Hierarchical Zeolites by Desilication - American Chemical Society

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Hierarchical Zeolites by Desilication: Occurrence and Catalytic Impact of Recrystallization and Restructuring Danny Verboekend,* Maria Milina, Sharon Mitchell, 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 ABSTRACT: The manifestation of zeolite recrystallization and the formation of amorphous aluminosilicate species during desilication are examined to better understand the properties of alkaline-treated hierarchical zeolites and their catalytic performance. This is achieved using a systematic experimental strategy, starting from treating the filtrate of alkaline-treated silicalite-1 in the presence of various external additives. No recrystallization is evidenced upon addition of tetrapropylammonium (TPA+) and/or aluminum hydroxide ions [Al(OH)4−], confirming the low probability of zeolite nucleation and/or growth during desilication. Conversely, ordered mesoporous materials (OMMs) form upon addition of cetyltrimethylammonium (CTA+) to the filtrate. By using other silicon sources, i.e., tetramethyl orthosilicate or the organosilane dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium, we verify the facile formation of amorphous materials during alkaline treatment of USY zeolites in the presence of hydrophobic micelle-forming alkyl moieties. A systematic characterization by X-ray diffraction, transmission electron microscopy, N2 and Ar adsorption, inductively coupled plasma optical emission spectroscopy, and Fourier transform infrared spectroscopy of pyridine adsorbed, demonstrates that zeolites exposed to base solutions containing CTA+ display weaker zeolitic properties, compared to those prepared using TPA+, and should be considered as hierarchical zeolite/OMM composites. Catalytic tests in the alkylation of toluene with isopropyl alcohol or benzyl alcohol evidence that CTA+-derived composites do not outperform the conventional USY zeolite. Only the hierarchical USY zeolite prepared by alkaline treatment in the presence of TPA+ yielded a superior catalytic performance.

1. INTRODUCTION Not long after synthetic zeolites were first catalytically employed in the 1950s and 1960s, the application of various chemical treatments proved to be essential to establishing their widespread implementation as superior solid acids.1 These modifications were principally aimed at enhancing catalytic performance by optimizing the stability of the zeolite framework and/or the concentration and strength of the acid sites. During the past decade, postsynthetic modifications have emerged as a powerful tool for introducing secondary porosity into any zeolite crystal.2−4 This auxiliary porosity is of crucial value, as the acid sites in zeolites are often poorly utilized because of access and diffusion constraints. The resulting hierarchical zeolites feature a strongly enhanced mass transfer and clearly outperform their conventional (purely microporous) counterparts.5−7 Of the postsynthetic methods available for the introduction of mesoporosity into commercial zeolites, desilication by controlled base leaching stands out as the most versatile and economic. Most commonly performed in aqueous sodium hydroxide solutions, this treatment can be tuned by varying the duration, temperature, and base concentration, or through the inclusion of additives termed external pore-directing agents (PDAs). With respect to the latter, the term “external” is used to differentiate from the inherent pore-directing role of © 2013 American Chemical Society

framework aluminum regulating zeolite dissolution and mesopore formation in alkaline media.2 The use of external PDAs is crucial for the preparation of all-silica8 and/or USY and beta9 zeolites in mesoporous form and can be additionally used to tailor the auxiliary porosity, surface composition, and acidic properties of hierarchical zeolites.2 The ability of external PDAs to direct mesopore formation derives from its interaction with the external surface of the zeolite crystal.8,9 Interestingly, various reactants commonly used to synthesize conventional zeolites, in particular, tetrapropylammonium (TPA+) and soluble metal hydroxide ions [e.g., Al(OH)4− or Ga(OH)4−], proved to be effective in directing mesopore formation during base leaching. Another branch of efficient PDAs consists of the micelle-forming tetraalkylammonium cations such as cetyltrimethylammonium (CTA+).9 These surfactants are known templates for the synthesis of ordered mesoporous materials (OMMs)10−12 and can induce a reassembly of (partially coordinated) dissolved Si and Al species derived from zeolite leaching. Accordingly, their application can yield either pure OMMs,13 zeolite/OMM Received: July 11, 2013 Revised: August 30, 2013 Published: September 12, 2013 5025

dx.doi.org/10.1021/cg4010483 | Cryst. Growth Des. 2013, 13, 5025−5035

Crystal Growth & Design

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Table 1. Sample Notation and Treatment Conditions sample code

starting solution

C (M)

T (°C)

t (h)

dissolved zeolite (g L−1)

zeolite added (g L−1)

initial pHa (−)

AT ATF HT1F HT2 HT2F

aqueous NaOH filtrate of AT-silicalite-1 filtrate of AT-silicalite-1 aqueous NH4OH filtrate of HT2-USY1

0.2 − − 0.37 −

65 65 140 150 150

0.5 0.5 20 10 10

− 13.3b 13.3b − 3.4c

33.3 33.3 − 15.6 −

13.3 11.6 11.6 10.5 9.5

a

Prior to any subsequent addition of PDAs and/or zeolite. bDerived from the solid yield of AT-silicalite-1. cDerived from the solid yield of HT2USY1.

composites,14,15 or less defined materials via pseudomorphic rearrangement16 or mesostructuring.17 Several parallels can be drawn between the conditions of zeolite desilication (primarily aimed at mesopore formation) and those of OMM or zeolite synthesis. The most important similarities comprise the required alkaline conditions and the presence of silicon (and/or aluminum) and templating species in solution. Hence, a relevant question is whether leached species could in situ, that is, during desilication, recrystallize into zeolite crystals or restructure into amorphous aluminosilicate species. Moreover, should such processes take place, it would be of eminent importance to precisely assess the nature of the secondary phase and to evaluate its potential impact in catalyzed reactions. This knowledge is relevant for fully controlling the effects of the alkaline treatments, improving our understanding of the properties of the resulting solids, and ascribing catalytic effects more accurately, hereby aiding in the rational design of optimal zeolite-based catalysts. Herein, we examine the occurrence of zeolite recrystallization and restructuring of amorphous materials during alkaline treatments in the presence of external PDAs, such as Al(OH)4−, TPA+, and CTA+. The latter is achieved using a systematic experimental strategy (Figure 1), treating filtrates derived from desilication of an all-silica silicalite-1 under atmospheric (65 °C for 30 min) and hydrothermal (140 °C for 20 h) conditions. Similarly, two USY zeolites with Si/Al ratios of 15 and 30 are treated in the presence of various filtrates, PDAs, and additional silicon sources, to generalize our findings. Representative samples are thoroughly characterized, highlighting the challenges in distinguishing hierarchical zeolites from OMMs based on nitrogen adsorption. Similarly, quantitative X-ray diffraction and Ar adsorption are established as being key in assessing the samples’ zeolitic nature, while TEM proves to be crucial for confirming the origin of the secondary porosity. Finally, we assess the composition and acidity of selected samples and evaluate their catalytic performance in the liquidphase alkylation of toluene with isopropyl alcohol or benzyl alcohol. In the latter reactions, a negative impact of the presence of a secondary OMM in the solid is established, leaving a highly crystalline hierarchical USY zeolite prepared with TPA+ as the only catalyst to outperform the conventional zeolite.

using the aqueous solutions, temperatures, and durations listed in Table 1. Atmospheric treatments were performed under magnetic stirring using an Easymax 102 instrument from Mettler Toledo. Hydrothermal treatments were conducted statically under autogenous pressure in 30 cm3 Parr autoclaves. When alkaline treatment was performed using the filtrate of another treatment, the letter F was added, i.e., ATF, HT1F, and HT2F. When additives, that is, TPABr (TPA, ABCR, 98%), CTABr (CTA, Acros, >99%), Al(NO3)3 (Al, AlfaAesar, 98%), dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (Si-QAC, OS, Aldrich, 72%), and tetramethylorthosilicate (TMOS, TS, Acros, 99%), were added to the alkaline solution, the corresponding code is indicated in the order of addition and is directly followed by the applied concentration. Similarly, when zeolites were introduced into the alkaline solution (see concentrations in Table 1), the corresponding abbreviations (silicalite-1, USY1, and USY2) were added to the sample code. For example, sample ATCTA0.2-TS0.2-USY2 was prepared by atmospheric alkaline treatment of USY2 zeolite (33.3 g L−1) in a mixture of 0.2 M NaOH, 0.2 M CTABr, and 0.2 M TS. The dried solids were calcined at 550 °C for 5 h (ramp rate of 5 °C min−1) after treatments to remove any residual organic species. Prior to acidity characterization and catalytic evaluation, the solids were converted into the protonic form by three consecutive ion exchanges in 0.1 M NH4NO3 (10 g of solid L−1) followed by the calcination protocol described above. 2.2. Characterization. Powder X-ray diffraction (XRD) patterns were acquired in a PANalytical X’Pert PRO-MPD diffractometer using Ni-filtered Cu Kα radiation (λ = 0.1541 nm). Data were recorded in the 2θ range of 3−60° with an angular step size of 0.05° and a counting time of 8 s per step. Transmission electron microscopy (TEM) was performed using FEI Tecnai F30 and Phillips CM12 microscopes operated at 300 and 100 kV, respectively. N2 isotherms at −196 °C were measured in a Quantachrome Quadrasorb-SI gas sorption analyzer. Prior to the measurement, the samples were degassed in vacuum at 300 °C for 10 h. The BET method18 was applied to calculate the total surface area, which is used for comparative purposes. The t-plot method19 was used to discriminate between micro- and mesoporosity. The mesopore size distribution was obtained by the Barret−Joyner−Halenda (BJH) model20 applied to the adsorption branch of the isotherm. Ar isotherms were recorded at −196 °C on a Micromeritics ASAP 2020 analyzer after in situ evacuation of the samples at 300 °C for 8 h. The hybrid nonlocal density functional theory (NLDFT) model21 describing argon adsorption in cylindrical micro- and mesopores was used to calculate the pore size distribution. Si and Al concentrations in the solids were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Horiba Ultima 2 instrument. Fourier transform infrared spectroscopy (FTIR) of adsorbed pyridine was conducted in a Bruker IFS 66 spectrometer (650−4000 cm−1, 2 cm−1 optical resolution, co-addition of 32 scans). Self-supporting wafers of zeolite (5 tons cm−2, 20 mg, 1 cm2) were degassed under vacuum (10−3 mbar) for 4 h at 420 °C, prior to adsorbing pyridine at room temperature. Gaseous and weakly adsorbed molecules were subsequently removed by evacuation at 200 °C for 30 min. The obtained spectra were normalized to the thickness of the wafers. The concentrations of Brønsted (B) and Lewis (L) acid sites were calculated from the band areas of adsorbed pyridine at 1545 and 1456 cm−1, using ε(B) and ε(L) extinction coefficients of 1.67 and 2.22 cm μmol−1, respectively.22

2. EXPERIMENTAL SECTION 2.1. Materials and Treatments. Silicalite-1 with a Si/Al ratio of 1060 (HSZ-890HOA, TOSOH) and USY zeolites with a Si/Al ratio of 15 (USY1, CBV 720, Zeolyst) and a Si/Al ratio of 30 (USY2, CBV 760, Zeolyst) were used as parent zeolites. In addition, all-silica SBA15 (Zeochem) and MCM-41 with a Si/Al ratio of 40 (Sigma-Aldrich) were used as reference materials. An overview of the synthetic strategies for the various solids and filtrates is provided in Figure 1. Alkaline treatments were performed under atmospheric (AT) or hydrothermal (HT1 and HT2) conditions 5026



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Figure 1. Overview of the reagents used and products formed upon various alkaline treatments. The reagents comprised nonzeolitic chemicals (blue), filtrates derived from alkaline-treated zeolites (orange), and pristine untreated zeolites (purple). The nonzeolitic chemicals comprised a base, a pore-directing agent (PDA), tetramethylorthosilicate (TS), and dimethyloctadecyl[3-(trimethoxysilyl)propyl]ammonium chloride (OS). After reaction under atmospheric (AT) or hydrothermal (HT) conditions (red), the obtained solids (green) and optionally the filtrate were isolated. The solids were consequently labeled AT or HT, which can be followed by the suffix F (in the case of a filtrate being used), -PDAx (type and molarity of PDA), -TSx (molarity of TS), -OSx (molarity of OS), and/or -zeolite [when a zeolite was included as a reagent (see the brackets in parts c, e, and f)]. Specific details of the treatments are listed in Table 1.

We conducted a number of experiments using the filtrate derived from alkaline treatment of silicalite-1 (Table 1). The latter zeolite exhibited an impurity-free MFI diffraction pattern (Figure 2a) and N2 isotherms (Figure 3a) and porous

2.3. Catalytic Evaluation. The alkylation tests of toluene with 2propanol or benzyl alcohol were undertaken in Ace pressure glass tubes (10 cm3 working volume) at 150 or 80 °C, respectively, under autogenous pressure. The powdered catalyst (25 mg) was added to mixtures of either toluene (47 mmol) and 2-propanol (1.15 mmol) or toluene (47 mmol) and benzyl alcohol (0.6 mmol). Ethylcyclohexane (0.3 mmol) served as an internal standard in both cases. Following the desired reaction time, the reactors were cooled and the collected liquid samples were analyzed using a gas chromatograph (HP 6890, HewlettPackard) equipped with an HP-5 capillary column and a flame ionization detector (FID). Reactants and products were calibrated using pure standards. The assignments were confirmed by gas chromatography and mass spectrometry (HP 5973, Hewlett-Packard).

3. RESULTS AND DISCUSSION The first two sections focus on the formation of solids from a silicon-containing filtrate obtained by alkaline treatment of silicalite-1 (Figure 1a). Various PDAs were added to this filtrate, followed by a number of atmospheric and hydrothermal treatments (Figure 1c,d). In section 3.1, using TPA+ and Al(OH)4− as PDAs, the manifestation of recrystallization, that is, the formation of zeolite crystals from dissolved zeolite fragments in the filtrate, is discussed. In section 3.2, restructuring, defined as the formation of ordered mesoporous materials from dissolved zeolite fragments in the alkaline treatment, is examined. Next, the influence of alternative silicon-containing sources on restructuring during alkaline treatment is studied in section 3.3 (Figure 1e,f). In section 3.4, the gained insights are extrapolated to the recently reported mesostructuring17 (Figure 1b,g,h). Finally, the composition and acidity of selected samples are analyzed, followed by the catalytic evaluation in acid-catalyzed alkylations (section 3.5). 3.1. Zeolite Recrystallization Using TPA+ and Al(OH)4− as Pore-Directing Agents. While the presence of TPA+ in an alkaline solution containing silicon species can give rise to the formation of silicalite-1 crystals, the presence of Al salts under the same conditions is known to yield ZSM-5 zeolites.10 Because these also comprise highly efficient pore-directing agents, we studied the possible recrystallization of zeolite crystals in alkaline solutions derived from desilicated silicalite-1. On the basis of the relatively low temperature (65 °C) and pressure (atmospheric), the short duration (30 min), and the relatively low concentration of dissolved Si species (∼15 g L−1), the formation of a zeolite phase during desilication should be considered improbable.23,24

Figure 2. X-ray diffraction patterns of selected (a) zeolitic and (b) amorphous samples.

properties (Table 2) characteristic of purely microporous materials. Upon conventional (atmospheric) alkaline treatment of the zeolite, sample AT-silicalite-1 was obtained in a 60% yield (Table 3). Next, aluminum nitrate and/or TPABr were added to the filtrate at concentrations that were optimal for the desilication of MFI zeolites,8 and the resulting solutions were subjected to atmospheric (samples ATF-TPA0.1 and ATFTPA0.1-Al0.003) or hydrothermal (samples HT1F-TPA0.1 and HT1F-TPA0.1-Al0.003) alkaline treatments. None of the latter 5027



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Figure 3. N2 isotherms (top) and BJH mesopore size distributions (bottom) of reference and alkaline-treated samples. The legends in the top graphs also apply to the graphs directly below them.

amorphous solids during desilication could be facilitated by the presence of a zeolite phase acting as a seed.10,23,24 To this end, we added USY2 zeolites to the filtrate and monitored the influence of the presence of TPA+ cations. A FAU zeolite was added, because its XRD pattern is easily distinguished from the MFI pattern originating from ZSM-5 or silicalite-1 crystals. In addition, while USY zeolites are relatively stable upon acid and steam treatment, they rapidly amorphize upon being exposed to alkaline media.9 Therefore, their addition to the filtrate also represents a suitable way to demonstrate the potential protective effects of the applied PDAs. In the absence of TPA+, a relative high solid yield was attained, which is expected on the basis of the relatively low pH of the filtrate (11.6), while the material completely amorphized and the porosity dramatically decreased [sample ATF-USY2 (Figures 2b and

Table 2. Porous Properties of Reference Zeolites and Ordered Mesoporous Materials sample

Vmicroa (cm3 g−1)

Vmicro,Arb (cm3 g−1)

Vmesoc (cm3 g−1)

Vpored (cm3 g−1)

Smesoa (m2 g−1)

MCM-41 SBA-15 silicalite-1 USY1 USY2

0.20 0.20 0.16 0.28 0.32

0.05 0.10 − − 0.30

1.39 1.17 0.02 0.23 0.22

1.59 1.37 0.18 0.51 0.54

1131 861 36 125 113

t-plot method. bCumulative volume of Ar adsorbed in ≤0.8 nm pores. cVmeso = Vpore − Vmicro. dN2 volume adsorbed at p/p0 = 0.99.

a

treatments resulted in the formation of solid products (Table 3), demonstrating that zeolite recrystallization during desilication does not occur. Of course, the nucleation of crystalline or

Table 3. Properties of Solids Derived from the Filtrate of Alkaline-Treated Silicalite-1 sample

yielda (%)

cryst.b (%)

Vmicroc (cm3 g−1)

Vmicro,Ard (cm3 g−1)

Vmesoe (cm3 g−1)

Vporef (cm3 g−1)

Smesoc (m2 g−1)

AT-silicalite-1 ATF ATF-TPA0.1 HT1F-TPA0.1 ATF-TPA0.1-Al0.003 HT1F-TPA0.1-Al0.003 ATF-CTA0.1 HT1F-CTA0.1 ATF-USY2 ATF-TPA0.1-USY2 ATF-CTA0.1-USY2

60 0 0 0 0 0 40 34 70g 65g 83g

91 − − − − − 0 0 0 67 44

0.16 − − − − − 0.26 0.44 0 0.29 0.35

− − − − − − 0.06 0.09 0 0.27 0.19

0.11 − − − − − 0.56 0.21 0.08 0.23 0.49

0.27 − − − − − 0.82 0.65 0.08 0.52 0.84

69 − − − − − 580 218 27 140 460

a Solid yield after treatment taking into account all included SiO2 species, i.e., from the filtrate and/or the zeolite phase. bCrystallinity determined by XRD. ct-plot method. dCumulative volume of Ar adsorbed in ≤0.8 nm pores. eVmeso = Vpore − Vmicro. fN2 volume adsorbed at p/p0 = 0.99. gFor reference, a yield of 70% would be attained assuming no zeolite dissolution and no deposition of SiO2 species from the filtrate.

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Figure 4. (a and c) Ar isotherms and (b and d) NLDFT cumulative pore volumes (Vpore) of reference and alkaline-treated samples as a function of pore size (dp). The legends in panels a and c also apply to panels b and d, respectively.

Figure 5. Transmission electron micrographs of reference and alkaline-treated samples.

inclusion of TPA+ in the filtrate leads to an enhanced dissolution of the USY2 zeolite in alkaline media. The latter is tentatively attributed to the formation of TPA−Si complexes.25 The enhanced dissolution constitutes further proof that zeolite recrystallization during desilication with TPA+ as PDA does not occur, because, should recrystallization occur,

3b and Table 3)]. In contrast, when TPA+ was incorporated into the filtrate prior to the addition of the USY zeolite (sample ATF-TPA0.1-USY2), the crystallinity and microporosity were largely preserved, demonstrating the protective role of the organic PDA. Moreover, the solid yield for ATF-TPA0.1-USY2 was slightly lower than for ATF-USY2, suggesting that the 5029



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Figure 6. Overview of the solids formed after various desilication treatments and their properties. The presence of tetrapropylammonium cations (TPA+) and/or soluble aluminum hydroxide ions [Al(OH)4−] in the Si-containing alkaline filtrate does not yield any solid product. Conversely, when CTA+ is present during the treatment, ordered mesoporous materials (OMMs) are formed. Accordingly, zeolites desilicated in the presence of TPA+ and/or Al(OH)4− transform into highly crystalline hierarchical porous zeolites, while those alkaline-treated in the presence of CTA+ are transformed into less crystalline zeolite/OMM composites.

sample.28 Application of the t-plot suggests that the solids displayed substantial microporosity (Vmicro ≥ 0.26 cm3 g−1) and mesoporosity [Smeso ≥ 218 m2 g−1 (Table 3)]. However, without adjusting the fitting of the t-plot, we also observed significant micropore volumes of 0.20 cm3 g−1 for MCM-41 and SBA-15. This highlights the difficulty of precisely quantifying the presence of zeolitic microporosity with this model, which is even more challenging in the case of the composites in which linear regions in the t-plot can be difficult to identify. Therefore, Ar adsorption was used to assess the microporosity in greater detail (Figure 4). The resulting pore size distributions, derived using NLDFT, evidenced that the CTA+-derived materials (ATF-CTA0.1 and HTF-CTA0.1) do not display well-defined micropore contributions and that, like for SBA-15 and MCM41, the volume of 80%) after 2 h. Conversely, MCM-41 displayed a minor conversion of IPA (∼20%) and a negligible selectivity (and yield) for cymenes because of the low concentration of Brønsted acid sites required to catalyze this reaction. Accordingly, it is no surprise that the samples that displayed low zeolitic character (i.e., largely decreased crystallinity, micropore volume, and concentration of Brønsted acid sites) and a large OMM fraction (ATF-CTA0.1-USY2 and ATOS0.2-USY2) exhibited a far inferior performance compared to that of USY2. The samples prepared by conventional desilication with CTA+ (AT-CTA0.2-USY2), although displaying a similar activity, yielded a lower YCym compared to that of USY2, as their SCym was substantially lower. Only sample ATTPA0.2-USY2 delivered a performance superior to that of USY2, displaying higher XIPA and SCym values.

In the reaction of toluene with benzyl alcohol (BA), only the conversion of benzyl alcohol (XBA) is displayed (Figure 10), as

Figure 10. (a) Conversion of benzyl alcohol (XBA) in the alkylation of toluene (T) with benzyl alcohol (BA) over reference and alkalinetreated samples after reaction for 1 h. (b) XBA vs time over the most active catalysts. Conditions: T = 80 °C, T/BA = 80, and Wcat = 25 mg.

the selectivity for (methyl)benzylbenzenes was 100% in all cases. Compared to the alkylation of toluene with isopropyl alcohol, this reaction can be catalyzed by relatively weak acid sites, as evidenced by the higher relative alkylation activity of MCM-41. Nevertheless, exactly the same trends as in the alkylation of toluene with isopropyl alcohol were obtained. In fact, the composites prepared with CTA+ proved to be even more inferior to the conventional USY2 zeolite, while the TPA+-treated sample (AT-TPA0.2-USY2) clearly outperforms 5033



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4. CONCLUSIONS The occurrence of recrystallization and/or restructuring from dissolved zeolitic species during alkaline treatment of conventional zeolites was studied. Zeolite recrystallization during desilication, with or without external additives such as tetraalkylammonium cations or aluminum hydroxide ions, does not occur. Conversely, when micelle-forming cationic surfactants are applied, the formation of ordered mesoporous material occurs concurrently with the introduction of intracrystalline mesopores. Accordingly, the materials obtained by treatments involving a base, micelle-forming cationic surfactants, dissolved Si species, and zeolite, such as pseudomorphic transformation and mesostructuring, should be considered as composites of hierarchical zeolites and ordered mesoporous materials (Figure 6). We demonstrate that in the studied reactions, the latter do not enhance catalytic activity, while highly crystalline single-phase hierarchical zeolites prepared with TPA+ yielded superior performance. Our results provide valuable insights regarding the solids formed during alkaline treatment, aid our understanding of their physicochemical implications, and highlight their potential in catalyzed reactions.

the other samples. Therefore, we can conclude that, in the studied reactions, only those prepared with TPA+ yield superior catalysts. The latter is assigned to their largely preserved intrinsic zeolitic properties, hence a higher number of Brønsted acid sites, and a more favorable nature of the external surface, i.e., derived from intracrystalline mesopores rather than from a secondary OMM. Therefore, in reactions requiring zeolitic acidity, the use of CTA+-treated zeolites is limited, as the inactive secondary phase dilutes the active zeolite phase. Naturally, in particular reactions, where the zeolitic acidity is best moderated to tune conversion and selectivities, e.g., catalytic cracking, the dilution of the zeolite phase using a less acidic secondary phase may be beneficial or even mandatory. It should be emphasized that the presence of a hierarchical zeolite phase together with an amorphous OMM phase renders it complicated for accurately ascribing possible benefits in catalyzed reactions. The latter is partially relevant taking into account that, based on the limited external surface of conventional zeolites (∼50 m2 g−1) and the large external surface of OMMs (often >1000 m2 g−1), the presence of a limited fraction of OMM in the bulk can leave a pronounced fingerprint on the N2 isotherm. This may result in the erroneous assignment of catalytic benefits, obtained by the bulk of the catalyst, i.e., hierarchical zeolite crystals, to a minor OMM constituent. We believe therefore that it is valuable that studies, involving the postsynthetic modification of zeolite crystals in alkaline solution with micelle-forming cations like CTA+ (see, for example, refs 14−17, 31, and 32), are complemented with a sample prepared under similar conditions, but with a tetraalkylammonium cation that does not form micelles, e.g., TPA+. The resulting solid should predominately feature a single-phase hierarchical zeolite, whose properties and catalytic performance should allow more accurate isolation of the value of the formed OMMs. For example, we anticipate that such a strategy should provide valuable insights regarding the recent works on “recrystallization”.14 The latter works focus, like mesostructuring (section 3.4), on the (partial) dissolution of conventional zeolite crystals by leaching in alkaline solutions containing CTA+, and the reassembly of dissolved species into OMMs. Depending on the degree of zeolite dissolution and OMM formation, the resulting solids can be classified into three groups. From the most zeolitic to purely OMMs, these are (i) coated mesoporous zeolites, (ii) micromesoporous nanocomposites, and (iii) mesoporous materials with zeolitic fragments in the walls, respectively. The first group is particularly interesting because its members often display the best (or close to the best) catalytic performance, while having high intrinsic zeolitic properties (crystallinity and microporosity) and only a minor amount of OMMs. Moreover, the alkaline solutions used to prepare such solids are typically relatively mild, e.g., 0.3−0.5 M NaOH, and correspond to those applied in desilication treatments aimed at mesopore formation.2 Accordingly, it is probable that the enhanced catalytic performance displayed by these materials was attained by the bulk of the sample, that is, the hierarchical zeolite crystals formed by desilication. In the latter process, the role of CTA+ species is expected to be primarily as a pore-directing agent optimizing the dissolution process, rather than to protect zeolite crystals from nonuniform leaching, as reported in ref 14.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Swiss National Science Foundation (Project 200021134572) is acknowledged for financial support. The Electron Microscopy Centre of the Swiss Federal Institute of Technology (EMEZ) is acknowledged for use of their facilities. Dr. J. C. Groen (Delft Solids Solutions B.V., Delft, The Netherlands) is acknowledged for collaboration on the characterization of porosity.

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REFERENCES

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Hierarchical Zeolites by Desilication - American Chemical Society

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