Enhanced Acidity and Accessibility in Al-MCM-41 through Aluminum

Article pubs.acs.org/cm

Enhanced Acidity and Accessibility in Al-MCM-41 through Aluminum Activation Roel Locus,† Danny Verboekend,*,† Ruyi Zhong,† Kristof Houthoofd,† Tony Jaumann,‡ Steffen Oswald,‡ Lars Giebeler,‡ Gino Baron,§ and Bert F. Sels*,† †

Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, Bus 2461, B-3001 Heverlee, Belgium Leibniz-Institute for Solid State and Materials Research Dresden, Institute for Complex Materials, Helmholtzstrasse 20, D-01069 Dresden, Germany § Department of Chemical Engineering, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium ‡

S Supporting Information *

ABSTRACT: Incorporating aluminum is the most widely applied and industrially relevant method to functionalize amorphous silica. However, established protocols yield predominately poorly distributed and inaccessible Al species, and as a result only ∼10−15% of the present aluminum gives rise to the acid sites, hampering the overall catalytic potential. Herein, the influence of alkaline activations with aqueous NaOH and NH4OH on the porosity, acidity, and catalytic properties of Al-MCM-41 is studied. By performing room temperature activations in 0.01−0.1 M NaOH or 0.5 M NH4OH, the Ostwald ripening of silica in alkaline media is exploited, which results in high mass retention yields (100−74%) and a controlled transformation of the 3.6 nm mesopores of the parent material to a broad pore range from 3 to ∼12 nm. Electron microscopy indicates the presence of additional interconnected intraparticle porosity, whereas no significant change in the shape and size of the original particles is observed. Elemental analysis reveals that the optimal alkaline activation with 0.05 M NaOH leads to a decrease in the Si/Al ratio at the surface, despite an increase in the bulk Si/Al ratio. 27Al magic angle spinning nuclear magnetic resonance spectroscopy demonstrates a large conversion of octahedral Al into tetrahedral Al, doubling the purely tetrahedral fraction from 30 to 60%. Pyridine-probed Fourier transformed infrared spectroscopy shows a doubling of the Brønsted and Lewis acidity after activation. The compositional and spectroscopic results are ratified by monitoring the relative accessibility of the acid sites, i.e., effective acidity (mol acid sites per mol Al). The alkaline activation enhances the effective acidity by increasing access to the Al sites trapped inside the pore wall and by reincorporation of the octahedral Al as accessible tetrahedral sites. As a result, an unprecedented effective acidity is obtained after the Al incorporation, which is substantiated using a novel accessibility concept. The catalytic potential of the activation protocol is demonstrated by quadrupling the catalytic activity for the acid-catalyzed alkylation of toluene with benzyl alcohol, an over-50% activity gain, a slightly enhanced selectivity, and a strongly reduced coking in the acid-catalyzed coupling of furfural with sylvan. surfactants as template in an alkaline synthesis,2−4 and the SBA family, which uses nonionic block copolymers in an acid synthesis.5,6 MCM-41 is the most widely applied OMS in many research applications such as catalysis, adsorption, separation processes, and controlled drug delivery.7−12 For many catalytic applications like petroleum refinery13 and also the emerging biomass refinery,14−16 large pore catalyst (supports) provide the opportunity to efficiently break down the bulky intermediates to more useful chemicals for further refinery or to synthesize oligomers or polymers. Some examples are hydrocracking of vacuum gasoil, hydrocracking, and hydro-

1. INTRODUCTION Silica, the most abundant material in the earth’s crust, exists under many forms both amorphous and crystalline. Highly porous amorphous silica can easily be synthesized as fumed silica or from sol−gel techniques.1 The latter is based on the unique hydrolysis and condensation properties of silica in aqueous solutions with different pH values.1 Particularly ordered mesoporous silica (OMS) are widely studied due to their easy synthesis, a high specific surface (typically around 1000 m2g−1), highly ordered unidirectional mesopores with a very narrow pore size distribution (which allow a straightforward material characterization), and a relatively good mechanical, thermal, and chemical stability.2 OMS are synthesized using organic micelles as a template. Two families of OMS are widely used: the MCM (M41S) family, which uses long cationic alkylammonium © 2016 American Chemical Society

Received: July 15, 2016 Revised: October 8, 2016 Published: October 11, 2016 7731

DOI: 10.1021/acs.chemmater.6b02874 Chem. Mater. 2016, 28, 7731−7743

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Chemistry of Materials isomerization of waxes to lubricants and oligomerization of C3− C5 olefins to C6−C18 hydrocarbons.13,17 In these reactions functionalized mesoporous silica form an attractive alternative, as their large mesopores are not hampered by diffusion and/or accessibility limitations, as can be the case for microporous aluminosilicate catalysts such as zeolites and SAPOs.13,18 A crucial step in the synthesis of a functional MCM-41 is the incorporation of heteroatoms in the structure via isomorphous substitution.19 By replacing tetravalent Si atoms in tetrahedral positions by trivalent Al atoms, Brønsted and Lewis acid sites are created.20−24 These sites are thermally rather stable and can be easily regenerated. This type of acidity is accordingly the most widely used type of solid acid in industry.25 Al-MCM-41 typically contains weak to moderate Brønsted acidity combined with Lewis acidity.22 These types of acid sites can be useful in reactions where the strong Brønsted acidity, for example, present in zeolites, is detrimental to the catalytic performance. Hence AlMCM-41 catalysts have been tested in the literature for many acid catalyzed reactions, for example, (hydro)cracking, olefin upgrading, and fine chemical synthesis, and a number of base catalyzed reactions.13,26−28 The substitution of Si by Al during the hydrothermal synthesis however is often inefficient, as the majority of the Al is incorporated into the pore walls and therefore is inaccessible for catalysis.20,29 As a result, Al-MCM-41s display rather low overall acidities and poor effective acidities (EA), i.e., the percentage of Al atoms that effectively creates an acid site, hereby lowering the catalytic potential. An exhaustive literature study (Table S1) reveals that Al-MCM-41 typically have an EA lower than 15%. Also postsynthesis grafting of pure-silica MCM-41 with Al can be used to introduce Al in MCM-41. However, similar to the aluminum introduced during hydrothermal synthesis, the obtained acidity and EA are low (0−15%, Table S1).30 In addition, the used postsynthesis techniques are often laborious, involving multiple steps, polycations, and/or organic solvents.30−33 Herein we explore the potential of alkaline activations on AlMCM-41 to enhance acidity and resolve accessibility issues. The physicochemical implications of the activations are thoroughly studied and the Ostwald ripening process of silica in alkaline media is exploited to achieve a widening of the mesopores without considerable mass losses. Furthermore, it is demonstrated that an optimized alkaline activation on Al-MCM-41 causes a transformation of Lewis and Brønsted acidity, more than doubling the effective acidity of the present Al compared to the parent Al-MCM-41 but also doubled compared to the maximal values achieved in the literature. In addition, the differences between the effects of alkaline treatment on amorphous (AlMCM-41) and crystalline aluminosilicates (zeolites) are highlighted. The results give rise to the definition of a new accessibility concept, strongly different than the conventional mass transport limitations encountered in zeolites. The increased acidity and the pore system modification are reflected in an up-to 4-fold boost of the activity toward acid-catalyzed reactions using substrates of distinct size, nature, and origin. These results open the door toward postsynthetic enhancement of the accessibility and catalytic performance of heteroatom-containing amorphous (meso)porous silicas.

water for 15 min. Then ethanol (Fisher) and ammonium hydroxide (Chemlab) were added and stirred for 15 min. Finally tetraethyl orthosilicate (TEOS, Acros) was added under stirring to obtain a synthesis mixture with the following molar composition 1TEOS: 0.3CTAB/11NH3/144H2O/58ethanol. The mixture was then aged for 2 h under stirring, filtered, washed, dried at 80 °C, and calcined (1 °C min−1 to 550 °C for 8 h). Al-containing MCM-41s were prepared by adding sodium aluminate (Riedel) to the mixture before the addition of TEOS. NaAlO2/TEOS ratios were 50, 19, and 10 tested and herein the total amount of Si and Al atoms is the same as the amount of Si atoms in the synthesis without Al. Alkaline activations were performed by immersing 0.333 g of MCM41 in 10 cm3 solutions of 0.01−1 M sodium hydroxide (Fisher) or 0.1− 0.5 M ammonium hydroxide (Chemlab). Samples are abbreviated as M(Si/Al)-base concentration. Herein “M” stands for MCM-41, and indicates the nature of the parent (P) sample. If the base in the manuscript is not specified, NaOH was used. “NH4OH” was added as a suffix when ammonium hydroxide was used. For example, an Al-MCM41 sample with Si/Al of 50 (in the gel) treated in 0.05 M NaOH will be abbreviated as “M50−0.05”. In addition, MCM-41 treated with NaOH solutions of 0.01 M are referred to as “weak”, 0.05 M as “optimal”, and 0.1 and 0.2 M as “severe”. An Al-MCM-41 with Si/Al of 19 treated in 0.5 M NH4OH would be referred to as “M19−0.5-NH4OH”. Prior to acidity characterizations and catalytic testing samples were converted from the sodium to the protonic form by 3 consecutive NH4+-ion exchanges of 6 h (1 g per 100 cm3 NH4NO3 solution of 0.1 M) with intermediate filtering and washing followed by a calcination as described above. 2.2. Material Characterization. Nitrogen physisorption isotherms were determined using a Tristar 3000 (Micromeritics) at −196 °C. Samples were degassed prior to analysis under N2 flow at 400 °C overnight. The total pore volume determined at p/p0 = 0.99 (Vpore) from the adsorption isotherm was further categorized by the fraction adsorbed at 0 < p/p0 < 0.4 (Vpore‑intrinsic) and at 0.4 < p/p0 < 0.99 (Vpore‑extra). Pore size distributions (PSDs) were determined using the NL-DFT (nonlocal density functional theory) model for cylindrical pores in metal oxides and the BJH (Barrett, Joyner, Halenda) model. Specific surface was determined using the BET model (Brunauer, Emmett, Teller). Argon physisorption isotherms were determined at −196 °C and −186 °C with a Quantasorb Autosorb AS-1 (Quantachrome). Samples were degassed prior to analysis under vacuum at 350 °C. PSDs were calculated from the adsorption isotherm at −186 °C using the NL-DFT model for silica/zeolites with cylindrical/spherical pores. Transmission electron microscopy (TEM) was carried out on a FEI Tecnai F30 with a field emission gun at 300 kV. Prior to analysis, samples were dispersed in ethanol and dropped on a copper grid with lacey carbon layer. The pore size of the parent sample was evaluated by fast Fourier transformation/inverse fast Fourier transformation with a background/noise reduction mask over the whole image. In a crosssection of the amplitude of this image the mean distance between the local maxima, belonging to lattice fringes which enclose the pores, was determined and pore sizes calculated. The pore size of the treated samples were estimated by analyzing individual pores inside the particle. For image processing, the program Digital Micrograph (Gatan Inc.) was used. The periodicity was assessed by measuring small-angle X-ray scattering (SAXS) on a SAXSess mc2 (Anton Paar) instrument with line-collimated Cu Kα radiation (λ = 1.5418 Å) and a 2D imaging plate detector. Bulk Si and Al contents (“SiICP” and “AlICP”, respectively) were determined using inductively coupled plasma optical emission spectroscopy (ICP-OES) using analytical wavelengths of λ = 251.611 nm for Si and λ = 396.153 nm for Al. In total, 50 mg of the samples was mixed with 250 mg of LiBO2 and heated for 10 min at 1000 °C. The molten material was then dissolved in 0.42 M HNO3 and further diluted. Surface Si and Al contents (“SiXPS” and “AlXPS”, respectively) were probed with X-ray photoelectron spectroscopy (XPS) using a Specs Phoibos 100 with Mg Kα radiation (1253.6 eV) at 300 W in an energy range of 0−1000 eV. The spectrometer is equipped with a hemispherical analyzer allowing high sensitivity and high-resolution experiments.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis. Parent (P) Al-MCM-41 was synthesized as stated in the literature.34,35 For a typical synthesis first, hexadecyltrimethylammonium bromide (CTAB, Acros) was dissolved in distilled 7732


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Chemistry of Materials Table 1. Mass Yields and Properties of Parent and Treated Al-MCM-41 sample M50-P M50-0.01 M50-0.05 M50-0.1 M50-0.2 M50-0.1-NH4OH M50-0.5-NH4OH

yielda (%)

SiICP/AlICPb (mol mol−1)

SiXPS/AlXPSc (mol mol−1)

[SiICP/AlICP]/[SiXPS/AlXPS] (/)

SBETd (m2g−1)

dporee (nm)

Vporef (cm3g−1)

Vpore‑intrinsicg (cm3g−1)

Vpore‑extrah (cm3 g−1)

100 100 89 73 27

54 67 66 54 22

97 89 62 110 40

1.8 1.3 0.9 2.0 1.9

1195 1111 624 639 616

3.6 3.0; 5.2 6.8 6.4 5.2

0.89 0.73 0.68 0.63 0.47

0.84 0.59 0.35 0.36 0.35

0.05 0.14 0.33 0.27 0.12

97 87

− −

− −

− 854

− 3.0; 5.5

− 0.68

− 0.47

− 0.21

− −

Mass after activation divided by mass before activation, corrected for mass losses through filtration (6 wt %). bICP-OES. cXPS. dSpecific surface (BET method). eMain pore diameter(s) based on the DFT pore size distribution. fTotal pore volume; N2 physisorption. gPore volume for 0 < p/p0 < 0.4; N2 physisorption. hPore volume for 0.4 < p/p0 < 1; N2 physisorption. a

Energy scale and binding energy were calibrated with Cu and Au foils at the binding energies of Cu 2p3/2 (932.67 eV) and Au 4f7/2 (84.00 eV), respectively. Because of large gas adsorption, the sample chamber could only be held at a base pressure of around 3 × 10−8 mbar after 1 h of pumping after degassing in a prevacuum chamber. Spectra were taken with a pass energy of 15 eV and a step size of 0.1 eV. The spectra are analyzed with the software CASA-XPS and the element concentrations are calculated with the standard sensitivity factors from the software. 27 Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected using a Bruker Avance400 spectrometer with a 9.4 T magnet. A total of 36 000 scans were accumulated with a recycle delay of 200 ms. The rotor had a spinning frequency of 10 kHz. The samples were hydrated for 48 h over a saturated salt solution prior to analysis and packed in a 4 mm MAS rotor. The integration of the tetrahedral Al peak was performed between 45 and 60 ppm. Pyridine-probed Fourier transformed infrared (PP-FTIR) spectroscopy was executed using a Nicolet 6700 spectrometer equipped with a DTGS detector. Samples were pressed into self-supporting plates and degassed at 400 °C for 1 h in vacuum before measurements. Lewis and Brønsted acid sites were analyzed using a pyridine probe. After evacuation, the samples were saturated by 4−5 pulses of ∼25 mbar of pyridine at 50 °C for 1 min. Physisorbed pyridine was removed by heating to 150 °C. The spectrum was collected at 150 °C after 20 min of equilibration. The absorptions at 1450 and 1550 cm−1 corresponded to the amount of Lewis and Brønsted acid sites, respectively. The extinction coefficients were determined by Emeis.36 2.3. Catalytic Testing. Catalytic tests were executed by reacting toluene with benzyl alcohol. In total, 0.025 g of catalyst was mixed with 4.98 cm3 (47 mmol) toluene (Fisher), 0.062 cm3 (0.6 mmol) benzyl alcohol (Sigma-Aldrich), and 0.048 cm3 (0.3 mmol) propyl cyclohexane (TCI Europe) as an internal standard. The catalyst powder was predried for 2 h at 300 °C (ramp 5 °C min−1) and the reaction was carried out in closed Schott bottles (Duran) for 1 h under stirring at 120 °C. The reaction mixture was filtered and analyzed with gas chromatography (Agilent 6850 series). The activated materials were also tested in the sylvan reaction with furfural. Typically 6 mmol of furfural, 13.2 mmol of 2-methylfuran (sylvan), and 20 mg of naphthalene were added in sequence to 50 mg of a predried catalyst in a 10 cm3 glass vial. The reactor was put into a preheated copper block at 50 °C under magnetic stirring for 6 h. The reaction solution was sampled after cooling the reactor in an icy H2O bath. After separation of solid catalyst by centrifugation, the supernatant reaction solution was analyzed by GC. To analyze the coking of the catalyst after reaction, thermogravimetric analyses (TGA) curves were obtained on a Q500 equipment (TA Instruments, Brussels, Belgium) by heating the samples from room temperature to 800 °C at 5 °C min−1 under O2.

morphology and the pore ordering (section 3.2). Next the effects on the coordination of the Al are investigated as well as the consequences toward the acidity and catalytic activity of the Alsites (section 3.3). Finally the implications of the activations on Al-MCM-41s are discussed in comparison with those on zeolites (section 3.4). 3.1. Mass Yields, Composition, and Porous Properties. The properties of the Al-MCM41s before and after alkaline activations are summarized in Table 1 and Table S2. Parent AlMCM-41s displayed the typical porous properties of MCM-41: uniform 3.6 nm mesopores (DFT) and high specific surfaces and pore volumes (SBET > 1100 m2g−1 and Vpore > 0.8 cm3g−1). The aluminum was almost fully incorporated in the parent MCM-41s during the synthesis as suggested by the similar Si/Al ratios of the bulk (Table 1 and Table S2) and those used during the synthesis gels. The Si/Al ratio at the surface in M50-P on the other hand was almost doubled compared to the bulk values (Table 1). The latter suggests that the aluminum in M50-P is not distributed evenly throughout the MCM-41 particles and/or that the Al is mostly encapsulated in the silica walls. On the basis of the mass yields (Figure 1a, Table 1), the effects of alkaline activations can be subdivided in three leaching regimes: first a weak leaching (0.01 M NaOH) where (almost) no mass loss occurred, second an optimal leaching (0.05 M NaOH) where ∼90% of the parent sample was preserved and finally a severe leaching (0.1−0.2 M NaOH) where only a 73% and 27%, respectively, of the parent material remained. Elemental analysis indicates the mass loss after optimal and severe activations is mainly due to loss of Si (Figure 1a−c). Weak activations induced a small Al loss roughly proportional to the Al content of the parent material. Higher NaOH concentrations however did not cause extra dealumination. For weak activations the loss of Si is still limited and smaller than the loss of Al, causing an increase in the bulk Si/Al ratio (Figure 1d). Only for activations with 0.1 M NaOH or higher, SiICP/AlICP values lower than in the parent materials were obtained. A substantially different trend was obtained from analysis of the surface composition (Table 1). After optimal activation with 0.05 M NaOH bulk and surface Si/Al ratios converge to similar values (∼65 mol mol−1). However, at higher NaOH concentrations the difference between the bulk and surface Si/Al ratio increased again. These results suggest that a pronounced redistribution of Al may occur within the sample and that the sample M50-0.05 could within such transformation be considered as optimal. The porous properties were assessed using nitrogen and argon physisorption (Figure 2a,c and Figures S3 and S4). Type IV nitrogen isotherms were observed, indicating capillary con-

3. RESULTS AND DISCUSSION The effects of the alkaline activations on the mass yields, the composition and the porous properties of the Al-MCM41s are first discussed (section 3.1), followed by an analysis of the 7733


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signal at lower pore sizes disappeared and the broad peak at higher pore sizes got more pronounced. A new maximum appeared in the PSDs at 6.8 (for M50-0.05) and at 6.4 nm (for M50-0.1). The size of the mesopores ranged from ∼5 to 13 nm (for M50-0.05) and to 12 nm (for M50-0.1). After severe alkaline activation with 0.2 M NaOH the pore size of the extra porosity decreased again and the PSD was also less broad, suggesting the pores are completely dissolved rather than enlarged. For M19 the pore enlargement is less distinct: only for M19-0.05 and M19-0.1 small maxima appeared at higher pore diameter. For M10 no extra porosity is visible (Table S2, Figure S3), indicating that a high Al content results in a different type of dissolution mechanism. The comparison of Ar physisorption isotherms at −196 °C and −186 °C (Figure S4) indicated the absence of ink bottle mesopores (that is, larger pores only accessible via smaller pores), since no cavitation was present in the desorption isotherms at −196 °C.37,38 To visualize the change in the porous system the pore volume (Vpore) was divided in two parts: Vpore‑intrinsic, being the Vpore for 0 < p/p0 < 0.4 and Vpore‑extra, for 0.4 < p/p0 < 1. Figure 2a shows that M50-P almost reaches the maximal Vpore at p/p0 of 0.4. Accordingly, Vpore‑intrinsic is used as an approximate indicator for the amount of parent “intrinsic” MCM-41-type pores. Vpore‑extra on the other hand is used to describe the extra porosity created by the activation. After optimal alkaline activation at 0.05 M, a maximal amount of extra porosity is created, while the intrinsic porosity has more than halved (Figure 3b). These trends are more pronounced when the effect on single particles is visualized by correcting for the mass yield (Figure S5). The specific surface (SBET) of M50 (Table 1) halves after optimal activations (0.05 NaOH) and then remains constant. This effect is caused by many small pores in a certain volume creating a larger surface than fewer larger pores. As described above the pore size increases significantly after optimal activations and thus the SBET decreases. For M19 and M10 (Table S2), a more linear decrease is visible since fewer large mesopores are created. The total pore volume (Vpore) of the samples roughly follows the mass yields: 70% remains after optimal activation (Figure 3a). The use of NH4OH in alkaline activations showed similar effects as NaOH, but higher concentrations were needed to obtain the same effects: 0.5 M NH4OH corresponded to the optimal NaOH activations (0.05 M). The use of a higher concentration may be related to the lower basic strength of NH4OH as compared to NaOH. Alkaline activation with NH4OH have as an advantage over activation with NaOH that the resulting material is already in the NH4-form. As such, the ion-exchanges series before the calcination to yield the desired protonic form can be avoided. The doubling of the pore size and the high mass yield after optimal activation is explained by a controlled Ostwald ripening process.1,39−42 At alkaline conditions, an equilibrium exists between silica dissolution and condensation, which are both driven by the OH−-ions in solution. This equilibrium shifts to the dissolution when the pH increases. The dissolution/condensation process is governed thermodynamically to minimize the overall energy by reducing the surface/volume ratio. Here, this process is represented by the gradual transformation of a highsurface area MCM-41 material to a more stable bulk silica. In practice, Ostwald ripening comes down to the inverse proportionality between SiO2 solubility and the curvature of the surface it dissolves: it is mostly known for the growth of larger (nonporous) silica particles in favor of smaller particles (positive,

Figure 1. (a) Mass yields (Y) before and after activation with aqueous NaOH. Elemental abundance of Si (b) and Al (c) after activation from ICP-OES (SiICP and AlICP) multiplied by the mass yields. (d) Si/Al ratios after activation (SiICP/AlICP). The legend in part a applies to the entire figure.

densation in cylindrical mesopores. After alkaline activation on M50-P hysteresis loops appeared, demonstrating the creation of pores larger than 3.9 nm. These results are confirmed by the PSDs (DFT model) visible in Figure 2b,d and Figure S3 (corresponding BJH PSDs and the NL-DFT PSD from Ar sorption can be found in Figure S4a,b,d). The sharp distribution signal caused by the parent mesopores (3.6 nm) gradually disappeared after activation and a broad range of mesopores was created. For weak activations the parent signal split up in two peaks: one sharp peak at lower pore sizes and one broader peak at higher pore sizes. After treatment at 0.05 and at 0.1 M NaOH, the 7734


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Figure 2. N2 physisorption isotherms (a,c) and related DFT pore size distributions (b,d) of parent and treated Al-MCM-41.

however, Ostwald ripening mainly causes a pore transformation rather than a dissolution. 3.2. Morphology and Ordering. To visualize the effect of the alkaline activations on the morphology of M50-P bright-field TEM micrographs were recorded (Figure 4). The parent material (Figure 4a) displays spherical particles with diameters distributed around 500 nm. The periodic mesopore ordering is visible as parallel lines in the particles (inset in Figure 4a). The use of fast Fourier transformation (omitted for clarity) and inverse fast Fourier transformation technique with an output mask to reduce noise enables a good estimate of the pore size of about 3 nm (as shown in Figure 4e−g) in the parent sample. After alkaline activations, the size of the particles is preserved. The spherical shape in principle remains with an increase of irregularities at the outside margin (Figure 4b,c). No periodic mesopore ordering is found after alkaline activation. The pores are slightly expanded and seem to become interconnected by the dissolution, which results in a diminishing periodicity. Nevertheless, the pore size was estimated at ∼3−5 nm. When 0.2 M NaOH was used, the particles become smaller and a reduced sphericity is observed (Figure 4d). Pore sizes are determined to approximately 4−6 nm indicating an advanced expansion and increasing irregularity due to higher dissolution of Si and Al during the severe activation conditions. The values for the pore sizes coincide well with the results from N2 physisorption. SAXS diffractograms (Figure 5) of the parent material showed a first order peak and a higher order peak, indicating the presence of a periodic ordering of the mesopores. The orientation of the mesopores perpendicular to the surface of the spherical particles implicates that the periodicity is not perfect, which explains the rather small reflections. After optimal activation, only a small fraction of the first order peak remained, corroborating the loss in periodicity and ordering, earlier suggested by the broad PSD present after alkaline activation. After activation with 0.2 M NaOH, no order remained. 3.3. Coordination, Acidity, and Functionality. To determine the effect of the alkaline activations on the Al coordination, 27Al MAS NMR spectroscopy experiments were performed (Figure 6). In amorphous aluminosilicates, roughly two general types of Al coordination occur: on one hand, during

Figure 3. (a) Trends after alkaline activations of the total pore volume (Vpore) for different Si/Al ratios. (b) Evolution of the intrinsic MCM-41type parent porosity (Vpore‑intrinsic) and the extra porosity generated by alkaline activation (Vpore‑extra) of M50-P.

convex curvatures), but it can also be considered for pores (negative, concave curvatures): larger pores are enlarged and smaller pores are reduced and eventually filled up. With higher NaOH concentration in solution, the equilibrium shifts from silica condensation to silica dissolution. This shift explains that in M50-0.2 the porosity is not enlarged substantially but rather the complete particle is dissolved. For weak and optimal activation, 7735


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Figure 4. (a−d) Bright-field TEM micrographs of M50 before and after alkaline activation (scale bars are 100 nm). The inset in part a shows the parent mesopores arranged parallel in periodic domains. The shape and size of the parent particles is retained for weak and optimal activations. (e−g) Illustrating the technique for calculation of an average pore size for the parent sample: (e) shows the original TEM micrograph, (f) fast Fourier transformation (omitted for clarity) and subsequent inverse fast Fourier transformation technique with an output mask (e) to reduce background noise. From part f, a cross section of the amplitude (g) is studied toward periodicity. The pore size is indicated with the double arrow and is calculated as the mean distance between the maxima (distance between vertical lines in part g = 2 nm).

signal (AlNMR) and the tetrahedral Al signal (Al(IV)NMR) were quantified. AlNMR (Figure 7a) is first compared to the Al content in the samples as determined by elemental analysis (AlICP, Figure S6). These trends were approximately the same, which demonstrates that the NMR visibility of Al is similar for the different MCM-41s, before and after activation. The decrease of the total AlNMR after weak activations is not visible in Al(IV)NMR (Figure 7b). Moreover an increase in the tetrahedral signal is observed. This observation thus shows that octahedral Al(VI) is dissolved and partially reincorporated into the structure as Al(IV) during the activation. It also suggests that during optimal alkaline activation, predominately Al(VI) species are removed from the solid. The coordination transformation is attributed to the appearance of Al species in alkaline solutions as a tetrahedral Al(OH)4− ion and the OH− ion driven incorporation of these species into silica materials during the Ostwald ripening process.46 The fraction of tetrahedral Al (Al(IV)NMR/AlNMR) almost doubles from 30−40% to 60−70% after activation (Figure 7c). This increase occurs for all Si/Al ratios. Moreover, assuming that the Al in the NMR spectra around 25 ppm is distorted tetrahedral Al(IV),22 around 85% of all Al species are tetrahedrally coordinated after activation. To monitor the acidity of these transformed Al sites in M50-P, pyridine-probed FTIR spectroscopy experiments were performed (Table 2, Figure 8a, and Figure S7). Upon optimal activation with 0.05 M NaOH, both Lewis and Brønsted acidity increased, respectively, from 25 (M50-P) to 44 μmol g−1 (M500.05) and from 11 (M50-P) to 25 μmol g−1 (M50-0.05). This increase was attained despite a small decrease in the total Al content. The increased acidity indicates a doubling of the EA from 14 to 31%. For M50-0.1, the acidity values are slightly higher than for the parent material, whereas for M50-0.2 they become lower. M50-0.5-NH4OH also showed an increase in acidity intermediary between M50-0.05 and M50-0.1. Temperature-programmed desorption (TPDs) of gaseous ammonia (NH3) was used as an additional technique to assess the acid properties of the samples (Figure S8). The peak around 350 °C

Figure 5. SAXS diffractograms of Al-MCM-41 samples before and after alkaline activation. Only the parent sample shows first and (small) second order reflections. In M50-0.05 and M50-0.1, the first order peak gradually disappears. The diffractograms are slightly shifted vertically to clearly indicate the trends in the reflections (y-axis shows log(scattered intensity) in arbitrary units).

the synthesis Al can be fully incorporated into the silica material by isomorphous substitution and where it displays a tetrahedral coordination (Al(IV)). On the other hand, high-temperature calcination causes transformation of these species to extrastructure Al clusters, which are not incorporated into the silica structure. These Al species display an octahedral Al coordination (Al(VI)).20−23 Al(IV) and Al(VI) in amorphous silica give different signals in 27Al MAS NMR at around 54 and 5 ppm, respectively, and thus can easily be distinguished. All three parent MCM-41 materials contain reasonable amounts of octahedral Al(VI). Strikingly, for all three AlMCM-41s a significant increase of the tetrahedral peak and a decrease in the octahedral peak was observed after alkaline activation (Figure 6). By integrating the spectra, the total Al 7736


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Figure 7. Al coordination in parent and activated Al-MCM-41 samples as measured with 27Al MAS NMR. (a) The total Al content corrected for mass differences (AlNMR). (b) The amount of tetrahedrally coordinated Al corrected for mass differences (Al(IV)NMR). The integration to calculate Al(IV) was performed between 60 and 45 ppm. (c) The fraction of tetrahedrally coordinated Al (Al(IV)NMR/AlNMR). The legend in part a applies to the entire figure. Yield corrected data for parts a and b are provided in Figure S5.

Table 2. Acidity and Catalytic Performance of Parent and Treated Al-MCM-41 sample M50-P M50-0.05 M50-0.1 M50-0.2 M50-0.5-NH4OH

Figure 6. 27Al MAS NMR spectra of M50 (a), M19 (b), and M10 (c) before and after alkaline activations. An increase in the tetrahedral Al(IV) signal around 54 ppm is visible and a decrease in the octahedral signal (around 5 ppm) after activation. The spectra are corrected for mass differences.

Ala (μmol g_‑1)

Bb (μmol g−1)

Lb (μmol g−1)

EAc (%)

XBAd (%)

255 221 251 599

11 25 17 10 17

25 44 24 18 29

14 31 16 5

8 32 17 17

a ICP-OES. bBrønsted (B) and Lewis (L) acid sites probed with PPFTIR. cEffective acidity calculated as (B + L)/Al. dConversion of benzyl alcohol (XBA) in acid-catalyzed alkylation of toluene.

of M50-0.05 increases with ∼50% compared to M50-P, confirming the increased acidity of the activated sample. To demonstrate the functionality of the formed acid sites, the samples were tested in the acid-catalyzed reaction of toluene with benzyl alcohol. The conversions of benzyl alcohol are shown in Table 2 and Figure 8b. The reaction with M50-0.05 attains a 4fold increased conversion compared to the parent material (32%

vs 8%). M50-0.1 and M50-0.2 cause a doubling of the conversions of up to 17%. The conversion follows the same trend as the number of Lewis and Brønsted acid sites after activation. In the weak and optimal activation range a linear trend between the Brønsted acid sites and the conversion is recognized (Figure 8c). As a proof of concept, also an Al-MCM-41 was 7737


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53% increase in the conversion of furfural compared to M50(2)P with a slight increase in selectivity toward FMBM (41−44%) (Figure 9a,b). The coke content after reaction was reduced with

Figure 9. Activity of parent M50(2)-P and the activated M50(2)-0.05 sample in the acid-catalyzed reaction of furfural (FFA) and 2methylfuran to 2,2′-(2-furylmethylene)bis(5-methylfuran) (FMBM). (a) FMBM yield (YFMBM) vs reaction time (t). (b) FMBM selectivity (SFMBM) vs FFA conversion (XFFA).

71% from 14 wt % in M50(2)-P to 4 wt % in M50(2)-0.05. This reduced coking is an important advantage of the larger mesopores in M50(2)-0.05. The results from sections 3.1−3.3 are schematically summarized for M50-P (Figure 10). Herein, the transformations of the Al-MCM-41 induced by the activations are visualized: the partial pore enlargement/partial pore filling controlled by Ostwald ripening, the mass retention after activation, and the change in Al coordination due to Al speciation chemistry in alkaline medium and effect on acidity. The results from inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS), NMR, and acidity measurements suggest that an optimal alkaline pH value exists for a (re)incorporation of Al into silica materials. With respect to the experimental activation conditions used in this contribution, this value corresponds to ∼12.7 in the starting solution (0.05 M NaOH). The high pH is important to ensure that the Al is in solution as a tetrahedrally coordinated Al(OH)4− complex. However, the solution should not be excessively alkaline as the increased silica solubility will lead to either an excessive dissolution (in the case of post-treatment) or an incomplete precipitation (in the case of the initial silica synthesis). For example, the pH variations occurring during the MCM-41 synthesis (Figure S10) may be directly related to the pronounced difference between the bulk and surface composition in M50-P (Table 1).

Figure 8. (a) Lewis (L) and Brønsted (B) acid sites before and after activation of Al-MCM-41 samples. A maximum for optimal activations is visible at 0.05 M NaOH. (b) The conversion (X) of benzyl alcohol in the acid catalyzed alkylation of toluene for M50-P after alkaline activation. The same trend as in part a is visible. (c) Conversion (X) as a function of the Brønsted acidity (B) for samples M50-P, M50-0.05, and M50-0.1.

prepared using a distinctly different synthetic approach (referred to as “M50(2)-P”), resulting in 1D ordered parallel mesopores (Table S9).43 These materials were also tested in the alkylation reaction. As was the case for the M50-derived samples, an activation with 0.05 M NaOH caused an increase in the catalytic activity, nearly doubling the conversion of benzyl alcohol (Table S9). As a second reaction, M50(2)-P was tested in the acidcatalyzed reaction of furfural with two 2-methylfuran (sylvan) molecules to 2,2′-(2-furylmethylene)bis(5-methylfuran) (FMBM). Furfural is an important platform chemical from biomass refinery, and FMBM can be used as diesel or jet fuel.44 FMBM has a diameter of ∼0.9 nm and thus diffusion limitations, even with the usual mesoporous materials like MCM-41, may occur. Optimal alkaline activation with 0.05 M NaOH induced a 7738


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can be used as leaching agent for a more controlled leaching.38,59 The similarity between the behavior of 12-MR zeolites and MCM-41 shows that the framework crystallinity of 12-MR zeolites does not ensure a superior stability toward alkaline media. The porous properties of conventional (untreated) zeolites and Al-MCM-41s differ substantially: Al-MCM-41 displays very high specific surfaces and pore volumes (typically above 1000 m2g−1 and 0.75 cm3g−1, respectively).2,3 In contrast, zeolites feature lower specific surface areas (∼400−700 m2 g−1) and pore volumes (∼0.25−0.45 cm3 g−1). By alkaline activation, the properties in the two materials converge to more comparable values, as Vpore and SBET decrease in Al-MCM-41 and increase in zeolites.52−57,60 Another striking feature that influences alkaline treatments in zeolites is the Si/Al ratio of the parent material. This influence is attributed to the role of Al in solution as a pore directing agent.56 When a zeolite is treated under alkaline conditions, both Si and Al are dissolved, but the Al brought into solution is deposited back onto the external surface.56,61 As a result only Si is removed from the solid. This chemically adsorbed Al is energetically more favorable than the Al species in solution and by covering the external surface thus prevents further dissolution.61 The higher the Al content in the parent material, the faster the external surface is fully covered with Al species and the less efficient the creation of extra porosity is. Accordingly, minimum Si/Al ratios have been identified to enable the creation of mesopores by alkaline treatment. In Al-MCM-41s, comparable results are found. M50 shows a large increase in Vpore‑extra after optimal activation (Figure 3), while in M19 and M10 limited extra porosity is formed (Table S2, Figure S3). Nevertheless, future studies may optimize the impact of the alkaline activation focusing on these Al-richer MCM-41s. Since Al is reincorporated at the surface of zeolites during alkaline treatments but Si stays in solution, the Si/Al ratio decreases during the activation. This decrease is dependent on the concentration of NaOH as silica dissolves more easily at higher pH values (Figure 11).54,62,63 In Al-MCM-41 a remarkable increase in Si/Al ratio is observed after weak and optimal alkaline activations with 0.01−0.05 M NaOH. Figure 1 showed that a substantial fraction of the Al is removed from the material in these mild activations. This observation may be explained by the presence of the octahedral extra-structure

Figure 10. Schematic representation of optimal alkaline activation of M50-P with 0.05 M NaOH. Change in pore size, mass retention (height of the drawing), loss of porous volume (pore blockings), and the types of Al are included. In M50-P, the difference in pore size is exaggerated to indicate Ostwald ripening enlarges bigger pores and reduces smaller pores.

3.4. Comparison of Alkaline Activations, Acidity, and Accessibility of Amorphous with Crystalline Aluminosilicates. The alkaline activation of Al-MCM-41 (an amorphous aluminosilicate) in this research may be compared to other alkaline activations developed to create hierarchical mesoporous zeolites (crystalline aluminosilicates, Table 3).16,45−49 This Table 3. Implications of Optimal Alkaline Treatments on Amorphous and Crystalline Porous Aluminosilicatesa parameter

amorphous

crystalline

specific surface Al content Al(IV) species acidity accessibility catalytic performance

− − ++ ++ ++b ++

+ + 0 − ++c ++

a

Ranges from decrease (−), to unchanged (0), to increase (+), to strong increase (++). bOn the basis of reducing the number of fully inaccessible Al atoms present in the pore walls and the increase of completely and locally accessible sites. cOn the basis of the reduction of nonlocal size and diffusion-based limitations.

comparison enables one to conclude that the alkaline treatments in both cases yield strongly enhanced catalytic performance. However, these improved catalytic properties stem from a fundamentally different physicochemical backgrounds. For alkaline activations on zeolites a first important distinction is that the desired benefits usually relate linearly with the applied alkalinity. However, for amorphous silicates, an optimum seems to exist (0.05 M for M50-P). Another large difference regards the microporosity of the starting materials. In zeolites, a sharp contrast in stability is visible between 10 membered rings (10MR) and 12-MR zeolites.16 In 10-MR zeolites (such as ZSM-5), optimal hierarchical structures are obtained after activation with aqueous NaOH at increased temperature without considerable amorphization of the zeolite framework.45,50−52 Conversely, 12MR zeolites (USY and beta) are very sensitive to alkaline media and dissolve and amorphize readily in aqueous NaOH even at room temperature.16,53−58 In this sense, MCM-41 behaves like a 12-MR ring zeolite in alkaline media, first as it readily dissolves at mildly basic conditions, second since a weaker base (NH4OH)

Figure 11. Composition of Al-MCM-41 and USY zeolites as a function of the NaOH concentration in the applied alkaline treatment. USY28 and USY17 regard USY zeolites with (in the parent materials) Si/Al ratios of 28 and 17, respectively.54,62,63 7739


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Chemistry of Materials Al(VI) in the Al-MCM-41 parents. It could be hypothesized that these Al(VI) species are easily removed from the Al-MCM-41 structure, (partially) converted to tetrahedral Al(OH)4− species, and then partially reincorporated in the solid. At higher NaOH concentrations, Al-MCM-41s follows the decreasing trend of the Si/Al ratio in zeolites, likely caused by the more severe alkaline conditions. The crystallinity of zeolites in general ensures a nearly complete tetrahedral incorporation of Al into the framework. This stands in great contrast to Al-MCM-41s where a substantial fraction of octahedral (extra-structure) Al is present after calcination.59,64,65 In addition, in OMS a large fraction of tetrahedral Al is completely embedded into the pore walls. As a result, only a small fraction of Al is present at the surface and can effectively be used as an acid site (Table S1). In zeolites the combination of micropores combined with the full tetrahedral coordination implies that virtually every Al present gives rise to a measurable Brønsted acid site (EA ≈ 100%).66 However, in zeolites the enhancement of the external surface after treatment implies a crystallinity loss which is associated with an acidity loss.67 In Al-MCM-41s on the other hand, alkaline activations create extra acidity, since the octahedral extra-structure Al is converted to accessible tetrahedral species and Al previously incorporated into the pore wall, is reincorporated at the new pore surface, causing a doubling of the EA compared to the parent. On the basis of the insights gathered within this work, the definitions of accessibility (limitations) of catalytic sites within micro- and mesoporous materials may be refined (Figure 12). Alkaline treatments increase the accessibility and thus the catalytic activity both in microporous zeolites and in OMS, albeit for difference reasons. In zeolites, low accessibility results mainly from size-based and molecular diffusion-based accessibility limitations (Figure 12d,e): larger molecules cannot enter the pores and can only react on the pore mouth (nonlocal size-based (in)accessibility). Smaller molecules, able to enter the pore, may face stringent diffusion limitations implied by the narrow pore sizes (nonlocal diffusion-based inaccessibility). The creation of mesopores in zeolites by alkaline treatment enhances the accessibility of the acid sites by either increasing the number of pore mouths, and/or by lowering transport limitations. In AlOMS on the other hand, the origin of the accessibility enhancement is quite different: in this case, first the inaccessibility is caused by the majority of Al being completely incorporated in the pore wall (Figure 12c, full inaccessibility). During the Ostwald ripening process, Al is leached from the pore wall and is preferentially reincorporated back onto the new pore wall before Si, where it is fully accessible.56,61 Finally a local (in)accessibility is proposed (Figure 12b): likely some Al sites are partially buried in the complex amorphous structure of the silica pore wall and thus less accessible or only accessible for molecules that have the suitable spatial or chemical interaction. The latter implies that local inaccessible sites will be mostly easily probed by small molecules with a strong attraction to the active site. With these insights, a hypothesis can be postulated to explain the results from acidity and catalytic measurements from section 3.3. Upon optimal treatment the amounts of fully inaccessible and local (in)accessible sites are reduced, whereas the amount of fully accessible sites is increased. As each probe molecule experiences a difference relative accessibility to the local accessible sites, the measured gain in acidity differs. These differences may explain why the relative acidity gain as measured with pyridine (2-fold-increase) is larger compared to measurements with ammonia (50% increase). Similarly, the 4-fold

Figure 12. Accessibility limitations in micro- and mesoporous materials. (a) Complete accessibility: the active site is available to reactants of any size which encounter no diffusional limitations. (b) Local (in)accessibility: active sites are partially incorporated into the solid structure, the reactants meet local accessibility limitations. In this case the accessibility depends on the nature (size and chemical interaction) of the reactant and the catalytic conditions. (c) Full inaccessibility: the complete incorporation of an active site into the solid structure rendering it inaccessible to any molecule. (d) Nonlocal size-based (in)accessibility: the active site is only accessible to molecules below a critical size. The degree of accessibility is similar for the majority of the sites and mostly originates from the ordering/crystallinity of the solid. The size-based access limitation is not necessarily near the active site. Accordingly, the chemical interaction between the active site and the reactant is less relevant. (e) Nonlocal diffusion-based inaccessibility: the limited mass transport inside the pores renders the active site poorly accessible. Full and local access limitations occur typically in the case of amorphous aluminosilicates, whereas nonlocal size-based and diffusionbased limitations occur frequently in the case of crystalline microporous materials, such as zeolites.

increase in catalytic activity may in turn relate to the larger size of benzylalcohol or the even bulkier product (methylbenzyl)benzene). However, we emphasize that the unravelling of the exact nature of the different types of acid sites (amount, strength, distribution, and accessibility) will be subject to future study. These studies may be aided by the use of alkylpyridines such as, lutidine, collidine, and 2,6-ditert-butylpyridine, which have been used to probe the (in)accessibility of acid sites in zeolites.68,69 In order to gain a better insight of the value of the obtained results, the properties of sample M50-0.05 were compared to the literature (containing the required composition, porosity, and acidity information) on the synthesis or postsynthesis of AlMCM-41 (Figure 13 and Table S1). The effective acidity (EA) of M50-0.05 (31%) is twice as high compared to the typical values obtained for existing Al-MCM-41s in the literature (Figure 13a). The absolute acidity of M50-0.05 is on the other hand somewhat lower than the maximal values obtained in the literature (Figure S11a). However, the samples with such high acidity appear predominately at very low Si/Al ratios, that is, in the region where severe reductions in porosity occur (Figure 13b,c and Figure S11d). Specifically, Al-MCM-41s with decreasing Si/Al ratios show an increasing loss in pore diameter up to 50% compared to the pure SiO2 MCM-41 synthesized with the same method. This decrease causes a loss of porous volume to up to 7740


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remains mostly unchanged (Figure 13b,c). This means the negative influences of the reduced porosity on the accessibility described above for standard Al-MCM-41s is absent after alkaline activation. Correcting the absolute acidity with a factor for the loss in porosity (using either dpore or Vpore) gives a more nuanced impression of the total acidity (Figure S11b,c,e), showing that the sample prepared by optimal alkaline activation (M50-0.05) compete with materials with low Si/Al.

4. CONCLUSIONS The catalytic potential of Al-containing amorphous silica may be systematically underused based on an overlooked type of inaccessibility of the heteroatom. A study on the consequences of alkaline activation on Al-containing MCM-41 materials was performed, aimed at enhancing its catalytic performance. Indepth physicochemical characterizations indicate that high mass yields and morphology preservation are combined with enlarged, interconnected mesopores after optimal alkaline activation, which is explained by a controlled Ostwald ripening. The alkaline activation gives rise to a pronounced shift of octahedral, extra-structure Al into tetrahedral Al species. These extra tetrahedral Al sites combined with inaccessible Al that was leached from inside the pore wall and subsequently reincorporated at the surface during Ostwald ripening, give rise to a doubling of the acidity and a 4-fold increase in catalytic activity. The effective acidity after optimal activation with 0.05 M NaOH is doubled compared to values of standard Al-MCM-41s from the literature. The remarkable properties of the activated Al-MCM41s should be of high interest in the conversion of relatively instable bulky feedstocks such as biomass and heavy fossil oil feedstocks where large pores and a large acid-site density should be combined with a moderate acidic strength. In addition to attaining enhancements in acidic and catalytic properties, the treatment of ordered mesoporous silicas, such as MCM-41, in base may constitute a promising approach to study the fundamentals behind this process. The results can open the door toward a more efficient functionalization of any (meso)porous silica with Al or with other catalytically valuable heteroatoms such as Sn, Zr, and Ti.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b02874. Effective acidity of Al-MCM-41s in the literature, nitrogen physisorption isotherms and PSDs of M19 and M10, table with mass yields and properties of M19 and M10, Ar physisorption derived NL-DFT PSDs, nitrogen adsorption-derived BJH PSDs, yield corrected data for Figure 3b and Figure 7a,b, Al-content from ICP after activation, pyridine-probed FTIR spectra of M50 after activation, NH3-TPD, and pH variation plot (PDF)

Figure 13. (a) The effective acidity (EA) vs Si/Al ratio of M50-0.05 compared to Al-MCM-41s in the literature. Effect of alumination on porous volume (b) and pore diameter (c) in MCM-41. Vpore and dpore are compared to pure silica analogues synthesized with the same method (Vpore‑parent and dpore‑parent). The legend in part b applies to all figures: full symbols are synthesized by direct Al incorporation, hollow symbols are postsynthesis aluminations. Data available in Table S1.

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AUTHOR INFORMATION

Corresponding Authors

70% and a loss of specific surface to up to 50%. This reduction in porosity is expected to decrease the (full, local, nonlocal sizebased, and possibly nonlocal diffusion-based) accessibility of the acid sites. Advantageously, after alkaline activation (M50-0.05) on the other hand, the pore size increases and the pore volume

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

The authors declare no competing financial interest. 7741


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(11) Athens, G. L.; Shayib, R. M.; Chmelka, B. F. Functionalization of Mesostructured Inorganic−Organic and Porous Inorganic Materials. Curr. Opin. Colloid Interface Sci. 2009, 14, 281−292. (12) De Vos, D. E.; Dams, M.; Sels, B. F.; Jacobs, P. A. Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations. Chem. Rev. 2002, 102, 3615−3640. (13) Corma, A.; Kumar, D. Possibilities of Mesoporous Materials in Catalysis. Stud. Surf. Sci. Catal. 1998, 117, 201−222. (14) Ennaert, T.; Van Aelst, J.; Dijkmans, J.; De Clercq, R.; Schutyser, W.; Dusselier, M.; Verboekend, D.; Sels, B. F. Potential and challenges of zeolite chemistry in the catalytic conversion of biomass. Chem. Soc. Rev. 2016, 45, 584−611. (15) Jacobs, P. A.; Dusselier, M.; Sels, B. F. Will Zeolite-Based Catalysis be as Relevant in Future Biorefineries as in Crude Oil Refineries? Angew. Chem., Int. Ed. 2014, 53, 8621−8626. (16) Verboekend, D.; Nuttens, N.; Locus, R.; Van Aelst, J.; Verolme, P.; Groen, J. C.; Perez-Ramirez, J.; Sels, B. F. Synthesis, characterisation, and catalytic evaluation of hierarchical faujasite zeolites: milestones, challenges, and future directions. Chem. Soc. Rev. 2016, 45, 3331−3352. (17) Corma, A.; Martinez, A.; Martinezsoria, V.; Monton, J. B. Hydrocracking of Vacuum Gasoil on the Novel Mesoporous MCM-41 Aluminosilicate Catalyst. J. Catal. 1995, 153, 25−31. (18) de Clippel, F.; Dusselier, M.; Van de Vyver, S.; Peng, L.; Jacobs, P. A.; Sels, B. F. Tailoring nanohybrids and nanocomposites for catalytic applications. Green Chem. 2013, 15, 1398−1430. (19) Tuel, A. Modification of Mesoporous Silicas by Incorporation of Heteroelements in the Framework. Microporous Mesoporous Mater. 1999, 27, 151−169. (20) Perego, C.; Millini, R. Porous Materials in Catalysis: Challenges for Mesoporous Materials. Chem. Soc. Rev. 2013, 42, 3956−3976. (21) Chen, C.-Y.; Li, H.-X.; Davis, M. E. Studies on Mesoporous Materials: I. Synthesis and Characterization of MCM-41. Microporous Mater. 1993, 2, 17−26. (22) Garrone, E.; Fajula, F. Acidity and Basicity of Ordered Silica-based Mesoporous Materials. In Acidity and Basicity; Karge, H. G.; Weitkamp, J., Eds. Springer-Verlag: Berlin, Heidelberg, Germany, 2008. (23) Kosslick, H.; Lischke, G.; Parlitz, B.; Storek, W.; Fricke, R. Acidity and Active Sites of Al-MCM-41. Appl. Catal., A 1999, 184, 49−60. (24) Suzuki, K.; Aoyagi, Y.; Katada, N.; Choi, M.; Ryoo, R.; Niwa, M. Acidity and Catalytic Activity of Mesoporous ZSM-5 in Comparison with Zeolite ZSM-5, Al-MCM-41 and Silica−Alumina. Catal. Today 2008, 132, 38−45. (25) Busca, G. Acid Catalysts in Industrial Hydrocarbon Chemistry. Chem. Rev. 2007, 107, 5366−5410. (26) Sayari, A. Mesoporous Materials. In The Chemistry of Nanostructured Materials; Yang, P., Ed. World Scientific Publ.: Singapore, 2003; pp 39−68. (27) Taguchi, A.; Schüth, F. Ordered Mesoporous Materials in Catalysis. Microporous Mesoporous Mater. 2005, 77, 1−45. (28) Martín-Aranda, R. M.; Č ejka, J. Recent Advances in Catalysis Over Mesoporous Molecular Sieves. Top. Catal. 2010, 53, 141−153. (29) Dědeček, J.; Ž ilková, N.; Č ejka, J. Experimental Study of the Effect of Si/Al Composition on the Aluminum Distribution in (Al)MCM-41. Microporous Mesoporous Mater. 2001, 44−45, 259−266. (30) Chen, L. Y.; Ping, Z.; Chuah, G. K.; Jaenicke, S.; Simon, G. A Comparison of Post-Synthesis Alumination and Sol-Gel Synthesis of MCM-41 with High Framework Aluminum Content. Microporous Mesoporous Mater. 1999, 27, 231−242. (31) Mokaya, R. Ultrastable Mesoporous Aluminosilicates by Grafting Routes. Angew. Chem., Int. Ed. 1999, 38, 2930−2934. (32) Mokaya, R.; Jones, W. Efficient Post-Synthesis Alumination of MCM-41 Using Aluminium Chlorohydrate Containing Al Polycations. J. Mater. Chem. 1999, 9, 555−561. (33) Liu, Y.; Pinnavaia, T. J. Aluminosilicate Mesostructures with Improved Acidity and Hydrothermal Stability. J. Mater. Chem. 2002, 12, 3179−3190.

ACKNOWLEDGMENTS W. Vermandel, I. Cuppens, Dr. E. Breynaert, and Dr. T. De Baerdemaeker are acknowledged for NH3 TPD measurements, ICP measurements, SAXS measurements, and DFT calculations, respectively. S. Kaschube is acknowledged for technical assistance with XPS measurements and Dr. T. Gemming for the possibility to use the TEM. D.V. acknowledges support from a FWO postdoctoral fellowship. R.Z. acknowledges the CSC D.V. Flanders Research Foundation (FWO). R.L. acknowledges the Interuniversity Attraction Poles (IAP) program of the Belgian Science Policy Office (Belspo).

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ABBREVIATIONS Al(IV), tetrahedral Al; Al(VI), octahedral Al; B, Brønsted; BJH, Barrett, Joyner, Halenda; dpore, most frequent pore diameter ((local) maxima); (NL-)DFT, (nonlinear) density functional theory; EA, effective acidity; ICP-(OES), inductively coupled plasma (optical emission spectroscopy); L, Lewis; (MAS) NMR, (magic angle spinning) nuclear magnetic resonance spectroscopy; OMS, ordered mesoporous silica; PP-FTIR, pyridine-probed Fourier transform infrared spectroscopy; PSD, pore size distribution; TEM, transmission electron microscopy; SAXS, small-angle X-ray scattering; SBET, specific surface area via Brunauer−Emmett−Teller method; V pore , pore volume; Vpore‑intrinsic, intrinsic MCM-41-type parent porosity; Vpore‑extra, the extra porosity induced by activation; XBA, benzyl alcohol conversion; XFU, furfural conversion; Y, mass yield (mass retention) after alkaline activation

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REFERENCES

(1) Brinker, C. J.; Scherer, G. W. Sol−Gel Science: The Physics and Chemistry of Sol−Gel Processing; Academic Press: San Diego, CA, 1990. (2) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Ordered Mesoporous Molecular Sieves Synthesized by a LiquidCrystal Template Mechanism. Nature 1992, 359, 710−712. (3) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, K. D.; Chu, C. T. W.; Olson, D. H.; Sheppard, E. W. A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates. J. Am. Chem. Soc. 1992, 114, 10834−10843. (4) Vartuli, J. C.; Schmitt, K. D.; Kresge, C. T.; Roth, W. J.; Leonowicz, M. E.; McCullen, S. B.; Hellring, S. D.; Beck, J. S.; Schlenker, J. L. Effect of Surfactant/Silica Molar Ratios on the Formation of Mesoporous Molecular Sieves: Inorganic Mimicry of Surfactant Liquid-Crystal Phases and Mechanistic Implications. Chem. Mater. 1994, 6, 2317− 2326. (5) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Ångstrom Pores. Science 1998, 279, 548−552. (6) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024−6036. (7) Ying, J. Y.; Mehnert, C. P.; Wong, M. S. Synthesis and Applications of Supramolecular-Templated Mesoporous Materials. Angew. Chem., Int. Ed. 1999, 38, 56−77. (8) Corma, A. Preparation and Catalytic Properties of New Mesoporous Materials. Top. Catal. 1997, 4, 249−260. (9) Lai, C.-Y.; Trewyn, B. G.; Jeftinija, D. M.; Jeftinija, K.; Xu, S.; Jeftinija, S.; Lin, V. S. Y. A Mesoporous Silica Nanosphere-Based Carrier System with Chemically Removable CdS Nanoparticle Caps for StimuliResponsive Controlled Release of Neurotransmitters and Drug Molecules. J. Am. Chem. Soc. 2003, 125, 4451−4459. (10) Davis, M. E. Ordered Porous Materials for Emerging Applications. Nature 2002, 417, 813−821. 7742


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of Recrystallization and Restructuring. Cryst. Growth Des. 2013, 13, 5025−5035. (54) de Jong, K. P.; Zečević, J.; Friedrich, H.; de Jongh, P. E.; Bulut, M.; van Donk, S.; Kenmogne, R.; Finiels, A.; Hulea, V.; Fajula, F. Zeolite Y Crystals with Trimodal Porosity as Ideal Hydrocracking Catalysts. Angew. Chem., Int. Ed. 2010, 49, 10074−10078. (55) Pérez-Ramírez, J.; Verboekend, D.; Bonilla, A.; Abelló, S. Zeolite Catalysts with Tunable Hierarchy Factor by Pore-Growth Moderators. Adv. Funct. Mater. 2009, 19, 3972−3979. (56) Verboekend, D.; Pérez-Ramírez, J. Desilication Mechanism Revisited: Highly Mesoporous All-Silica Zeolites Enabled Through Pore-Directing Agents. Chem. - Eur. J. 2011, 17, 1137−1147. (57) Verboekend, D.; Vilé, G.; Pérez-Ramírez, J. Mesopore Formation in USY and Beta Zeolites by Base Leaching: Selection Criteria and Optimization of Pore-Directing Agents. Cryst. Growth Des. 2012, 12, 3123−3132. (58) Verboekend, D.; Vilé, G.; Pérez-Ramírez, J. Hierarchical Y and USY Zeolites Designed by Post-Synthetic Strategies. Adv. Funct. Mater. 2012, 22, 916−928. (59) Van Aelst, J.; Haouas, M.; Gobechiya, E.; Houthoofd, K.; Philippaerts, A.; Sree, S. P.; Kirschhock, C. E. A.; Jacobs, P.; Martens, J. A.; Sels, B. F.; Taulelle, F. Hierarchization of USY Zeolite by NH4OH. A Postsynthetic Process Investigated by NMR and XRD. J. Phys. Chem. C 2014, 118, 22573−22582. (60) Pérez-Ramírez, J.; Abelló, S.; Bonilla, A.; Groen, J. C. Tailored Mesoporosity Development in Zeolite Crystals by Partial Detemplation and Desilication. Adv. Funct. Mater. 2009, 19, 164−172. (61) Zhai, D.; Zhao, L.; Liu, Y.; Xu, J.; Shen, B.; Gao, J. Dissolution and Absorption: A Molecular Mechanism of Mesopore Formation in Alkaline Treatment of Zeolite. Chem. Mater. 2015, 27, 67−74. (62) Keller, T. C.; Arras, J.; Wershofen, S.; Pérez-Ramírez, J. Design of Hierarchical Zeolite Catalysts for the Manufacture of Polyurethane Intermediates. ACS Catal. 2015, 5, 734−743. (63) Nuttens, N.; Verboekend, D.; Deneyer, A.; Van Aelst, J.; Sels, B. F. Potential of Sustainable Hierarchical Zeolites in the Valorization of αPinene. ChemSusChem 2015, 8, 1197−1205. (64) Gil, B.; Mokrzycki, Ł.; Sulikowski, B.; Olejniczak, Z.; Walas, S. Desilication of ZSM-5 and ZSM-12 Zeolites: Impact on Textural, Acidic and Catalytic Properties. Catal. Today 2010, 152, 24−32. (65) Fernandez, C.; Stan, I.; Gilson, J.-P.; Thomas, K.; Vicente, A.; Bonilla, A.; Pérez-Ramírez, J. Hierarchical ZSM-5 Zeolites in ShapeSelective Xylene Isomerization: Role of Mesoporosity and Acid Site Speciation. Chem. - Eur. J. 2010, 16, 6224−6233. (66) Thibault-Starzyk, F.; Stan, I.; Abelló, S.; Bonilla, A.; Thomas, K.; Fernandez, C.; Gilson, J.-P.; Pérez-Ramírez, J. Quantification of Enhanced Acid Site Accessibility in Hierarchical Zeolites − The Accessibility Index. J. Catal. 2009, 264, 11−14. (67) Mitchell, S.; Milina, M.; Verel, R.; Hernández-Rodríguez, M.; Pinar, A. B.; McCusker, L. B.; Pér ez-Ramírez, J. Aluminum Redistribution during the Preparation of Hierarchical Zeolites by Desilication. Chem. - Eur. J. 2015, 21, 14156−14164. (68) Corma, A.; Fornés, V.; Forni, L.; Márquez, F.; Martínez-Triguero, J.; Moscotti, D. 2,6-Di-Tert-Butyl-Pyridine as a Probe Molecule to Measure External Acidity of Zeolites. J. Catal. 1998, 179, 451−458. (69) Bordiga, S.; Lamberti, C.; Bonino, F.; Travert, A.; ThibaultStarzyk, F. Probing zeolites by vibrational spectroscopies. Chem. Soc. Rev. 2015, 44, 7262−7341.

(34) Grün, M.; Lauer, I.; Unger, K. K. The Synthesis of Micrometerand Submicrometer-Size Spheres of Ordered Mesoporous Oxide MCM-41. Adv. Mater. 1997, 9, 254−257. (35) de Clippel, F.; Dusselier, M.; Van Rompaey, R.; Vanelderen, P.; Dijkmans, J.; Makshina, E.; Giebeler, L.; Oswald, S.; Baron, G. V.; Denayer, J. F. M.; Pescarmona, P. P.; Jacobs, P. A.; Sels, B. F. Fast and Selective Sugar Conversion to Alkyl Lactate and Lactic Acid with Bifunctional Carbon−Silica Catalysts. J. Am. Chem. Soc. 2012, 134, 10089−10101. (36) Emeis, C. A. Determination of Integrated Molar Extinction Coefficients for Infrared Absorption Bands of Pyridine Adsorbed on Solid Acid Catalysts. J. Catal. 1993, 141, 347−354. (37) Garcia-Martinez, J.; Xiao, C.; Cychosz, K. A.; Li, K.; Wan, W.; Zou, X.; Thommes, M. Evidence of Intracrystalline Mesostructured Porosity in Zeolites by Advanced Gas Sorption, Electron Tomography and Rotation Electron Diffraction. ChemCatChem 2014, 6, 3110−3115. (38) Van Aelst, J.; Verboekend, D.; Philippaerts, A.; Nuttens, N.; Kurttepeli, M.; Gobechiya, E.; Haouas, M.; Sree, S. P.; Denayer, J. F. M.; Martens, J. A.; Kirschhock, C. E. A.; Taulelle, F.; Bals, S.; Baron, G. V.; Jacobs, P. A.; Sels, B. F. Catalyst Design by NH4OH Treatment of USY Zeolite. Adv. Funct. Mater. 2015, 25, 7130−7144. (39) Otterstedt, J.-E.; Brandreth, D. A. Small Particles Technology; Springer-Verlag: Berlin, Germany, 1998. (40) Boudot, M.; Gaud, V.; Louarn, M.; Selmane, M.; Grosso, D. Sol− Gel Based Hydrophobic Antireflective Coatings on Organic Substrates: A Detailed Investigation of Ammonia Vapor Treatment (AVT). Chem. Mater. 2014, 26, 1822−1833. (41) Takahashi, R.; Nakanishi, K.; Soga, N. Insight on Structural Change in Sol−Gel-Derived Silica Gel with Aging under Basic Conditions for Mesopore Control. J. Sol-Gel Sci. Technol. 2005, 33, 159−167. (42) Wang, D. P.; Zeng, H. C. Creation of Interior Space, Architecture of Shell Structure, and Encapsulation of Functional Materials for Mesoporous SiO2 Spheres. Chem. Mater. 2011, 23, 4886−4899. (43) Lim, M. H.; Stein, A. Comparative Studies of Grafting and Direct Syntheses of Inorganic−Organic Hybrid Mesoporous Materials. Chem. Mater. 1999, 11, 3285−3295. (44) Li, G.; Li, N.; Wang, Z.; Li, C.; Wang, A.; Wang, X.; Cong, Y.; Zhang, T. Synthesis of High-Quality Diesel with Furfural and 2Methylfuran from Hemicellulose. ChemSusChem 2012, 5, 1958−1966. (45) Verboekend, D.; Pérez-Ramírez, J. Design of Hierarchical Zeolite Catalysts by Desilication. Catal. Sci. Technol. 2011, 1, 879−890. (46) Verboekend, D.; Pérez-Ramírez, J. Towards a Sustainable Manufacture of Hierarchical Zeolites. ChemSusChem 2014, 7, 753−764. (47) Na, K.; Somorjai, G. Hierarchically Nanoporous Zeolites and Their Heterogeneous Catalysis: Current Status and Future Perspectives. Catal. Lett. 2015, 145, 193−213. (48) Verboekend, D.; Keller, T. C.; Milina, M.; Hauert, R.; PérezRamírez, J. Hierarchy Brings Function: Mesoporous Clinoptilolite and L Zeolite Catalysts Synthesized by Tandem Acid−Base Treatments. Chem. Mater. 2013, 25, 1947−1959. (49) Verboekend, D.; Milina, M.; Pérez-Ramírez, J. Hierarchical Silicoaluminophosphates by Postsynthetic Modification: Influence of Topology, Composition, and Silicon Distribution. Chem. Mater. 2014, 26, 4552−4562. (50) Groen, J. C.; Bach, T.; Ziese, U.; Paulaime-van Donk, A. M.; de Jong, K. P.; Moulijn, J. A.; Pérez-Ramírez, J. Creation of Hollow Zeolite Architectures by Controlled Desilication of Al-Zoned ZSM-5 Crystals. J. Am. Chem. Soc. 2005, 127, 10792−10793. (51) Groen, J. C.; Zhu, W.; Brouwer, S.; Huynink, S. J.; Kapteijn, F.; Moulijn, J. A.; Pérez-Ramírez, J. Direct Demonstration of Enhanced Diffusion in Mesoporous ZSM-5 Zeolite Obtained via Controlled Desilication. J. Am. Chem. Soc. 2007, 129, 355−360. (52) Groen, J. C.; Moulijn, J. A.; Pérez-Ramírez, J. Desilication: on the Controlled Generation of Mesoporosity in MFI Zeolites. J. Mater. Chem. 2006, 16, 2121−2131. (53) Verboekend, D.; Milina, M.; Mitchell, S.; Pérez-Ramírez, J. Hierarchical Zeolites by Desilication: Occurrence and Catalytic Impact 7743


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