 h, a mixture of phases MFIMOR was obtained. These previous results seem to support the liquid-phase mechanism of synthesis according to which those factors leading to an increase in the rate of dissolution of the starting gel accelerate the rate of crystallisation. It should then be expected that a reduction of the Si/Al ratio in the starting gel would facilitate the rate of gel dissolution with a concomitant increase in the rate of crystallisation. In fact, exp. S-8, S-2 and S-9 clearly illustrate this effect. Synthesis results seem to support the following crystallisation sequence: AMOR ! AMOR MFI ! MFI ! MFI MOR ! MOR X-ray diffractograms of Fig. 1 illustrate the previous transformation sequence for a series of preparations. As seen, no crystalline phases other than that corresponding to MFI were observed for the solids resulting from exp. S-2 and S-11 (Fig. 1(b) and (c)). Similarly, only the MOR phase was observed for the product of exp. S-3 (Fig. 1(d)). The diffractogram of the quasi-amorphous solid obtained from exp. S-7 is shown in Fig. 1(a). The fact that the phase MOR appears at the end of the crystallisation sequence suggests that this zeolite is thermodynamically more stable than MFI in the present crystallisation system.
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
33
Fig. 1. X-ray diffractograms of the following preparations quoted in Table 1 illustrating the phase-transformation sequence for the crystallisation system under study: (a) S-7 (AMOR); (b) S-2 (low-crystallinity MFI); (c) S-11 (high-crystallinity MFI); (d) S-3 (MOR).
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F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
Interestingly, the present synthesis procedure can be used to prepare indistinctly either MFI or MOR from the same synthesis gel by only changing operation conditions such as temperature or crystallisation time. Worthy to mention is the fact that a synthesis carried out under conditions similar to those used in exp. S-2 but without stirring led to an amorphous material. Based on the previous observations, we have made some combinations of different reaction parameters in an attempt to produce MFI zeolites with higher Si/Al ratios at low crystallisation time. Results of exp. S-11 to S-14 in Table 1 show the feasibility of preparing MFI-type zeolites at a crystallisation time of only 24 h from a template-free crystallisation gel. The absence of any appreciable background in their respective diffractograms (not shown) evidenced the high purity of these materials. Some characteristics of the synthetic MFI zeolites are given in Table 2. The numbers in the sample designation are referred to the experiences in Table 1. Contrarily to what is usually observed during the TPAsynthesis, the Si/Al ratio of the resulting zeolites do not correspond to that of the starting gel. The higher the Si/Al ratio of the starting gel the higher the composition difference with respect to the resulting zeolite. In fact, the solid yields de®ned as grams of solid product obtained per gram of silica in the starting gel were 0.98 and 0.50 for S-2 and S-14, respectively. From these values and the unit cell composition of
the respective zeolites, the ratio SiO2 in the zeolite to SiO2 in the gel can be obtained. The calculated values were 0.84 and 0.43 for S-2 and S-14, respectively, clearly indicating that the incorporation of silica is limited. It is well known that the addition of bulky cations to the synthesis gel leads to silicon-rich zeolites. Conversely, the absence of bulky charge-compensating cations such as TPA should disfavour the incorporation of silica to yield aluminium-rich zeolites. There seems to be an upper limit for the Si/Al ratio of the resulting MFI-zeolites in the range from 22 to 29, regardless of the Si/Al ratio of the synthesis gel, under the synthesis conditions used in the present work. Micrographs of MFI-samples: MFI-2 (Si/Al12) and MFI-11 (Si/Al25) and that of the mordenite obtained from exp. S-3 are shown in Fig. 2. Spherulitic crystals of sub-micron size (maximum crystallite size of about 1 micron) are observed for MFI-2 (Fig. 2(a)). By contrast, the synthesis at a higher temperature and alkalinity led to larger crystals of an average crystal size close to 4 microns (Fig. 2(b)), only after 24 h crystallisation. Most of these crystals showed the MFI-typical hexagonal morphology. However, a small proportion of crystal clusters with a ¯aky habit such as those signalled with letter ``a'' in Fig. 2(b) was also observed. EDX analysis of these unusual crystals gave an average Si/Al ratio of 43, whereas that of the typical hexagonal crystals was 25,
Fig. 2. Electron micrographs of: (a) MFI-2 (Si/Al12); and (b) MFI-11 (Si/Al25), silicon-rich crystals with flaky habit are marked with letter ``a'' on the micrograph; (c) MOR (Si/Al8) obtained from exp. S-3.
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
in close agreement with the Si/Al ratio obtained from bulk chemical analysis. Small crystals of MFI-2 produce line-broadening of the X-ray re¯ections as can be observed by contrasting Fig. 1(b) with (c). Highlycrystalline large-crystals mordenite with the surface topography and habits shown in Fig. 2(c) was obtained from exp. S-3. An average Si/Al ratio of about 8 was determined for these mordenite crystals from EDX analysis. The n-hexane cracking activity of highly pure and crystalline unmodi®ed MFI-zeolites have been shown to increase linearly with increasing the framework aluminium (FAL) content [23]. On the other hand, the rate constant for cracking of n-hexane has been estimated as to be about 104 times as higher as that of amorphous aluminosilicates. Therefore, this catalytic test can be safely used to evaluate the framework composition of MFI-zeolites even in the presence of amorphous materials as long as corrections are made to express the activity in terms of the zeolitic phase alone. The conversion of n-hexane remained fairly constant with time-on-stream over the 60 min period studied. The observed (ÿrH) and corrected (ÿrH)* rate of n-hexane cracking are given in Table 2. By plotting the latter parameter against the number of aluminium atoms per unit cell (Al/CU), a linear correlation was obtained. The best straight line calculated by including the extrapolated point zero activity at zero FAL was ÿrH 0:046 Al=UC ÿ 0:004
R2 0:99:
(5)
Aluminium rich MFI-zeolite such as MFI-2 conforms the base of the octane-booster designated ST5 in a previous patent [20]. To illustrate the n-hexane cracking capacity of this material, a comparison with three different commercially available octane promoters was done. The reaction conditions and results were given in Example 5 of the patent above cited [20]. For the sake of completion and due to the relevance of these results a summary of the salient features is reproduced: The conversion of n-hexane was much higher (46 vol%) than two of the commercial additives used (17 and 29 vol%) and very much like the third one (45 vol%) containing an active support. For the latter, the percentage by weight of coke was higher (0.33)
35
than that of ST5 (0.08). This unexpected behaviour (high cracking conversion with low coke production) can be explained in terms of the higher proportion of framework Al of ST5 as compared to the others. Frequency shifts of infrared stretch bands with changing framework composition have been usually observed for a variety of zeolites. In all cases, good linear correlation between the frequency and FAL have been observed. Such a dependence can, in principle, be used to estimate the zeolite framework composition in the presence of extra-framework aluminium species (EFAL). In addition to the synthetic zeolites of Table 2, MFI samples synthesised with TPA and having Si/Al ratios between 30 and 60 were also included to obtain the following correlation between the asymmetric stretch band frequency () (at around 1100 cmÿ1) and the number of aluminium atoms per unit cell: FAL 1099 ÿ =2:83:
(6)
The above correlation was then used to determine the number of framework aluminium atoms per unit cell (FAL) of the steam-treated MFI-samples. However, it should be considered that the accuracy in determining FAL depends on the precision in measuring , according to the following error expression: jFALj=FAL jj= 1099 ÿ :
(7)
It can be observed that as the frequency approaches the value of 1099 cmÿ1 (decreasing FAL), the relative error FAL in FAL increases. In our case, the precision in measuring was 1 cmÿ1. Such an uncertainty causes a relative error ranging from 6% for the less dealuminated sample (FAL5.6) to 25% for the silica-richest zeolite (FAL1.4). Therefore, care should be taken when using the FAL data obtained from the previous expression for interpreting catalytic results, particularly for the highly dealuminated zeolites. Steaming time and temperature used during the mild hydrothermal treatment, applied to the samples MFI-2 (Si/Al12) and MFI-14 (Si/Al29), are given in Table 3 together with the observed frequency shift (), the FAL values obtained from Eq. (6), the corrected n-hexane transformation rate (ÿrH)* and the speci®c transformation rate of n-hexane (STR). The in¯uence of the treatment time and temperature on the extend of dealumination can be better visua-
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F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
Table 3 Extend of dealumination under mild and severe steaming and catalytic behaviour of mild-steamed synthetic MFI-2 and MFI-14 zeolites Sample
t (min)
T (K)
(cmÿ1)
FAL
(ÿrH)* (mmol/m2 h)
STR (mol/h FAL)
MFI-2 MFI-2a MFI-2b MFI-2c MFI-2d MFI-2e MFI-2f MFI-14 MFI-14a MFI-14b MFI-14c MFI-14d MFI-14e MFI-14f
± 10 60 120 10 60 12 ± 10 60 120 10 60 120
± 873 873 873 1023 1023 1023 ± 873 873 873 1023 1023 1023
1078 1083 1086 1087 1091 1093 1095 1090 1091 1093 1093 1092 1094 1095
7.4 5.6 4.6 4.2 2.8 2.1 1.4 3.2 2.7 2.1 2.1 2.6 1.8 1.4
0.34 0.30 0.65 0.23 ± ± ± 0.21 0.21 0.21 0.13 ± ± ±
99 116 300 110 ±
Fig. 3. Framework aluminium per unit cell (FAL) as determined from IR measurements as a function of the steaming time for samples MFI-2 (Si/Al12) and MFI-14 (Si/Al29) steamed at low (873 K) and high (1023 K) temperatures.
lised in Fig. 3. In addition to the hydrothermal treatment at 873 K, results at more severe conditions (1023 K) have also been included for comparison. As seen, mild dealumination (at 873 K) removed only 43% of the aluminium atoms from the aluminium-rich sample MFI-2 after 120 min of treatment as compared with almost 80% at 1023 K. At the latter temperature, the number of structural tetrahedral atoms remaining after 120 min was 1.40.35. By contrast, the siliconricher sample MFI-14 with a Si/Al ratio of 25 showed only 62% of dealumination at the more severe conditions (1023 K and 120 min). Coincidentally, the resulting dealuminated product also contained
94 156 225 181 ± ± ±
1.40.35 FAL suggesting an upper limit for the removal of framework aluminium regardless of the initial zeolite composition. Previous calculation of the FAL distribution as a function of the aluminium loading has shown that at a value close to 1.5 FAL, nearly 90% of the Al sites correspond to the environment Al (0Al) [15]. These sites are known to be more dif®cult to remove via hydrolysis than sites Al (1Al). For mordenites, it has also been shown that the last one Al framework atom remaining after strong dealumination treatments is strongly bound to the MOR framework [24]. Based on the FAL distribution curves as a function of the AlIV loading [14], it can be observed that the sample MFI-2 with more than seven FAL per unit cell contains 100% of sites Al (1Al) which are easier to remove. The speci®c transformation rate of n-hexane (STR) can be taken as an index of the acid strength. As seen in Table 3, mild steaming caused a signi®cant increase of STR for both MFI-2 and MFI-14. A graphic representation of the relative speci®c transformation rate of n-hexane (RSTR) as a function of the steaming time is shown in Fig. 4. It can be observed that the sample MFI-2 with the highest Al content showed a more pronounced activity enhancement after 60 min of mild steaming, as much as three times as the one observed for the unsteamed MFI-2 zeolite. Three separate portions of the sample MFI-2b, showing the highest activity enhancement, were treated with HCl solutions of increasing concentration:
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
Fig. 4. Relative specific transformation rate for n-hexane (RSTR) cracking as a function of the steaming time for the samples MFI-2 (Si/Al12) and MFI-14 (Si/Al29).
0.25, 1.60 and 3.20 M. Previously, it was veri®ed that no dealumination of the parent sample MFI-2 occurred after treated in the same way with the more concentrated acid solution. The effect of these extractive acid treatments on the catalytic activity can be observed from Table 4. Progressive removal of the extraframework aluminium species (EFAL) led to a proportional decrease in the catalytic activity con®rming their participation in conforming the enhanced catalytic sites. It has been largely invoked that EFAL species can act as Lewis sites [14,16,18]. Some authors [14,18] have proposed a sort of synergism between an adjacent Lewis and BroÈnsted pair to yield the so-called ``superacid'' site. A different approach suggests that such Lewis-like EFAL species are per se capable of polarising paraf®n molecules and therefore Table 4 Effect of the acid concentration on the composition and catalytic activity of the activity-enhanced MFI-2b sample Sample
HCl conc. (M)
Al/UC a
(ÿrH)* (mmol/m2 h)
MFI-2b MFI-2b (0.25) MFI-2b (1.60) MFI-2b (3.20)
± 0.25 1.60 3.20
7.3 6.8 6.2 5.2
0.65 0.60 0.52 0.38
a
As determined from bulk chemical analysis.
37
be responsible for the observed enhancement of the nhexane cracking activity [16]. Conventional cationic EFAL species such as those formed during steaming of faujasite-like zeolites have been shown to be easily extracted with acid solution of concentration as low as 0.1 M. In the present work, however, even with the more concentrated acid solution (3.20 M), only a partial extraction of EFAL was observed suggesting a strong interaction of these species with the zeolite structure. The Mobil group [15], postulated that hexa-coordinated Al atoms generated by a partial hydrolysis of Al (1Al) sites, still bounded to the zeolite framework, were responsible for the enhanced n-hexane cracking activity observed after a mild hydrothermal treatment. This model can satisfactorily explain both the resistance to acid extraction of EFAL and the higher activity enhancement showed by the Al-rich sample MFI-2 having the higher population of Al (1Al) sites. 4. Conclusions A series of MFI-like zeolites were prepared from a wholly inorganic reaction gel, without seeding, in only 24 h of crystallisation at 463 K. The resulting solids have Si/Al ratios ranging from 12 to 29 and acceptably good crystalline fractions and sorption capacities. The fact that a good linear correlation was found between the modi®ed n-hexane transformation rate and the Al content as determined by bulk chemical analysis con®rms the purity and the framework composition of the synthetic MFI-preparations. Aluminium-rich MFI zeolites such as MFI-2 (Si/Al12), contain a higher density of intrinsic strong-acid sites than conventional MFI materials usually prepared with Si/Al ratio higher than 18. This fact can explain the higher n-hexane conversion with much lower coke production observed for ST5 (an octane-booster additive based an aluminium-rich MFI-zeolites) as compared with different commercial FCC octane-boosters, under the same reaction conditions. Interestingly, mordenite can also be prepared from the same reaction gel by only changing either temperature or reaction time. The following crystallisation sequence was observed suggesting that MOR is
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F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
thermodynamically more stable than MFI: AMOR ! AMOR MFI ! MFI ! MFI MOR ! MOR The sample MFI-2 with higher Al content was easier to dealuminate by either mild or severe steaming than the sample MFI-14 with the lower Al loading. However, after severe steaming, the amount of AlIV remaining was the same for both zeolites suggesting a very strong bounding of the last about 1.5 Al per unit cell, where the maximum Al (0Al) sites have been reported to occur. Mild hydrothermal treatment, as has been previously reported, caused an activity enhancement for n-hexane cracking. We have found that this effect is more pronounced for the aluminium-rich sample having 100% of Al (1Al) sites. Partial removal of EFAL led to a concomitant decrease of the activity supporting their participation in the nature of the enhanced active sites. We have observed that these species seem to be strongly bound to the MFI-framework. The kinetic model advanced by Lago et al. [15] to explain the observed enhancement of n-hexane cracking activity over mild steamed MFI-like zeolites can satisfactorily explain the present results. Acknowledgements This work was supported by CDCH-UCV project 03.12.3331.95. References [1] C. Chang, A. Silvestry, J. Catal. 47 (1977) 249. [2] J. Haggin, Chem. Eng. News 60 (1982) 9. [3] J. Biswas, I.E. Maxwell, Appl. Catal. 58 (1990) 1.
[4] J.S. Buchanan, Appl. Catal. A 171 (1998) 57. [5] R. Argauer, G. Landolt, US Patent 3 702 886 (1972). [6] P.A. Jacobs, J.A. Martens, Stud. Surf. Sci. Catal. 33 (1987) 134. [7] F.Y. Dai, M. Susuki, H. Takahashi, Y. Saito, Proceedings of the Seventh International Zeolite Conference, Kodansha, Tokyo, 1986, p. 223. [8] F.Y. Dai, M. Susuki, H. Takahashi, Y. Saito, Zeolite synthesis, ACS Symp. Ser. 398 (1989) 224. [9] G. Bellusi, G. Perego, A. Carati, U. Cornaro, V. Fattore, Stud. Surf. Sci. Catal. 37 (1988) 37. [10] V. Nayak, J.B. Moffat, Appl. Catal. 47 (1989) 363. [11] V.P. Shiralkar, A. Clearfield, Zeolites 9 (1989) 363. [12] W. Schwieger, K.H. Bergk, D. Freude, M. Hunger, H. Pfeifer, Zeolite synthesis, ACS Symp. Ser. 398 (1989) 275. [13] T.A.J. Herdenberg, L. Martens, P. Mesman, H.C. Muller, C.P. Nicolaides, Zeolites 12 (1992) 685. [14] A. Ashton, S. Batmanian, D. Clark, J. Dwyer, F. Fitch, A. Hinhcliffe, F.J. Machado, Stud. Surf. Sci. Catal. 20 (1985) 101. [15] R.M. Lago, W.O. Haag, R.J. Mikovsky, D.H. Olson, S.D. Hellring, K.D. Schmitt, G.T. Kerr, in: Proceedings of the Seventh International Conference on Zeolites, Kodansha, Tokyo, 1986, p. 677. [16] V.L. Zholobenko, L.M. Kustov, V.B. Kasansky, E. Loeffler, U. Lohser, Ch. Peuker, G. Oehlmann, Zeolites 10 (1990) 304. [17] T. Masuda, Y. Fujikata, S. Mukai, K. Hashimoto, Appl. Catal. A 172 (1998) 73. [18] C. Mirodatos, D. Barthomeuf, J. Chem. Soc., Chem. Commun. (1981) 39. [19] S.M. Campbell, D.M. Bibby, J.M. Coddington, R.F. Howe, R.H. Meinhold, J. Catal. 161 (1996) 338. [20] N. MartõÂnez, J. Lujano, N. Alvarez, F.J. Machado, C.M. LoÂpez, US Patent 5 254 327 (1993). [21] D.W. Breck, Zeolite Molecular Sieves: Structure, Chemistry and Use, chapter 4, Wiley, New York, 1974, p. 246, and references given therein. [22] C.M. LoÂpez, F.J. Machado, J. Goldwasser, B. MeÂndez, K. RodrõÂguez, M. RamõÂrez-Agudelo, Zeolites 19 (1997) 133. [23] D.H. Olson, R.M. Lago, W.O. Haag, J. Catal. 61 (1980) 390. [24] M.J. van Niekerk, J.C. Fltcher, C.T. O'Connor, J. Catal. 138 (1992) 150.
Template-free synthesis and catalytic behaviour of aluminium-rich MFI-type zeolites Francisco J. Machadoa,*, Carmen M. LoÂpeza, MarõÂa A. Centenob, Caribay Urbinab a
Centro de CataÂlisis PetroÂleo y PetroquõÂmica, Escuela de QuõÂmica, Universidad Central de Venezuela, Apartado 47102, Caracas 1020-A, Venezuela b Centro de MicroscopõÂa ElectroÂnica, Facultad de Ciencias, Universidad Central de Venezuela, Apartado 47102, Caracas 1020-A, Venezuela Received 12 August 1998; received in revised form 9 November 1998; accepted 9 November 1998
Abstract A systematic study of MFI-like zeolites synthesis starting from template-free reaction gels was carried out. It was possible to prepare either MFI- or MOR-like zeolites from the same synthesis gel by only changing either temperature or crystallisation time. An upper limit for the incorporation of silica in the MFI framework was observed regardless of the silica-to-alumina ratio of the starting gel, the resulting solids having Si/Al ratios ranging from 12 to 29. A good linear correlation between the corrected rate for n-hexane cracking and the Al loading con®rmed the composition and purity of the zeolitic MFI-phase obtained. Hydrothermal treatments applied to two MFI zeolites with different Si/Al ratio caused a higher percentage of dealumination for the one containing the highest Al loading, but the number of Al atoms per unit cell remaining after severe (1023 K) steaming was the same (1.40.35) in both cases. Enhancement of the catalytic activity for n-hexane cracking was observed for the samples steamed under mild (873 K) conditions with this effect being more pronounced for the aluminiumrich zeolite. Treatments of the sample showing the highest activity enhancement with HCl solutions of increasing concentration caused a progressive removal of extraframework Al species (EFAL) with a concomitant decrease in the catalytic activity. However, only a partial extraction of EFAL was observed, even with the more concentrated acid solution, suggesting a strong interaction of these species with the zeolite framework. # 1999 Elsevier Science B.V. All rights reserved. Keywords: MFI-zeolites; Template-free synthesis; Steaming; Cracking activity; Dealumination
1. Introduction MFI-type zeolites have been successfully used as commercial catalysts in several hydrocarbon transformations proceeding via carbenium ions [1,2]. Particularly, this structure has been used as gasoline octane-booster additive [3]. Silicon-rich steamed *Corresponding authors. Fax: +58-2-6052220; e-mail: [email protected]
ZSM-5 zeolites have been recently studied to rationalise the low gasoline loss per octane gain observed commercially for high-silica ZSM-5 FCC additives [4]. Normally, the synthesis of these zeolites comprises the use of expensive and corrosive organic alkylammonium ion as a structure directing template [5]. Therefore, special reactor materials and additional separation operations are required. Moreover, for the use of these materials as catalysts, organic com-
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00383-4
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F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
pounds inside pores and channels of the zeolite must be removed. It is then desirable to prepare MFIzeolites without the use of organic templates. Several authors [6±12] have reported template-free synthesis of ZSM-5. In most cases the range of composition of the starting gel is very narrow, the crystallisation times are high (up to several days) and seeding procedures are frequently used. The resulting zeolites have high silica-to-alumina ratio, typically higher than 25. It has been shown that the catalytic activity for nhexane cracking is proportional to the framework aluminium (FAL). This reaction has been used to evaluate the crystallinity of ZSM-5 zeolite [13] and should also be good to estimate the tetrahedral Al loading. Low silica to alumina ratio should provide a high acid site density increasing the catalytic activity. Regeneration of catalysts usually implies high temperature in the presence of wet-air. Under these conditions, framework aluminium atoms can be removed. The coexistence of intra and extraframework aluminium can enhance the catalytic activity as has been previously shown [14±17]. The optimal FAL to EFAL ratio to reach the highest n-hexane cracking activity was shown to be the unity for a ZSM-5 with a starting Si/Al ratio of 30 [14], supporting the superacid site model of Mirodatos and Barthomeuf [18]. However, this situation might change with the initial framework composition of the zeolite. In fact, it was shown that ZSM-5 with high aluminium content is less resistant to dealumination [19]. Additional proposals on the possible nature of sites responsible for the observed enhancement of cracking activity of mild steamed ZSM-5 zeolites have been advanced [15±17]. Thus, the group from Mobil [15] proposed that paired Al framework atoms are required for the formation of enhanced sites. Zholobenko et al. [16] concluded that Lewis sites associated with EFAL species were responsible for such a behaviour. More recently, Masuda et al. [17] observed a dependence between this phenomenon and the amount of distorted octahedral Al atoms on the outer surface of the MFI crystals suggesting that accessibility to acid sites instead of acid strength is the major in¯uencing factor on the catalytic activity. Based on the previous review, it should be noted that the nature of the enhancement of catalytic sites after mild hydrotreatment is, by no means, a settled issue.
This work is intended to study the effect of mild steaming over the n-hexane cracking activity as a function of the Si/Al ratio of the starting MFI-zeolite in an attempt to contribute to the understanding of the true nature of the enhanced catalytic sites. A detailed synthesis procedure for preparing aluminium-rich template-free MFI-type zeolites at a relatively low crystallisation time and conventional temperature is disclosed. Based on the previous characteristic study, the unexpected catalytic properties shown by the octane-booster additive based on this material, designated ST5 in a previous patent [20], is rationalised. 2. Experimental The following starting materials were used for the synthesis: 56 wt% Al2O3 sodium aluminate (from Aldrich), 40 wt% SiO2 colloidal silica (Ludox AS40 from Dupont), 97 wt% sodium hydroxide (from Aldrich) and demineralised water. To prepare synthesis gels, an aqueous sodium aluminate solution was ®rstly prepared by adding the sodium aluminate to an aqueous sodium hydroxide solution. Then this aluminate solution was added to the colloidal silica with continuous stirring until a homogeneous gel was obtained. The composition of the resulting gels, expressed as mole ratios of components, as well as some other synthesis conditions are given in Table 1. Gels were then transferred to stainless-steel autoclaves provided with rocking-like agitation systems and heated at temperatures ranging from 403 to 473 K for crystallisation times between 24 and 72 h. The resulting solids recovered by ®ltration were copiously washed with demineralised water and dried at 393 K overnight. Hydrothermal treatment was carried out in a tubular reactor made up of quartz using samples of about 2 g of zeolite. First, the zeolite was heated at the steamtreatment temperature by increasing the temperature at a heating rate of 5 K/min under a dry nitrogen ¯ow of 60 ml/min. Then, a nitrogen stream saturated with water at about 323 K was passed trough the hydrogen form of the zeolite sample at a ¯ow rate of 30 ml/min at either 873 K (mild treatment) or 1023 K (severe treatment) for times ranging from 0 to 120 min. To study the in¯uence of the zeolite composition on the properties of the steamed product, two zeolite samples
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
31
Table 1 Synthesis conditions used during the preparation of template-free MFI-like zeolites Exp.
Tc (K)
tc (h)
Starting gel composition SiO2/Al2O3
S-1 S-2 S-3 S-4 S-5 S-6 S-7 S-8 S-9 S-10 S-11 S-12 S-13 S-14
403 438 473 438 438 438 438 438 438 438 463 463 463 463
48 48 48 48 48 72 24 48 48 48 24 24 24 24
18 18 18 18 18 18 18 40 10 18 40 60 80 120
prepared with the lowest and the highest gel silica-toalumina ratio (S-2 and S-14 in Table 1) were chosen for this experience. Chemical analyses for Al and Si were performed using atomic absorption spectrometry in a Varian Techtron model AA6. Samples were previously fused with lithium metaborate and dissolved in diluted nitric acid before the analyses. X-ray diffractograms were obtained with a Philips diffractometer PW 1730 using Co K radiation Ê ) operated at 35 kV and 20 mA and (1.790255 A scanning speed of 28 2/min. Diffraction lines between 208 and 408 2 were taken to determine the fractional crystallinity (Cryst.) in the usual way. The sample resulting from the preparation of S-12 in Table 1 (MFI-12 in Table 2) with the highest intensity summation of the chosen lines was used as the reference (100% crystalline).
Result
Na2O/SiO2
H2O/SiO2
0.10 0.10 0.10 0.08 0.19 0.10 0.10 0.10 0.10 0.10 0.16 0.16 0.16 0.16
18 18 18 18 18 18 18 18 18 18 43 43 43 43
AMOR MFI MOR AMOR MOR MORMFI AMORMFI AMOR MOR MFI MFI MFI MFI MFI
N2-speci®c surface areas (SSA), expressed as m2/g of catalyst, were measured on a Micromeritics 2200 sorptometer at a liquid nitrogen temperature. All the samples were pre-treated at 623 K under vacuum overnight. Mid-IR spectra where structural T-O vibration occurs (below 1200 cmÿ1) were acquired with the aid of a Perkin Elmer 1760-X FTIR spectrometer using the KBr-sample pellet technique for specimen preparation. Spectra were recorded at room conditions. Scanning electron micrographs were taken on a Hitachi S-500 microscope operated at 20 keV and 50 mA. Samples were Au-coated for 15 min on an Eiko Engineering instrument. Average particle size (APS) was evaluated with the aid of a Ladd Microcomputer Graphic Data Analyser system from Ladd Research Industries. Local crystal composition was
Table 2 Some characteristics of the MFI zeolites obtained from experiences in Table 1 Sample
SSA (m2/g)
Cryst.
(Al/CU)a
(ÿrH) (mmol/g h)
(ÿrH)* (mmol/m2 h)
APS (mm)
MFI-2 MFI-11 MFI-12 MFI-13 MFI-14
373 364 310 352 294
0.55 0.74 1.00 0.93 0.82
7.3 4.2 4.0 3.2 3.7
67.2 52.3 59.8 39.8 39.8
0.33 0.19 0.19 0.12 0.17
1 4 ± ± 4
See text for more details about definition, units and labels of the heading parameters. As determined from bulk chemical analysis.
a
32
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
determined by electron probe microanalysis on an EDX detector Kevex 7000 System. The transformation of n-hexane was carried out at 653 K in a continuous ¯ow tubular reactor made up of glass under atmospheric pressure. Dry nitrogen was used as a carrier gas at a molar ratio N2/n-hexane of 2.0 and n-hexane partial pressure of 0.34 atm. The space velocity (WHSV) calculated at 298 K and 1 atm was 22.0 hÿ1 (grams of n-hexane per hour per gram of catalyst). Reaction products taken over a period of 60 min were analysed in a line by FID-GC using an noctane/Porasil C packed-column of 2.5 m length and 1/8 in. diameter. Fractional conversion of n-hexane (X) was evaluated from the corrected chromatographic areas for each product (Ai) as follows: X X (1) X Ai ÿ An-hexane = Ai : The rate of n-hexane transformation (ÿrH) expressed in mmol of n-hexane converted per hour per gram of catalyst can be obtained from the following expression: ÿrH 103 WHSV X=Mn-hexane ;
(2)
where Mn-hexane stands for the molecular mass of nhexane. A modi®ed transformation rate (ÿrH)* to take into account differences in both speci®c surface area and crystallinity of the catalysts studied, expressed in mmol of n-hexane converted per hour per m2, was de®ned as follows: ÿrH ÿrH = Cryst: SSA:
(3)
The speci®c transformation rate of n-hexane (STR), expressed as molecules of n-hexane transformed per hour per framework aluminium, was calculated as follows: ÿ3
STR 10 ÿrH Mz = Cryst: FAL;
(4)
where Mz represents the formula weight of the zeolite unit cell (5760 g/mol), and FAL stands for the number of framework Al atoms per unit cell.
3. Results and discussion The in¯uence of the synthesis conditions on the resulting solid phase can be observed from the experi-
ences (exp.) summarised in Table 1. By comparing exp. S-1 to S-3, it can be observed that by increasing the temperature of crystallisation from 403 to 473 K, the solid product switches from an amorphous phase (AMOR) to the mordenite phase (MOR) leading, at the intermediate temperature of 438 K, to the MFI phase. Successive phase transformation during the synthesis of molecular sieves has been largely observed [21,22]. Such a behaviour, associated to solid-phase metastability, has been discussed in terms of free energy relations between the different solid phases as a function of temperature [21]. Similarly, increasing the alkalinity of the reaction medium (Na2O/SiO2 from 0.08 to 0.19), the same transformation sequence was observed as illustrated by the exp. S-4, S-2 and S-5. The effect of the crystallisation time can be observed by comparing exp. S-7, S-2 and S-6. As shown, the solid product changes from quasiamorphous to MFI by increasing the crystallisation time from 24 to 48 h. At the highest crystallisation time studied of 72 h, a mixture of phases MFIMOR was obtained. These previous results seem to support the liquid-phase mechanism of synthesis according to which those factors leading to an increase in the rate of dissolution of the starting gel accelerate the rate of crystallisation. It should then be expected that a reduction of the Si/Al ratio in the starting gel would facilitate the rate of gel dissolution with a concomitant increase in the rate of crystallisation. In fact, exp. S-8, S-2 and S-9 clearly illustrate this effect. Synthesis results seem to support the following crystallisation sequence: AMOR ! AMOR MFI ! MFI ! MFI MOR ! MOR X-ray diffractograms of Fig. 1 illustrate the previous transformation sequence for a series of preparations. As seen, no crystalline phases other than that corresponding to MFI were observed for the solids resulting from exp. S-2 and S-11 (Fig. 1(b) and (c)). Similarly, only the MOR phase was observed for the product of exp. S-3 (Fig. 1(d)). The diffractogram of the quasi-amorphous solid obtained from exp. S-7 is shown in Fig. 1(a). The fact that the phase MOR appears at the end of the crystallisation sequence suggests that this zeolite is thermodynamically more stable than MFI in the present crystallisation system.
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
33
Fig. 1. X-ray diffractograms of the following preparations quoted in Table 1 illustrating the phase-transformation sequence for the crystallisation system under study: (a) S-7 (AMOR); (b) S-2 (low-crystallinity MFI); (c) S-11 (high-crystallinity MFI); (d) S-3 (MOR).
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F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
Interestingly, the present synthesis procedure can be used to prepare indistinctly either MFI or MOR from the same synthesis gel by only changing operation conditions such as temperature or crystallisation time. Worthy to mention is the fact that a synthesis carried out under conditions similar to those used in exp. S-2 but without stirring led to an amorphous material. Based on the previous observations, we have made some combinations of different reaction parameters in an attempt to produce MFI zeolites with higher Si/Al ratios at low crystallisation time. Results of exp. S-11 to S-14 in Table 1 show the feasibility of preparing MFI-type zeolites at a crystallisation time of only 24 h from a template-free crystallisation gel. The absence of any appreciable background in their respective diffractograms (not shown) evidenced the high purity of these materials. Some characteristics of the synthetic MFI zeolites are given in Table 2. The numbers in the sample designation are referred to the experiences in Table 1. Contrarily to what is usually observed during the TPAsynthesis, the Si/Al ratio of the resulting zeolites do not correspond to that of the starting gel. The higher the Si/Al ratio of the starting gel the higher the composition difference with respect to the resulting zeolite. In fact, the solid yields de®ned as grams of solid product obtained per gram of silica in the starting gel were 0.98 and 0.50 for S-2 and S-14, respectively. From these values and the unit cell composition of
the respective zeolites, the ratio SiO2 in the zeolite to SiO2 in the gel can be obtained. The calculated values were 0.84 and 0.43 for S-2 and S-14, respectively, clearly indicating that the incorporation of silica is limited. It is well known that the addition of bulky cations to the synthesis gel leads to silicon-rich zeolites. Conversely, the absence of bulky charge-compensating cations such as TPA should disfavour the incorporation of silica to yield aluminium-rich zeolites. There seems to be an upper limit for the Si/Al ratio of the resulting MFI-zeolites in the range from 22 to 29, regardless of the Si/Al ratio of the synthesis gel, under the synthesis conditions used in the present work. Micrographs of MFI-samples: MFI-2 (Si/Al12) and MFI-11 (Si/Al25) and that of the mordenite obtained from exp. S-3 are shown in Fig. 2. Spherulitic crystals of sub-micron size (maximum crystallite size of about 1 micron) are observed for MFI-2 (Fig. 2(a)). By contrast, the synthesis at a higher temperature and alkalinity led to larger crystals of an average crystal size close to 4 microns (Fig. 2(b)), only after 24 h crystallisation. Most of these crystals showed the MFI-typical hexagonal morphology. However, a small proportion of crystal clusters with a ¯aky habit such as those signalled with letter ``a'' in Fig. 2(b) was also observed. EDX analysis of these unusual crystals gave an average Si/Al ratio of 43, whereas that of the typical hexagonal crystals was 25,
Fig. 2. Electron micrographs of: (a) MFI-2 (Si/Al12); and (b) MFI-11 (Si/Al25), silicon-rich crystals with flaky habit are marked with letter ``a'' on the micrograph; (c) MOR (Si/Al8) obtained from exp. S-3.
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
in close agreement with the Si/Al ratio obtained from bulk chemical analysis. Small crystals of MFI-2 produce line-broadening of the X-ray re¯ections as can be observed by contrasting Fig. 1(b) with (c). Highlycrystalline large-crystals mordenite with the surface topography and habits shown in Fig. 2(c) was obtained from exp. S-3. An average Si/Al ratio of about 8 was determined for these mordenite crystals from EDX analysis. The n-hexane cracking activity of highly pure and crystalline unmodi®ed MFI-zeolites have been shown to increase linearly with increasing the framework aluminium (FAL) content [23]. On the other hand, the rate constant for cracking of n-hexane has been estimated as to be about 104 times as higher as that of amorphous aluminosilicates. Therefore, this catalytic test can be safely used to evaluate the framework composition of MFI-zeolites even in the presence of amorphous materials as long as corrections are made to express the activity in terms of the zeolitic phase alone. The conversion of n-hexane remained fairly constant with time-on-stream over the 60 min period studied. The observed (ÿrH) and corrected (ÿrH)* rate of n-hexane cracking are given in Table 2. By plotting the latter parameter against the number of aluminium atoms per unit cell (Al/CU), a linear correlation was obtained. The best straight line calculated by including the extrapolated point zero activity at zero FAL was ÿrH 0:046 Al=UC ÿ 0:004
R2 0:99:
(5)
Aluminium rich MFI-zeolite such as MFI-2 conforms the base of the octane-booster designated ST5 in a previous patent [20]. To illustrate the n-hexane cracking capacity of this material, a comparison with three different commercially available octane promoters was done. The reaction conditions and results were given in Example 5 of the patent above cited [20]. For the sake of completion and due to the relevance of these results a summary of the salient features is reproduced: The conversion of n-hexane was much higher (46 vol%) than two of the commercial additives used (17 and 29 vol%) and very much like the third one (45 vol%) containing an active support. For the latter, the percentage by weight of coke was higher (0.33)
35
than that of ST5 (0.08). This unexpected behaviour (high cracking conversion with low coke production) can be explained in terms of the higher proportion of framework Al of ST5 as compared to the others. Frequency shifts of infrared stretch bands with changing framework composition have been usually observed for a variety of zeolites. In all cases, good linear correlation between the frequency and FAL have been observed. Such a dependence can, in principle, be used to estimate the zeolite framework composition in the presence of extra-framework aluminium species (EFAL). In addition to the synthetic zeolites of Table 2, MFI samples synthesised with TPA and having Si/Al ratios between 30 and 60 were also included to obtain the following correlation between the asymmetric stretch band frequency () (at around 1100 cmÿ1) and the number of aluminium atoms per unit cell: FAL 1099 ÿ =2:83:
(6)
The above correlation was then used to determine the number of framework aluminium atoms per unit cell (FAL) of the steam-treated MFI-samples. However, it should be considered that the accuracy in determining FAL depends on the precision in measuring , according to the following error expression: jFALj=FAL jj= 1099 ÿ :
(7)
It can be observed that as the frequency approaches the value of 1099 cmÿ1 (decreasing FAL), the relative error FAL in FAL increases. In our case, the precision in measuring was 1 cmÿ1. Such an uncertainty causes a relative error ranging from 6% for the less dealuminated sample (FAL5.6) to 25% for the silica-richest zeolite (FAL1.4). Therefore, care should be taken when using the FAL data obtained from the previous expression for interpreting catalytic results, particularly for the highly dealuminated zeolites. Steaming time and temperature used during the mild hydrothermal treatment, applied to the samples MFI-2 (Si/Al12) and MFI-14 (Si/Al29), are given in Table 3 together with the observed frequency shift (), the FAL values obtained from Eq. (6), the corrected n-hexane transformation rate (ÿrH)* and the speci®c transformation rate of n-hexane (STR). The in¯uence of the treatment time and temperature on the extend of dealumination can be better visua-
36
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
Table 3 Extend of dealumination under mild and severe steaming and catalytic behaviour of mild-steamed synthetic MFI-2 and MFI-14 zeolites Sample
t (min)
T (K)
(cmÿ1)
FAL
(ÿrH)* (mmol/m2 h)
STR (mol/h FAL)
MFI-2 MFI-2a MFI-2b MFI-2c MFI-2d MFI-2e MFI-2f MFI-14 MFI-14a MFI-14b MFI-14c MFI-14d MFI-14e MFI-14f
± 10 60 120 10 60 12 ± 10 60 120 10 60 120
± 873 873 873 1023 1023 1023 ± 873 873 873 1023 1023 1023
1078 1083 1086 1087 1091 1093 1095 1090 1091 1093 1093 1092 1094 1095
7.4 5.6 4.6 4.2 2.8 2.1 1.4 3.2 2.7 2.1 2.1 2.6 1.8 1.4
0.34 0.30 0.65 0.23 ± ± ± 0.21 0.21 0.21 0.13 ± ± ±
99 116 300 110 ±
Fig. 3. Framework aluminium per unit cell (FAL) as determined from IR measurements as a function of the steaming time for samples MFI-2 (Si/Al12) and MFI-14 (Si/Al29) steamed at low (873 K) and high (1023 K) temperatures.
lised in Fig. 3. In addition to the hydrothermal treatment at 873 K, results at more severe conditions (1023 K) have also been included for comparison. As seen, mild dealumination (at 873 K) removed only 43% of the aluminium atoms from the aluminium-rich sample MFI-2 after 120 min of treatment as compared with almost 80% at 1023 K. At the latter temperature, the number of structural tetrahedral atoms remaining after 120 min was 1.40.35. By contrast, the siliconricher sample MFI-14 with a Si/Al ratio of 25 showed only 62% of dealumination at the more severe conditions (1023 K and 120 min). Coincidentally, the resulting dealuminated product also contained
94 156 225 181 ± ± ±
1.40.35 FAL suggesting an upper limit for the removal of framework aluminium regardless of the initial zeolite composition. Previous calculation of the FAL distribution as a function of the aluminium loading has shown that at a value close to 1.5 FAL, nearly 90% of the Al sites correspond to the environment Al (0Al) [15]. These sites are known to be more dif®cult to remove via hydrolysis than sites Al (1Al). For mordenites, it has also been shown that the last one Al framework atom remaining after strong dealumination treatments is strongly bound to the MOR framework [24]. Based on the FAL distribution curves as a function of the AlIV loading [14], it can be observed that the sample MFI-2 with more than seven FAL per unit cell contains 100% of sites Al (1Al) which are easier to remove. The speci®c transformation rate of n-hexane (STR) can be taken as an index of the acid strength. As seen in Table 3, mild steaming caused a signi®cant increase of STR for both MFI-2 and MFI-14. A graphic representation of the relative speci®c transformation rate of n-hexane (RSTR) as a function of the steaming time is shown in Fig. 4. It can be observed that the sample MFI-2 with the highest Al content showed a more pronounced activity enhancement after 60 min of mild steaming, as much as three times as the one observed for the unsteamed MFI-2 zeolite. Three separate portions of the sample MFI-2b, showing the highest activity enhancement, were treated with HCl solutions of increasing concentration:
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
Fig. 4. Relative specific transformation rate for n-hexane (RSTR) cracking as a function of the steaming time for the samples MFI-2 (Si/Al12) and MFI-14 (Si/Al29).
0.25, 1.60 and 3.20 M. Previously, it was veri®ed that no dealumination of the parent sample MFI-2 occurred after treated in the same way with the more concentrated acid solution. The effect of these extractive acid treatments on the catalytic activity can be observed from Table 4. Progressive removal of the extraframework aluminium species (EFAL) led to a proportional decrease in the catalytic activity con®rming their participation in conforming the enhanced catalytic sites. It has been largely invoked that EFAL species can act as Lewis sites [14,16,18]. Some authors [14,18] have proposed a sort of synergism between an adjacent Lewis and BroÈnsted pair to yield the so-called ``superacid'' site. A different approach suggests that such Lewis-like EFAL species are per se capable of polarising paraf®n molecules and therefore Table 4 Effect of the acid concentration on the composition and catalytic activity of the activity-enhanced MFI-2b sample Sample
HCl conc. (M)
Al/UC a
(ÿrH)* (mmol/m2 h)
MFI-2b MFI-2b (0.25) MFI-2b (1.60) MFI-2b (3.20)
± 0.25 1.60 3.20
7.3 6.8 6.2 5.2
0.65 0.60 0.52 0.38
a
As determined from bulk chemical analysis.
37
be responsible for the observed enhancement of the nhexane cracking activity [16]. Conventional cationic EFAL species such as those formed during steaming of faujasite-like zeolites have been shown to be easily extracted with acid solution of concentration as low as 0.1 M. In the present work, however, even with the more concentrated acid solution (3.20 M), only a partial extraction of EFAL was observed suggesting a strong interaction of these species with the zeolite structure. The Mobil group [15], postulated that hexa-coordinated Al atoms generated by a partial hydrolysis of Al (1Al) sites, still bounded to the zeolite framework, were responsible for the enhanced n-hexane cracking activity observed after a mild hydrothermal treatment. This model can satisfactorily explain both the resistance to acid extraction of EFAL and the higher activity enhancement showed by the Al-rich sample MFI-2 having the higher population of Al (1Al) sites. 4. Conclusions A series of MFI-like zeolites were prepared from a wholly inorganic reaction gel, without seeding, in only 24 h of crystallisation at 463 K. The resulting solids have Si/Al ratios ranging from 12 to 29 and acceptably good crystalline fractions and sorption capacities. The fact that a good linear correlation was found between the modi®ed n-hexane transformation rate and the Al content as determined by bulk chemical analysis con®rms the purity and the framework composition of the synthetic MFI-preparations. Aluminium-rich MFI zeolites such as MFI-2 (Si/Al12), contain a higher density of intrinsic strong-acid sites than conventional MFI materials usually prepared with Si/Al ratio higher than 18. This fact can explain the higher n-hexane conversion with much lower coke production observed for ST5 (an octane-booster additive based an aluminium-rich MFI-zeolites) as compared with different commercial FCC octane-boosters, under the same reaction conditions. Interestingly, mordenite can also be prepared from the same reaction gel by only changing either temperature or reaction time. The following crystallisation sequence was observed suggesting that MOR is
38
F.J. Machado et al. / Applied Catalysis A: General 181 (1999) 29±38
thermodynamically more stable than MFI: AMOR ! AMOR MFI ! MFI ! MFI MOR ! MOR The sample MFI-2 with higher Al content was easier to dealuminate by either mild or severe steaming than the sample MFI-14 with the lower Al loading. However, after severe steaming, the amount of AlIV remaining was the same for both zeolites suggesting a very strong bounding of the last about 1.5 Al per unit cell, where the maximum Al (0Al) sites have been reported to occur. Mild hydrothermal treatment, as has been previously reported, caused an activity enhancement for n-hexane cracking. We have found that this effect is more pronounced for the aluminium-rich sample having 100% of Al (1Al) sites. Partial removal of EFAL led to a concomitant decrease of the activity supporting their participation in the nature of the enhanced active sites. We have observed that these species seem to be strongly bound to the MFI-framework. The kinetic model advanced by Lago et al. [15] to explain the observed enhancement of n-hexane cracking activity over mild steamed MFI-like zeolites can satisfactorily explain the present results. Acknowledgements This work was supported by CDCH-UCV project 03.12.3331.95. References [1] C. Chang, A. Silvestry, J. Catal. 47 (1977) 249. [2] J. Haggin, Chem. Eng. News 60 (1982) 9. [3] J. Biswas, I.E. Maxwell, Appl. Catal. 58 (1990) 1.
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