Maria
Milina
,
Sharon
Mitchell
,
Zair Domínguez
Trinidad
,
Danny
Verboekend
and
Javier
Pérez-Ramírez
*
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Wolfgang-Pauli-Strasse 10, CH-8093, Zurich, Switzerland. E-mail: jpr@chem.ethz.ch; Fax: +41 44 633 14 05
First published on 22nd December 2011
Desilication of conventional zeolites in alkaline medium generates variable intracrystalline mesoporosity, but inevitably changes other properties such as the Si/Al ratio and aluminium distribution. Assessing the individual effects of porosity and composition on the catalytic performance of desilicated zeolites is relevant for their optimal design. Herein, we decouple the respective impacts in the acid-catalysed alkylation of toluene (or cyclohexylbenzene) with benzyl alcohol. These reactions experience strong accessibility constraints to the micropores, providing high sensitivity to the properties of the developed mesopore surface. Through strategic comparison of alkaline-treated ZSM-5 zeolites prepared with and without subsequent acid treatment, we show that while acidity is important, the alkylation activity is dominated by the mesoporous surface area. The selectivity to (methylbenzyl)benzene does not depend on the available external surface. Excessively large mesopore volumes are detrimental to the activity, and variation in micropore volume has a minimal effect. Acid-treated mesoporous zeolites exhibit higher catalytic activities primarily due to textural enhancements by removal of aluminium-rich amorphous debris. The catalytic results are rationalised on the basis of extensive characterisation (AAS, N2 sorption, XRD, TEM, 27Al MAS NMR, FTIR, NH3-TPD, and adsorption of toluene and cyclohexylbenzene).
Interconnected mesopores can be efficiently introduced in a scalable manner by desilication of zeolites in alkaline media.7,8 A major concern with post-synthetic methods are the compositional changes experienced during demetallation.7 Upon alkaline treatment, silicon is preferentially extracted into solution, while aluminium species realuminate on the external surface, playing an important role in pore direction.9,10 Alkaline-treated zeolites exhibit a decreased bulk Si/Al ratio, micropore volume and crystallinity, and an increased amount of Lewis acid sites.11–16
To avoid possible alteration of the catalytic properties arising from alkali-induced surface realumination, sequential acid treatments are increasingly applied. The additional step aims to restore the original framework composition.9,10 Acid washing of alkaline-leached ZSM-5 is accompanied by simultaneous enhancements in micropore volume, mesoporous surface area, and crystallinity.11 These concomitant changes were never systematically studied in catalysed reactions. As such the implications of realuminated species are still not understood. This particularly applies to zeolites of a low Si/Al ratio at high base concentrations, where larger amounts of aluminium-rich amorphous debris are generated.11
Herein, the alkylations of toluene and cyclohexylbenzene with benzyl alcohol (Scheme 1) are taken as model reactions to decouple the relative impacts of compositional and porosity effects on desilicated zeolites. Mesoporous ZSM-5 zeolites have been advantageously applied for both the alkylation17–19 and the related acylation20 of aromatic molecules. The choice of access-limited reactions provides optimal sensitivity to the redistribution of aluminium upon alkaline treatment and acid washing through confinement of the alkylation activity to the mesoporous surface. Property–function relationships are established, through detailed sample characterisation using multiple techniques. We address important design aspects of mesoporous zeolites such as the aluminium content and acid site speciation, the optimal extent and appropriate descriptors of mesoporosity, and the impact of residual aluminium-rich species on adsorption and catalysis.
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Scheme 1 Alkylation of (a) toluene and (b) cyclohexylbenzene with benzyl alcohol. |
Sample | NaOH/M | HCl/M | Yield/% | Si/Ala/− | S meso /m2 g−1 | S BET /m2 g−1 | V micro /cm3 g−1 | V meso /cm3 g−1 |
---|---|---|---|---|---|---|---|---|
a AAS. b t-plot method. c BET method. d V meso = Vpore − Vmicro. e Values in parentheses represent the overall yield (alkaline and acid treatments). The code of the samples includes the nominal Si/Al molar ratio of the parent (P) zeolite and the treatment conducted: AT (alkaline treatment), AW (acid washing), and ST (steaming). | ||||||||
Z10-P | — | — | — | — | 102 | 394 | 0.12 | 0.34 |
Z15-P | — | — | — | 15 | 76 | 412 | 0.14 | 0.15 |
Z15-ST | — | — | 100 | 15 | 112 | 415 | 0.13 | 0.23 |
Z15-AT1 | 0.6 | — | 70 | 10 | 147 | 444 | 0.13 | 0.38 |
Z15-AT1-AW1 | 0.6 | 0.02 | 94 (66)e | 12 | 158 | 588 | 0.15 | 0.40 |
Z25-P | — | — | — | 25 | 76 | 461 | 0.17 | 0.31 |
Z40-P | — | — | — | 39 | 78 | 468 | 0.17 | 0.28 |
Z40-ST | — | — | 100 | 39 | 72 | 456 | 0.17 | 0.13 |
Z40-AT1 | 0.1 | — | 84 | 34 | 135 | 497 | 0.15 | 0.19 |
Z40-AT1-AW2 | 0.1 | 0.1 | 95 (80) | 34 | 135 | 498 | 0.15 | 0.21 |
Z40-AT2 | 0.2 | — | 67 | 25 | 288 | 589 | 0.13 | 0.40 |
Z40-AT2-AW2 | 0.2 | 0.1 | 91 (61) | 32 | 296 | 573 | 0.12 | 0.46 |
Z40-AT3 | 0.3 | — | 45 | 17 | 417 | 637 | 0.09 | 0.78 |
Z40-AT3-AW2 | 0.3 | 0.1 | 89 (40) | 32 | 440 | 675 | 0.10 | 0.84 |
Z40-AT3-AW2-ST | 0.3 | 0.1 | 100 (40) | 32 | 484 | 648 | 0.06 | 0.96 |
Z40-AT4 | 0.5 | — | 18 | 7 | 336 | 519 | 0.08 | 0.98 |
Z40-AT4-AW2 | 0.5 | 0.1 | 85 (15) | — | 415 | 635 | 0.09 | 1.00 |
Z40-AT5 | 0.7 | — | 11 | 4 | 294 | 349 | 0.02 | 0.94 |
Z40-AT5-AW2 | 0.7 | 0.1 | 69 (8) | — | 381 | 639 | 0.11 | 1.02 |
Z200-P | — | — | — | — | 73 | 398 | 0.15 | 0.24 |
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Fig. 1 N2 isotherms of selected zeolites. Insets: BJH mesopore size distributions. |
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Fig. 2 27Al MAS NMR spectra of selected zeolites. |
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Fig. 3 NH3-TPD profiles of selected zeolites. |
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Fig. 4 FTIR spectra of selected zeolites. |
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Fig. 5 Influence of the molar Si/Al ratio of the parent zeolites on the conversion of benzyl alcohol (BA) after 25 min in the alkylation with toluene (T). The zero conversion on the steamed zeolites is indicated by the solid symbols. Conditions: T = 433 K, P = 5 bar, T/BA = 80, 0.6 wt.% zeolite. |
The steam-induced extraction of framework aluminium permits determination of the influence of acidity modification on the alkylation activity. Following steam treatment, no major variation in sample porosity was observed (Table 1). The 27Al MAS NMR spectra, however, evidenced an increased intensity of the band at 0 ppm, and the appearance of an additional broad signal visible as a shoulder to the band at 59 ppm in the steamed zeolites (Fig. 2). These observations are related to dealumination of the framework, the condensation of dislodged aluminium as extra-framework species, and to greater variation in the coordination environment of the remaining lattice aluminium.23,24 Accordingly, a decrease in the infrared absorbance at 3600 cm−1 (associated with Brønsted acid sites)25 and an increase in the absorbances at 3650 and 3780 cm−1 (associated with extra-framework aluminium species)25 were seen in the FTIR spectrum of Z15-ST and Z40-ST (Fig. 4). Furthermore, the contribution above 700 K in the NH3-TPD profiles vanished (Fig. 3). No benzyl alcohol conversion was observed over the steamed zeolites (Fig. 5), strongly suggesting that weakly acidic extra-framework aluminium is not catalytically active.
The introduction of mesoporosity was clearly reflected in the N2 adsorption isotherms, by an enhanced uptake at intermediate and high relative pressures (Fig. 1). Analysis of the BJH pore size distributions (Fig. 1, inset) evidenced mesopore sizes centred around 10 nm for Z40-AT3. For Z15-AT1, a broader mesopore size distribution was observed. The intracrystalline origin of the mesoporosity has been confirmed by TEM (not shown). Due to the higher external area, the intensity of the silanol band at 3740 cm−1 in the IR spectra of desilicated samples increased (Fig. 4). The XRD patterns of the treated zeolites evidenced retention of the MFI structure in all cases (not shown).
Modification of the porous properties was accompanied by a reduction in the bulk Si/Al ratio (Table 1). No indication of an increase in the number of Brønsted acid sites is evidenced by IR spectroscopy (Fig. 4). The rise in the intensity of the low-temperature contribution (ca. 570 K) in the NH3-TPD profile (Fig. 3) reveals an increased presence of sites of moderate acidic strength, which are thought to be of Lewis-type and concentrated at the mesopore surface.26,27 No substantial evidence of octahedrally-coordinated aluminium was observed in the 27Al MAS NMR (Fig. 2) or IR (Fig. 4) spectra, indicating the different nature in comparison with the extra-framework aluminium formed upon steaming.
The mild acidic conditions (pH ≈ 5) of ion-exchange (with aqueous ammonium nitrate) also contribute to variation in the porous properties. The relative enhancements in Vmicro and Smeso brought about by ion exchange and acid washing of alkaline-treated zeolites are compared in Fig. 6. For the zeolite desilicated at low base concentration (0.1 M NaOH), similar improvements were seen for both treatments. The micropore volume was most significantly affected, increasing from 0.12 to 0.15 cm3 g−1, owing to the exchange of charge-balancing and to the removal of residual sodium ions. Only minor variation in the mesoporous surface area was observed. In contrast, for the zeolite desilicated at higher base concentration (0.5 M NaOH) acid washing was more beneficial improving Smeso by 135 m2 g−1 with respect to the 56 m2 g−1 increase induced by ion exchange.
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Fig. 6 Influence of ion exchange and acid washing on the porous properties of Z40 zeolites treated at different NaOH concentrations. Alkaline-treated zeolites are represented by grey bars, sequentially alkaline-treated and ion-exchanged zeolites by open bars, and sequentially alkaline-treated and acid-washed zeolites by black bars. |
The toluene adsorption isotherm of the parent Z40-P exhibited a single sharp uptake at low pressure (p < 2 mbar), related to micropore filling and characteristic of type I behaviour (Fig. 7a). The mesoporous zeolites displayed slightly higher initial uptakes of toluene compared to the parent sample. In contrast, analysis by N2 adsorption evidenced lower micropore volumes (0.10 cm3 g−1 compared to 0.17 cm3 g−1, respectively). This demonstrates an additional adsorption contribution, which should be related to the mesopore introduction. At higher pressures the adsorbed amount increased progressively. The uptake of Z40-AT3-AW2 was higher than that of Z40-AT3 at all relative pressures, consistent with the enhanced textural properties of the washed sample. Assuming a bulk liquid density of toluene, the total pore volumes estimated are similar to those derived from N2 adsorption. This indicates that both the micro- and mesopores were completely filled, confirming their full accessibility.
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Fig. 7 Adsorption isotherms of (a) toluene at 298 K and (b) cyclohexylbenzene at 321 K on parent and mesoporous zeolites. |
For the bulkier cyclohexylbenzene, which is not expected to penetrate into the micropores, significantly lower uptakes were observed for all zeolites (Fig. 7b). The mesoporous samples exhibited higher uptakes than the parent zeolite. In contrast to the adsorption of toluene, the initial adsorption of cyclohexylbenzene was similar for both the alkaline-treated and acid-washed zeolites, reflecting the negligible impact of micropore volume. However, the higher mesoporosity of the washed sample resulted in increased uptake at higher pressures.
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Fig. 8 Influence of post-synthetic modification on the conversion of benzyl alcohol in the alkylation with toluene. Reaction conditions as in the caption of Fig. 5. |
Fig. 9 summarises the relationship between toluene alkylation activity, the porous properties of mesoporous zeolites derived from Z40-P (Vmicro, Smeso, and Vmeso), and treatment efficiency as a function of the NaOH concentration applied in their preparation. The negligible dependence of catalytic activity on the micropore volume is consistent with the access-limited nature of the alkylation reaction. Accordingly, the strong interdependence between Smeso and XBA is directly related to the larger surface area available for alkylation. The activity (XBA at t = 10 min) mirrors the corresponding variation in the mesoporous surface area. Both reach a maximum for the sample treated at 0.3 M NaOH. In contrast, the trend in XBA deviates from that of Vmeso. This is most noticeable at increased values of mesopore volume, demonstrating that the generation of large mesopores is not advantageous in liquid-phase alkylation. Therefore, the mesopore surface seems to be a more suitable descriptor in the design of hierarchical zeolite catalysts than the mesopore volume. The positive impact of acid washing on the catalytic activity increased with the severity of the initial alkaline treatment (Fig. 9b). This further evidences the benefits of removing the aluminium-rich debris which blocks potentially active Brønsted acid sites at the pore mouths. To account for the loss of raw material which occurs during preparation, the conversion of benzyl alcohol was factored by the treatment yield of hierarchical zeolites (Fig. 9c). Taking this into consideration, Z40-AT2-AW2, desilicated with 0.2 M NaOH, was identified as the optimal mesoporous zeolite catalyst derived from Z40-P in this study.
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Fig. 9 Effect of the NaOH concentration on (a) the porous properties of the alkaline-treated Z40, (b) the conversion of benzyl alcohol during the alkylation with toluene over alkaline-treated (AT) zeolites before (grey circles) and after (solid squares) sequential acid washing (AW) after 10 min of reaction, and (c) the benzyl alcohol conversion factored by the overall treatment yield for the AT–AW zeolites. Values of the parent zeolite are represented at [NaOH] = 0 M. Reaction conditions as in the caption of Fig. 5. |
The catalytic properties of the hierarchical zeolites were also characterised in the alkylation of cyclohexylbenzene (Fig. 10). The larger substituted aromatic molecule was expected to provide even greater sensitivity to differences in the mesoporous surface structure than toluene. Remarkably, although the conversion of benzyl alcohol over Z40-AT3 was 30% lower than that observed with toluene, the catalytic activity of Z40-AT3-AW2 (after 25 min) was nearly equivalent in both alkylations. This permitted further insight into the property–function relationships of the hierarchical zeolite catalysts. Acid sites present on the mesoporous surface of the modified zeolites have sufficient strength to catalyse both alkylation reactions. The higher conversions with toluene over the unwashed sample implied that residual aluminium species restrict access to active sites on the mesopore surface of alkaline-treated zeolites. The removal of such species by acid washing unblocks pore mouths and improves the active site accessibility, thereby reducing the molecular size-dependence of the catalytic activity. Comparatively, the relative enhancement of XBA upon mild acid treatment of Z40-AT3 was 9 and 45% for toluene and cyclohexylbenzene, respectively.
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Fig. 10 Conversion of benzyl alcohol in the alkylation with toluene (solid bars) and cyclohexylbenzene (open bars) over hierarchical zeolites. Conditions: T = 433 K, P = 5 bar, t = 25 min, T/BA = 80, CHB/BA = 130, 0.6 wt% zeolite. |
Consistent with previous work,29 a clear trend is observed between alkylation activity and secondary mesoporosity (Fig. 11a). Interestingly, the linear dependence is distinct for zeolites obtained from Z15-P or Z40-P. This relates to the difference in framework aluminium content, which is 2.6 times higher in Z15-P. A three times larger mesoporous surface area is required to reach equivalent benzyl alcohol conversions for the Z40-derived hierarchical zeolites. This implies that mesoporosity introduction into aluminium-rich zeolites potentially offers greater catalytic improvements. The absence of any clear correlation with the bulk Si/Al ratio proves that moderately acidic species residual from alkaline treatment are less active than framework aluminium in the context of alkylation. In addition, the amount of strong acid sites in the remaining zeolite framework is apparently unaffected by the treatments applied. The selectivity for (methylbenzyl)benzene was around 75%, independent of the bulk Si/Al ratio, the textural properties, or the presence of amorphous aluminium-rich debris (Fig. 11b). The main by-product of the reaction, dibenzyl ether (selectivity of 25%), results from the self-etherification of benzyl alcohol. Polyalkylated compounds were not observed, as expected from the use of an excess of toluene (T/BA = 80). The selectivity relationship between ortho, meta, and para isomers was estimated to be 4:
1
:
5 for all zeolites. The similar selectivities verify that no shape selectivity effects, related to the participation of micropores, influence the product distribution. In agreement with our earlier observations, steaming significantly reduced the activity of the most active mesoporous zeolite (Z40-AT3-AW2 in Fig. 11a). However, the steamed sample retained a higher activity (XBA = 35% after 10 min) than the best performing purely microporous zeolite.
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Fig. 11 (a) Conversion of benzyl alcohol and (b) selectivity to (methylbenzyl)benzene in the alkylation of toluene over zeolites derived from Z15-P (triangles) and Z40-P (circles) with respect to the mesoporous surface area (Smeso) after 10 min of reaction. Parent zeolites are indicated by open symbols, alkaline-treated zeolites by grey symbols, and acid-washed zeolites by black symbols. The black diamond symbol represents the drop in activity upon steaming of mesoporous Z40-AT3-AW2. Reaction conditions as in the caption of Fig. 5. |
The catalytic trends described in this work have all been derived from liquid-phase alkylations of aromatic molecules. It should be pointed out that these observations can differ for other reaction systems, with distinct acidity requirements, and extent of micropore utilisation, or under different process conditions. Furthermore, more detailed evaluation of the acidic properties is required to understand the variation in acid site speciation occurring upon post-synthetic modification. These aspects will be addressed in future work.
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