Hierarchical Silicoaluminophosphates by Postsynthetic Modification

Article pubs.acs.org/cm

Hierarchical Silicoaluminophosphates by Postsynthetic Modification: Influence of Topology, Composition, and Silicon Distribution Danny Verboekend,*,† Maria Milina, and Javier Pérez-Ramírez* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1, CH-8093, Zurich, Switzerland S Supporting Information *

ABSTRACT: AlPO-5, SAPO-5, SAPO-11, and SAPO-34 silicoaluminophosphates (SAPOs) are exposed to various acid and base treatments aimed at mesopore formation and investigation of associated physicochemical modifications. SAPOs amorphize strongly in aqueous NaOH, requiring the use of organic bases (e.g., tetrapropylammonium hydroxide or diethylamine) to preserve the crystallinity during base treatment. In acid media (HCl, H4EDTA, and Na2H2EDTA), SAPO-11 remains fully crystalline, while SAPO-34 strongly amorphize. No clear influence of the framework topology is established. The high resistance in alkaline media and low stability in acid media of SAPO-34 is attributed to its relatively high silicon content. Base treatment of SAPOs leads predominately to the formation of intercrystalline porosity. Still, an up to 4-fold increase in external surface and pore volume in SAPO-11 are achieved. Besides the formation of secondary porosity, base treatment of SAPOs induces a variety of (correlated) physicochemical changes. For example, the silicon distribution clearly influences the dissolution behavior of SAPO-11 in alkaline media, as zeolitic-like Si-domains are more resistant than AlPO domains. As a result, base leaching is selective to phosphorus and leads, depending on the silicon distribution, to either aluminum or silicon enrichment. The resulting changes in bulk composition can be directly related to the secondary porosity, as the Si and Al enrichment takes place predominately on the external surface. Acidity characterization (TPD of ammonia and IR spectroscopy of pyridine or 2,6-di-tert-butylpyridine adsorbed) shows that base treatment of SAPO-11 slightly reduces the concentration of Brønsted sites, while the number of Lewis sites is substantially increased. Moreover, the amount of Brønsted acid sites associated with the external surface is largely enhanced. The behavior of zeotypes, that is, AlPOs, SAPOs, and zeolites, in acid and basic aqueous solutions is generalized, highlighting the role of charge balancing cations. Catalytic evaluation of SAPO-11 shows the potential of base-treated samples in the alkylation of benzyl alcohol with toluene.

1. INTRODUCTION About a decade after the success of synthetic aluminosilicate zeolites in adsorption and catalysis, the class of crystalline silicoaluminophosphates was conceived.1,2 First, the partial substitution of P5+ for Si4+ in Al-rich zeolites was achieved.3 This was followed by the synthesis of Si-free aluminophosphates (AlPOs), representing a breakthrough in the preparation of crystalline microporous materials.4 Subsequently, a minor substitution of Si 4+ for P 5+ in AlPOs gave rise to silicoaluminophosphates (SAPOs)5 combining zeolitic features as high hydrothermal stability, accessible micropores, and shape selectivity, with novel framework structures and a weak-tomoderate acidity. While some applications as molecular sieves and supports for optical devices have been reported, the main use of SAPOs is as solid acid catalysts.6,7 Of the latter, the best examples are SAPO-11, commercially used as hydroisomerization catalyst, and SAPO-34, employed in the methanol-toolefins reaction. Additionally, applications in shape-selective partial oxidation of paraffins have been demonstrated by incorporating redox active elements in SAPOs.6 Academic interest in the preparation of SAPO catalysts has predominately focused on tuning hydrothermal synthesis.1,6−10 © 2014 American Chemical Society

For example, variation of the components in the synthesis gel and crystallization conditions enables to tailor the type of framework and its composition,6,7 the Si distribution (hence acidity),8 the crystal size,9 and the morphology10 of the resulting solid. Moreover, the inclusion of various metals, like Cu, Co, Mn, and Fe, in the synthesis gel leads to SAPO frameworks with redox properties.6,7 Alternatively, during the past decade, various approaches were devised to tailor the hydrothermal synthesis protocol in order to obtain hierarchically structured SAPOs.11−16 The latter is achieved by addition of organic porogens to the synthesis gel, which are subsequently removed by combustion. The resulting hierarchical SAPOs combine the intrinsic micropores with an auxiliary level of (meso)porosity, aiming at reducing access and diffusion limitations and subsequently enhancing their performance in catalyzed reactions. In contrast to the efforts in tuning the hydrothermal synthesis, little knowledge exists on the postsynthetic Received: May 16, 2014 Revised: July 16, 2014 Published: July 21, 2014 4552

dx.doi.org/10.1021/cm501774s | Chem. Mater. 2014, 26, 4552−4562

Chemistry of Materials

Article

Table 1. Topological Details, Composition, and Origin of the Parent Materials

a

material

topologya

AlPO-5 SAPO-5 SAPO-11 SAPO-11 SAPO-34

AFI (1D) AFI (1D) AEL (1D) AEL (1D) CHA (3D)

micropore size (nm)

codeb

bulk composition (−)

origin

× × × × ×

AP5-P SP5-P SP11A-P SP11B-P SP34-P

Al0.50P0.50O2c Si0.16Al0.49P0.35O2d Si0.05Al0.50P0.45O2e Si0.06Al0.43P0.51O2e Si0.14Al0.45P0.41O2e

synthesized after example 5 in ref 4 synthesized after ref 10 supplied by Clariant supplied by ACSMaterial supplied by ACSMaterial

0.73 0.73 0.40 0.40 0.38

0.73 0.73 0.65 0.65 0.38

Dimensionality of the micropore network in parentheses. b“-P” denotes “parent”. cFrom ref 4. dFrom ref 10. eICP-OES.

modification of SAPOs. The latter is striking since postsynthetic modifications of zeolites proved critical to their widespread success.1 For example steam and acid treatments enable to tune the bulk and framework composition (Si/Al ratio) of zeolites, herewith their stability and acidity and subsequently enhancing performance.17 In addition, various strategies were reported to synthesize highly efficient hierarchical zeolites using acid and base treatments.18,19 Particularly, the secondary porosity obtained by alkaline treatment, or desilication,18 has received substantial attention, producing superior zeolite catalysts in a plethora of reactions.20 In the case of SAPOs, studies on postsynthetic modifications concerned mostly the removal of the organic structure-directing agent after hydrothermal synthesis or the exchange of various charge-balancing cations in the zeotype.21 One study focused on acid treatment of SAPO-5,22 aimed at assessing its thermal stability. Still, the possibility to employ acid or base treatments to tailor the (meso)porosity or (framework) composition of SAPOs remains completely unexplored. Sparked by the vast potential in the preparation of hierarchical zeolites by postsynthetic design, we herein explore the opportunities and challenges of modifying SAPOs in aqueous solutions. By using strategic acid and base treatments on AlPO-5, SAPO-5, SAPO-11, and SAPO-34, basic criteria are established to successfully leach (silico)aluminophosphates while preserving their crystallinity. In a case study on SAPO11, we show the interplay between the external surface, composition, topology, and the silicon distribution. Trends on SAPOs and zeolites are generalized, identifying the extraframework cation type and content as a key parameter. Finally, by using the alkylation of toluene with benzyl alcohol, we demonstrate the potential of two distinct hierarchical SAPOs in catalytic applications.

Table 2. Sample Notation and Treatment Conditions treatment

reagent

C (M)

SLRa (g L−1)

t (h)

T (°C)

B1 B2 B3 B4 B5 B6 A1 A2 A3

NaOH NaOH+TPABr TPAOH DEA DEA DEA+NaCl HCl H4EDTA Na2H2EDTA

0.2 0.2 + 0.2 0.2 0.4 1 0.4 + 1 0.1 0.11 0.11

33 33 33 33 33 33 67 67 67

0.5 0.5 0.5 0.5 0.5 0.5 4 4 4

65 65 65 65 65 65 100 100 100

a

Solid-to-liquid ratio: weight of silicoaluminophosphate per unit volume of treatment solution.

isotherm. Si, Al, and P concentrations in the solids were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Horiba Ultima 2 instrument equipped with photomultiplier tube detection. The relative abundance of Si, Al, and P on the surface of the solids was ascertained using X-ray photoelectron spectroscopy (VG Thermo Escalab 220i-XL) using an Al Kα nonmonochromatic source. Scanning electron microscopy (SEM) was carried out using a LEO Gemini 1530 microscope operated at 1 kV. Transmission electron microscopy (TEM) was performed using a FEI Tecnai F30 microscope operated at 100 kV. 27Al, 29Si, and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was conducted in a Bruker Avance 400 spectrometer equipped with a 4 mm probe head and 4 mm ZrO2 rotors using the conditions provided in Table S1. Temperature-programmed desorption of ammonia (NH3TPD) was carried out in a Autochem 2910 II instrument from Micromeritics. The zeolite (100 mg) was pretreated at 400 °C in He flow (20 cm3 min−1) for 2 h. Afterward, 10 vol % NH3 in He (20 cm3 min−1) was adsorbed at 200 °C for 30 min followed by He purging at the same temperature for 1 h. This procedure was repeated three times. Desorption of NH3 was monitored in the range 200−700 °C using a heating rate of 10 °C min−1. Fourier transform infrared (FTIR) spectroscopy was performed using a Bruker Optics Vertex 70 spectrometer equipped with a high-temperature DRIFT cell (Harrick) and an MCT detector. Spectra were recorded under a nitrogen atmosphere at 200 °C, in the range of 650−4000 cm−1, by coaddition of 200 scans and with a nominal resolution of 4 cm−1. Prior to the measurement, the samples were dried at 300 °C in a N2 flow for 1 h. FTIR of adsorbed pyridine or 2,6-di-tert-butylpyridine was conducted in a Bruker IFS 66 spectrometer (650−4000 cm−1, 2 cm−1 optical resolution, coaddition of 32 scans). Self-supporting wafers of the samples (1 cm2) were degassed at 10−3 mbar and 420 °C for 4 h prior to analysis. Following adsorption at room temperature, weakly bound molecules were evacuated at 200 °C for 30 min (pyridine) or 150 °C for 1 h (2,6-di-tert-butylpyridine). The total concentrations of Brønsted (BPy) and Lewis (LPy) acid sites were calculated from the band areas of adsorbed pyridine at 1545 and 1454 cm−1, respectively, using extinction coefficients ε(BPy) = 1.67 cm μmol−1 and ε(LPy) = 2.22 cm μmol−1.23 The concentration of Brønsted acid sites associated with the external (mesopore) surface of the zeolites was assessed from the band area of adsorbed 2,6-di-tert-butylpyridine (BDTBPy) at 1530 cm−1 assuming ε(BDTBPy) = 1.67 cm μmol−1.

2. EXPERIMENTAL SECTION Materials and Treatments. The sample coding of the parent materials, their origin, and basic topological details are provided in Table 1. Base (code “Bx”) and acid (code “Ax”) treatments were performed in an Easymax 102 instrument from Mettler Toledo under magnetic stirring using the conditions summarized in Table 2 and were followed by filtration, extensive washing with distilled water, and drying at 65 °C for 12 h. Afterward samples were calcined at 550 °C for 5 h (heating rate of 5 °C min−1). Samples treated in NaOH were subjected to a 3-fold 12 h ion exchange in 0.1 M NH4NO3 (10 g L−1) prior to calcination. Characterization. Powder X-ray diffraction (XRD) patterns were acquired in a PANalytical X’Pert PRO-MPD diffractometer using Nifiltered 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. Nitrogen sorption at −196 °C was carried out in a Micromeritics TriStar II instrument. Prior to the measurement, the samples were degassed in vacuum at 300 °C for 3 h. The t-plot method was used to discriminate between micro- and mesoporosity. The mesopore size distribution was obtained by the Barrett−Joyner− Halenda (BJH) model applied to the adsorption branch of the 4553



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Catalytic Testing. The alkylation of toluene with benzyl alcohol was undertaken in glass-pressure tubes (10 cm3 working volume, autogenous pressure) at 160 °C. The catalysts (25 mg) were added to mixtures of toluene (47 mmol), benzyl alcohol (1 mmol), and ethylcyclohexane (0.7 mmol, internal standard). 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 a HP-5 column and a flame ionization detector.

3. RESULTS AND DISCUSSION 3.1. Acid and Base Treatments of Silicoaluminophosphates. To explore general compositional and topological trends, several common SAPOs were studied. In addition to the differences in framework topologies, these SAPOs possessed distinct compositions, going from Si-free (AlPO-5) to moderate Si content (SAPO-11) to relatively Sirich (SAPO-5 and SAPO-34) (Table 1). In addition, two SAPO-11 materials of distinct silicon distribution are included (see also Section 3.2). These materials were exposed to various acid and base solutions (Table 2). The latter treatments were adopted from postsynthetic strategies established to turn purely microporous zeolites into to their hierarchical analoges.19 For instance, NaOH treatment (B1) is traditionally applied to introduce mesoporosity in MFI zeolites with Si/Al ratios of 10−200.24 Alternatively, NaOH + TPABr (B2) solutions enabled preparation of hierarchical USY and beta zeolites with preserved crystallinity. In this case, TPA+ is required as USY and beta zeolites strongly amorphize in aqueous solutions containing only inorganic bases as NaOH or NH4OH.25 Moreover, TPAOH (B3) and DEA (B4) yielded hierarchical USY zeolites featuring crystallinities even higher than those obtained using NaOH and TPABr (B2).26 HCl treatment A1 is derived from a mild treatment that is often used for removal extra-framework aluminum and amorphous debris from steamed or base-leached MFI zeolites.24 Treatment A2 (using H4EDTA) is an acid treatment used to prepare hierarchical Y zeolites (Si/Al ∼ 2.5).27 Finally, Na2H2EDTA treatment (A3) was selected as this proved successful to convert the highly Alrich zeolites (X and A, Si/Al ∼ 1.5) to the hierarchical form.28 We have evaluated the influence of the treatments on the solids in this section by monitoring the developed auxiliary porosity (external surface, Smeso, and mesopore volume, Vmeso) and the preserved intrinsic properties of SAPOs (crystallinity and micropore volume, Vmicro). For clarity’s sake, we have focused the discussion on the SAPO-11. The X-ray diffractograms (Figure 1) show that both SAPO-11 samples comprise the typical AEL reflections. However, SP11A-P displays double features at 9, 13, 16° 2θ. In addition, minor peaks occur at ca. 19.5° 2θ. The latter became particularly pronounced in both SAPO-11 samples after acid treatment (vide inf ra). Such variations of the XRD pattern have been observed for SAPO1129−31 and may be due to the coexistence of AEL phases with different compositions. The micropore volumes of SP11A-P and SP11B-P were equal to 0.03 cm3 g−1 (Table 3). Such relatively low micropore volumes (maximum values are ca. 0.08 cm3 g−1) are often reported for SAPO-1121,31 and may be caused by blockage of the unidirectional elliptical 10-membered ring (10-MR) micropores by, for example, impurities or the intergrowth of two AEL crystallites. The chemical composition shows that for SP11A-P a suitable substitution of Si for P in the framework was achieved (Table 1). Conversely, for SP11B-P a minor excess of P in the sample suggests the presence of Si− O−P bonds, which may be in the form of amorphous

Figure 1. X-ray diffraction patterns of SAPO-11 zeotypes. The asterisks indicate the tridymite phase.

Table 3. Treatment Yields and Porous Properties of SAPO11 Zeotypes sample SP11A-P SP11A-B1 SP11A-B2 SP11A-B3 SP11A-B4 SP11A-B6 SP11A-A1 SP11A-A2 SP11A-A3 SP11B-P SP11B-B1 SP11B-B2 SP11B-B3 SP11B-B4 SP11B-B5 SP11B-A1 SP11B-A2 SP11B-A3

yielda (%) 75 76 60 52 82 85 69 82 65 65 58 48 14 78 57 68

Vmicrob (cm3 g−1)

Vmesoc (cm3 g−1)

Smesob (m2 g−1)

0.04 0 0 0.04 0.03 0 0.03 0.04 0.01 0.03 0 0 0.08 0.07 0.03 0.08 0.08 0.09

0.29 0.18 0.17 0.39 0.46 0.09 0.27 0.33 0.28 0.03 0.06 0.05 0.14 0.18 0.29 0.09 0.20 0.11

140 62 56 141 173 25 90 90 78 25 21 14 70 89 195 32 66 37

a

Gram of solid after treatment per gram of starting material. bt-plot method. cVmeso = Vpore − Vmicro.

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amount of protons. The treated samples featured fully preserved and sometimes enhanced crystallinity (SP11A-A1). Also the micropore volume of the acid-treated samples was preserved. In the case of SP11B, the base and acid treatments induced an enhanced microporosity. The latter effect is tentatively related to the removal impurities from the sample, freeing the remainder of the microporosity. Figure 2 shows that some acid-treated samples displayed enhanced (meso)pore volumes (SP11A-A1) and/or external surface areas (SP11BA2). Hence, some degree of mesopore formation in SAPOs can be achieved using acid or base treatments. Treatments B1 and B4 were also applied to AlPO-5 (AP5-P) and SAPO-5 (SP5-P). The resulting samples confirmed that NaOH leads to the formation of tridymite, whereas organic bases can be used to substantially leach the zeolite while maintaining crystallinity and microporosity (Figure S1 and Table S2). The XRD patterns of these solids concomitantly reveal that an impurity in AP5-P was selectively removed. For SAPO-34, treatment in NaOH (B1) reduced the crystallinity only partially (Figure S1), and the microporosity was largely preserved (Table S2). Surprisingly, upon contact with TPAOH (B3), SAPO-34 completely amorphized. Conversely, when treated in DEA (B4), a weight loss of 15% was achieved, while completely preserving the structure. The preservation of the SAPO-34 structure using diethylamine is tentatively attributed to its relatively small size, enabling it to enter (part of) the micropore cavities. The degree of dissolution could not be increased by enhancing the DEA concentration (see SP34-B5). On the other hand, the dissolution of SAPO-34 was increased by increasing the concentration of NaOH (Figure S2a). While the resulting materials lost most of their crystallinity (data not shown), the leaching completely removed phosphorus from the sample, yielding an amorphous aluminosilicate (Figure S2b). SAPO-34 completely amorphized upon treatments A1 and A2, based on the lack of crystallinity (Figure S1) and complete loss of microporosity (Table S2). Accordingly, SAPO-34 is clearly more sensitive to the acid treatment compared to SAPO-11. 3.2. Morphology, Coordination, Composition, and Acidity of Base-Treated SAPO-11. Morphology. The physicochemical properties of parent and selected base-treated SAPO-11 samples were studied in further detail. The appearance of the two parent materials (SP11A-P and SP11B-P) as demonstrated by SEM was roughly similar, with crystals in the size range of ca. 1 μm (Figure 3). TEM revealed that SP11A-P comprised, in addition to larger particles, a substantial amount of small crystals (Figure 4). In contrast, SP11B-P displayed predominately larger crystals and accordingly also a smaller external surface. The distinct morphology of the parent samples should be related to the use of different template agents during their hydrothermal synthesis.35 Upon base treatment, the smaller crystals became more abundant (SP11A-B4), while no intracrystalline porosity could be detected. Also in the case of SP11B-B4, despite surface roughness, no clear signs of intracrystalline porosity occurred. The efficiency of a postsynthetic method aimed at generation of secondary porosity can be expressed by relating the increase of external surface (ΔSmeso) to the associated weight loss upon alkaline treatment (Figure 5).34 In the case of zeolites, the developed external surface areas were about 2−8 m2 g−1 per percentage of weight loss, depending strongly on the crystal size and framework topology.18 For SAPOs, the values are roughly 5 times lower (ca. 1 m2 g−1 %−1 at 50% yield), and a clear influence of the framework topology did not occur. This

impurities. Obviously, these SAPO-11 samples deserve further in-depth characterization to unravel their exact constitution. However, to keep our contribution concise, discussion is focused on the major physicochemical changes occurring within the samples. Nevertheless, it should be stressed that, besides demonstrating the role of acid and base treatments in the preparation of hierarchical SAPOs, the postsynthetic strategies in this contribution form a powerful tool to aid to the understanding of the exact constitution of silicoaluminophosphates.32 Upon base treatment in NaOH (B1), a significant dissolution occurred for SAPO-11 (Table 3), suggesting that, unlike for zeolites,27 a high framework aluminum content does not inhibit alkaline-mediated dissolution. Moreover, XRD analysis evidenced that SP11A-P was completely converted to a dense tridymite phase after treatment in NaOH. In the case of SP11B, this transformation was less pronounced. The high sensitivity of SAPO-11 to alkaline media is striking as 10-MR 2D (ferrierite)33 and 10-MR 1D (ZSM-22)34 zeolites are rather inert in alkaline conditions, requiring enhanced temperature (85 °C) and alkalinity (>0.6 M NaOH) to experience dissolution. Furthermore, unlike for the fragile USY and beta zeolites,25 the addition of TPABr to the alkaline solution (B2) did not enable retention of the crystallinity of the SAPO-11 samples upon base treatment. In fact, in the case of SP11B, it seemed to enhance the amorphization of the framework. Conversely, the use of organic bases like TPAOH (B3) or DEA (B4) enabled full retention of the crystallinity and microporosity. In addition, the Smeso and Vpore of the samples could be enhanced, as can be seen from selected isotherms and derived BJH mesopore distributions (Figure 2). The latter was particularly pronounced for SP11B-B3 and SP11B-B4 (Table 3). Acid treatments (A1-A3) on the SAPO-11 samples also initiated substantial dissolution, which was most severe for H4EDTA and may be related to the highest stoichiometric

Figure 2. N2 isotherms (left column) and derived BJH mesopore size distributions (right column) of SAPO-11 zeotypes. 4555



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topologies compared to SAPO-11, e.g. ZSM-22, and SAPO-34, i.e. SSZ-13, were also not optimally prepared by base treatment due to either low mesopore formation efficiency (ZSM-22)34 or severe amorphization (SSZ-13).37 Coordination and Composition. The 29Si MAS NMR spectra reveal that the silicon coordination was very different in the parent SAPO-11 samples (Figure 6). In both solids, two

Figure 3. Scanning electron micrographs of SAPO-11 zeotypes. The scale bar shown applies to all micrographs.

Figure 6. 29Si MAS NMR spectra of SAPO-11 zeotypes.

contributions were discerned, centered around −92 ppm and −110 ppm. The former, attributed to Si(Al4) species, occurs when silicon is incorporated homogeneously in the SAPO framework.35 The peak at −110 ppm, relating to Si(Si4) species, indicates the presence of (zeolitic) silica-rich domains. SP11A-P comprises mostly the SAPO phase, while SP11B-P distinctly features both SAPO and zeolitic phases. The SAPO11 zeotypes were exposed to alternating DEA concentrations to gain a profound insight in the implications of the treatments. The composition and porous properties of the treated SAPO11 samples are presented in Figure 7, showing that the dissolution of SP11A-P was linear, while for SP11B-P a

Figure 4. Transmission electron micrographs of SAPO-11 zeotypes. The scale bar shown applies to all micrographs.

Figure 5. Relationship between the introduced mesopore surface area (ΔSmeso = Smeso,Bx − Smeso,P) and the weight loss upon base treatment of SAPO-11 (using DEA), SAPO-34 (using NaOH), and octadecasil.36

overall low efficiency is attributed to the absence of active (meso)pore-directing agents (PDAs) in the alkaline treatment (vide inf ra). Accordingly, the mesopore surface is based predominately on the formation of intercrystalline cavities. As a result, like was observed in the base treatment of octadecasil,36 the ΔSmeso increases in an exponential trend with the weight loss. Still, zeolites of similar framework

Figure 7. Yield, composition, and porous properties of parent and alkaline-treated SP11A (a) and SP11B (b). The parent materials are represented at 0 M DEA. 4556



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surface area. This is achieved by multiplying the average surface area of a T atom (ST)38 with the total amount of T atoms on the external surface. Since Si and Al will be on the external surface, the number of T atoms on the surface is derived by taking the sum of aluminum and silicon atoms and deducting the amount of aluminum required to form the Al−O−P bonds in the AlPO domains. The resulting amount of T atoms (nAlnP+nSi) multiplied with ST yields a calculated external surface area (Smeso,calc, in m2 g−1). Although the accuracy of this calculation may be subject to refinement, Figure 8b reveals a strikingly linear between the measured and calculated external surfaces. Second, the surface composition was probed by XPS (Table 4), confirming that the surface of the SAPO-11 samples is enriched particularly in Si and deficient in P. The latter trend increases strongly upon base treatment. Strikingly, in line with earlier work,30,39 the surface composition of the parent and base-treated SAPO-11 could be estimated using the bulk composition and the following relations: Sisurf = 4*Sibulk, Alsurf = Albulk, and Psurf = 0.5*Pbulk. Acidity. The acidity of the SAPO-11 samples was evaluated using FTIR in the OH-stretching region and TPD of NH3 (Figure 9). A hydroxyl band at about 3620 cm−1 resulting associated with Si−OH−Al groups is observed in all spectra, except for SP11B-P. The latter may result from impurities that mask the 3620 cm−1 band.25 The absorbances at 3790 and 3674 cm−1 can be ascribed to the stretching vibrations of surface P− OH and Al−OH groups, respectively.30,40 The band at ca. 3740 cm−1 most probably originates from terminal Si−OH groups, although a contribution from external Al−OH groups cannot be excluded.40 The FTIR spectra show that the absorbance associated with Brønsted acid sites (3620 cm−1) was largely preserved. Conversely, the band at 3740 cm−1 appeared enhanced upon base treatment in DEA, supporting the enrichment of the external surface with silicon. The NH3TPD profiles show a single desorption peak around 300 °C, while little changes in position and intensity of the peak occurred after base treatment. Integration of the NH3-TPD profiles yields a total acidity of ca. 0.20 mmol g−1. Dividing this number by the silicon content evidence that many Si atoms do not give rise to acid centers. The low amount of “acidic” silicon (ca. 25%) could be related the substantial Si concentration on the surface (Figure 8) as well as the presence of Si-rich domains. The latter particularly applies to the case of SP11B-P and SP11B-B4. The assessment of the concentration of Brønsted and Lewis acid sites by IR spectroscopy of pyridine adsorbed resulted in significantly lower values than those measured by NH3-TPD (Table 4). This may be attributed to access restrictions that the pyridine molecules experience in 1D zeotypes.34 Still, the measurements show that in both cases the DEA-treatment led to a slightly decreased number of Brønsted acid sites (by ca. 10%) and an increased concentration of Lewis acid sites (most noticeably for SP11-B4). Conversely, both acid-treated samples SP11A-A1 (16 μmol g−1) and SP11B-A2 (72 μmol g−1) displayed Brønsted acidities lower than that of the base-treated samples. In agreement with the similar desorption temperatures in NH3-TPD (Figure 9b), desorption of adsorbed pyridine at varying temperatures did not evidence a change of acid strength upon acid or base treatment (Figure S5). The concentration of Brønsted acid sites associated with the external surface was studied by FTIR of 2,6-di-tertbutylpyridine adsorbed (Table 4). This bulky molecule is unable to enter the SAPO-11 micropores and accordingly gives an indication of the accessibility of the acid centers. In

relatively steep loss occurred at 0.2 M DEA. This can be explained, in line with the 29Si MAS NMR experiments, by the existence of two types of phases in the SP11B-P, resulting into the selective leaching of the more sensitive phase. The latter is confirmed by XRD showing the enhancement of the peaks at 9, 13, 16, and 19.5° 2θ (Figure 1b, Figure S3). 29MAS NMR shows that the contribution around −110 ppm has grown in relative abundance after base leaching (SP11B-B4). This trend was less pronounced for SP11A-B4 (Figure 6). Also the 27Al and 31P spectra indicate that, unlike in the case of SP11A-P, alkaline treatment of SP11B-P leads to changes in the coordination of Al and P (Figure S4). The selective leaching of the SAPO phase implies that the XRD reflections at 9, 13, 16, and 19.5° 2θ relate to the zeolitic phase. Interestingly, as observed after acid treatment with H4EDTA (SP11B-A2), these diffraction lines were further enhanced by increasing the DEA concentration (sample SP11B-B5, Figure S3). The selective leaching of the SAPO-phase, being relatively Prich, was further corroborated by the depletion of phosphorus from SP11B-P (Figure 7b). Hence, as for SAPO-34 (Figure S2), the base leaching of SAPO-11 is selective to phosphorus (Figure 7). In addition, the selective removal of phosphorus implies that the external surface of the base-treated SAPO-11 samples are enriched in Si and Al. Indeed, the phosphorus fraction showed a distinct negative trend as a function of the external surface (Figure 8a). In turn, the combined fractions of silicon and aluminum resulted in a mirror trend. The surface depletion of phosphorus can be further corroborated using two independent approaches. First, assuming that silicon and aluminum are preferentially on the surface, the bulk composition can be used to theoretically derive the external

Figure 8. Relation between the external surface (Smeso) and the bulk composition of SAPO-11 zeotypes (a). The relation between the Smeso and the calculated external surface (Smeso,calc) (b). The Smeso,calc is derived from relating the bulk composition (nAl-nP+nSi) with the specific surface area of a T atom (ST = πrT2, where rT represents the radius based on the tetrahedral bond length38). 4557



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Table 4. Composition and Acidity of SAPO-11 Zeotypes sample

bulk compositiona (−)

surface compositionb (−)

Sia (mmol g−1)

NH3c (mmol g−1)

NH3/Si (−)

BPyd (μmol g−1)

LPyd (μmol g−1)

BDTBPyd (μmol g−1)

SP11A-P SP11A-B4 SP11B-P SP11B-B4

Si0.05Al0.50P0.45O2 Si0.06Al0.54P0.40O2 Si0.06Al0.43P0.51O2 Si0.10Al0.44P0.46O2

Si0.19Al0.54P0.27O2 Si0.23Al0.57P0.20O2 Si0.24Al0.46P0.30O2 Si0.38Al0.45P0.17O2

0.79 0.92 0.98 1.62

0.22 0.23 0.20 0.20

0.27 0.25 0.20 0.12

41 36 108 85

43 68 34 38

11 19 5 8

a

ICP-OES. bXPS. cAmmonia uptake. dConcentration of Brønsted (B) or Lewis (L) acid sites determined by IR spectroscopy of adsorbed pyridine (Py) or 2,6-di-tert-butylpyridine (DTBPy).

Figure 9. Infrared spectra in the OH-stretching region (a) and NH3TPD profiles (b) of SAPO-11 zeotypes.

agreement with the greater Smeso, SP11A-P displayed a higher amount of Brønsted acid sites on the surface in comparison SP11B-P. Following the treatment with DEA, an increase in the surface acidity was evidenced in both samples. However, whereas more than 50% of the total amount of Brønsted acid sites were accessible to di-tert-butypyridine in SP11A-B4, only ca. 10% of the acid sites were probed in SP11B-B4. The results in this section highlight that, while maintaining the crystallinity and acidity, optimized base treatments enable to incur substantial changes in meso- and macroporosity and the composition of SAPOs. 3.3. Generalizing the Behavior of Zeotypes in Acid and Basic Media. Generalization of the properties and behavior of zeotypes is commonly performed using the composition of the T atoms in the framework.19,41−45 Hence, we have positioned various zeotypes according to their composition (Figure 10a). On this plot, AlPOs represent a single point, zeolites a line, and the SAPOs and the Psubstituted zeolites represent the entire area in between. The SAPO-11 and SAPO-34 treated in DEA are included in Figure 10b. While the composition of SAPO-34 is not severely modified, those of the SAPO-11 materials are substantially altered. In those cases, the lowering of the position on the plot

Figure 10. Ternary compositional plot containing AlPOs,4 SAPOs,5 Psubstituted zeolites,3 and zeolites (a). The formula applies to the materials within the red line. A zoom of the ternary plot with the parent and DEA-treated SAPOs of this study (b) (SP11A, open triangle, SP11B, open square, and SP34, open circle). The arrows indicate the compositional changes incurred by the DEA treatments.

indicates the selective removal of phosphorus. For SP11A, this is accompanied primarily by Al-enrichment, while for SP11B a preferential Si-enrichment occurred. Of course, this plot does not take into account that not all T-atoms may be perfectly incorporated or distributed in the framework. For example, aluminum fractions higher than 0.5 (hence Løwenstein forbidden) have been reported5 and also occur in the case of the SP11A samples (Table 4). This phenomenon can be explained by Al-enrichment on the external surface. In the case of zeolites, Løwenstein-forbidden samples do not typically occur as the lowest Si/Al framework ratio is often relatively far from unity, i.e. Si/Al ∼ 1.3. Moreover, related Al-rich zeolites 4558



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Figure 11. Relative stability of zeotypes in alkaline media. Stability increases going from AlPO to SAPO, to all-silica zeolite, to low-silica zeolite. As the stability decreases, the sensitivity to mineral salts (e.g., NaCl or KBr) increases.

(e.g., A, X) commonly grow in large crystals with relatively low external surface areas. The stability of SAPOs and zeolites in acid and base media is most easily rationalized using the compositional extremes in Figure 10a, that is, Al0.5P0.5O2, H0.5Si0.5Al0.5O2, and Si1O2. This is justified as most zeotypes are positioned in the proximity of the line connecting those extremes. Based on the results in this contribution and those previously obtained on zeolites, we have classified their relative stabilities. In basic media, the stability varies from very high (H0.5Si0.5Al0.5O2), to moderate (SiO2), to low (Al0.5P0.5O2) (Figure 11). In acid media, the stability varies from high (Si1O2), to moderate (Al0.5P 0.5O2), to low (H0.5Si0.5Al0.5O2). These trends can be explained using the elemental composition of the framework, i.e. the susceptibility of the nearest neighbor bond to break. In basic media the stability should accordingly follow Al−O−Si ≫ Si−O−Si > Al−O−P. In acid media, the stability is given by Si−O−Si > Al−O−P > Si−O−Al. In the case of base leaching, the relatively low stability of the Al−O−P bond relates well to the selective phosphorus removal from alkaline-treated SAPO-11 and SAPO-34. For zeolites, the micropore size has a pronounced complementary influence on the dissolution behavior, i.e. 12MR zeolites are substantially more sensitive to alkaline media compared to zeolites of 8 or 10 MRs.19 However, likely caused by their high sensitivity, no clear relation between the stability of SAPOs in basic media and their framework topology was established. Although evaluation of the framework T atom composition provides a suitable argumentation for the observed stability trends, they do not account for the type and abundance of the extra-framework charge-balancing counter cations (CBCCs) located in the micropores. The latter play an important role in postsynthetic modifications and hydrothermal synthesis, e.g. as is the case in the “charge density mismatch” approach developed to enhance the control over the crystallization process of zeolites.46 For the sake of simplicity, we have focused the below discussion primarily on zeotypes in the protonic form. When the CBCC density (indicated as ‘H+’) is evaluated as a function of the position on the Al0.5P0.5O2−H0.5Si0.5Al0.5O2− Si1O2 trajectory, a symmetrical profile is obtained (Figure 12a). The latter can be directly related to the conditions used in the hydrothermal synthesis of zeolites. For example, there is a clear relation between the CBCC density and the need for organic templates, i.e. structure-directing agents, during hydrothermal synthesis. Such templates are required mostly where the CBCC abundance is low, while CBCC-rich materials are made in the absence of organics. In addition, the plot highlights that

Figure 12. Relation between the zeotype T atom composition and the relative charge-balancing countercation density (H+) (a). The compositions follow the Al0.5P0.5O2 to H0.5Si0.5Al0.5O2 to SiO2 trajectory displayed in Figure 10a. Zeotypes with high H+ are typically hydrothermally synthesized in the absence of organic templates. P-L and P-A refer to phosphorus-substituted zeolites L and A,3 respectively. Relative stability of SAPOs and zeolites in aqueous solutions as a function of pH and H+ (b). Zeotypes comprising large CCBC density are most stable in high pH, while low CBCC zeotypes are more stable at low pH. The solutions in this plot are based on inorganic acids or bases (e.g., NaOH and HCl). The relative stability refers to the occurrence of (i) dissolution and (ii) amorphization. In the case of SAPOs, these events take place simultaneously. In contrast, for zeolites, the micropore size has a strong influence on the degree of amorphization taking place during dissolution.

zeotypes of a particular framework topology, but of completely different framework compositions, e.g. SSZ-13 and SAPO-34, are synthesized at similar CBCC densities. The CBCC density can also be related to the behavior of SAPOs and zeolites in acid or alkaline media. Figure 12b shows that zeotypical materials comprising a relatively limited amount of CBCCs are relatively unstable at high pH. Similarly, zeotypical materials 4559



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comprising an abundance of CBCC are relatively unstable at low pH. Of course, this plot does not account for the stability difference between Al0.5P0.5O2 and Si1O2. However, importantly, it relates the conditions used in hydrothermal synthesis with the optimal conditions for postsynthetic modifications. This intimate relationship can be illustrated using several examples. For instance, many CBCC-rich zeolites are hydrothermally synthesized in the absence of organic templates (e.g., Y28 and L19) (Figure 12a). This implies that they are highly stable in alkaline media (Figure 12b) and accordingly require an initial acid treatment prior conversion to the hierarchical form (by base treatment). Conversely, zeolites with lower CBCC densities (ZSM-5,24 beta,25 ZSM-2234) are hydrothermally produced with organic structure-directing agents. After template removal, these are less stable in alkaline media and do not require acid treatment prior to mesopore formation by base treatment. The relationship is further corroborated by the optimal base treatment conditions established for the SAPOs in this contribution. Unlike most zeolites, SAPOs are commonly hydrothermally synthesized in the absence of mineral salts like KBr or NaBr.1−5 Accordingly, it is not surprising that the optimal base leaching of SAPOs occurs in the absence of such salts (Figure 11). This is exemplified by the amorphous solid obtained after exposing SP11A-P to a diethylamine solution containing 1 M NaCl (SP11A-B6, Table 3). Similarly, based on the behavior in postsynthetic modifications, the hydrothermal conditions can be (at least qualitatively) predicted. For instance, low-CBCC high-silica USY is not easily directly obtained by hydrothermal synthesis. It can be deduced that, since USY is most preferably prepared by base treatment in (sodium-free) solutions of organic bases,26 its hydrothermal synthesis should be also performed in the absence of mineral salts. We expect that the tight relationship between the synthesis and postsynthesis conditions may also enable to enhance the efficiency of the base leaching of SAPOs. An efficient (acid or base) treatment of zeotypes aimed at mesopore formation occurs when three conditions are met: (i) the solid partially dissolves, (ii) the intrinsic properties of the remaining solid are preserved, and (iii) the dissolution is directed, for example using ‘(meso)pore-directing agents’.47 The base treatments of SAPOs with organic base, such as DEA or TPAOH, successfully tackle the first two points, while the last remains suboptimal. The key to the identification of active PDAs for the SAPOs may be derived from approaches or ingredients applied in hydrothermal synthesis. For instance, in the case of zeolites, two basic ingredients in their hydrothermal synthesis, i.e. Al(OH)4− and tetraalkyl-ammonium cations, are essential to efficiently base treat zeolites, acting as active PDAs.47 In the case of SAPOs, the use of mixtures of bases (NaOH and TEAOH48 or TMAOH and TPAOH5) or the inclusion of large amines15 may represent attractive strategies. 3.4. Catalytic Evaluation of Base-Treated SAPO-11. The performance of the SAPO-11 samples was evaluated in the model alkylation reaction of benzyl alcohol with toluene, which, due to strong access-limitations in 10-MR zeotypes, is very sensitive to the developed mesoporosity.49 Figure 13 shows the conversion of benzyl alcohol (XBA) as a function of time over the conventional and treated SAPO-11 samples. In line with the higher Smeso and accessibility of the acid sites, SP11A-P showed significantly higher alkylation activity compared to SP11B-P. In both cases, the tridymite phase obtained by NaOH (B1 samples) did not yield any activity. Conversely, in the case of

Figure 13. Conversion of benzyl alcohol (XBA) versus time in the alkylation of toluene (T) with benzyl alcohol (BA) over SAPO-11 zeotypes (SP11A (a), SP11B (b)). Conditions: T = 160 °C, T/BA = 47, and Wcat = 25 mg.

SP11B, the DEA-treatment (B4) proved highly beneficial, enhancing the conversion 10-fold. The activity of SP11A-B4 was only around 50% compared to that of SP11A-P. The latter is unexpected as the accessibility was almost doubled, while the total amount and strength of the acid sites was virtually unchanged. Acid treatment (SP11A-A1) yielded a similar performance as the DEA-treated sample. In contrast, the acid treatment (SP11B-A2) enabled a 5-fold enhancement of the catalytic activity. These results show that mesopore formation in SAPOs by optimized base treatments can strongly enhance the catalytic performance. Moreover, it proves that the performance of a conventional SAPO, and subsequently the potential of postsynthetic modifications, depends strongly on the conditions of hydrothermal synthesis of the conventional parent material. Herein, the Si distribution appears to be a crucial criterion. Of course, further catalytic evaluation of these samples is required to yield tighter property-function relationships. However, such studies are beyond the scope of this contribution. For example, to more carefully examine their catalytic potential, these hierarchical SAPO-11 materials require testing in the hydroisomerization of n-alkanes.

4. CONCLUSIONS The physicochemical implications of postsynthetic modifications of SAPOs using acid or base treatments were extensively studied. Both types of treatments can be used to enhance the external surface and change the elemental composition, while preserving the intrinsic properties. Critical parameters in the modification of SAPOs using aqueous solutions are the silicon content and silicon distribution. In the case of base treatment, 4560



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the efficiency of mesopore formation is considered suboptimal. Nevertheless, by recognizing the need for active mesoporedirecting agents during base leaching, we pave the way to a more efficient preparation of hierarchical SAPOs by postsynthetic design. The stabilities of SAPOs in aqueous solutions are generalized and related to those of zeolites, highlighting an important role of charge-balancing counter cations. Catalytic evaluation in a model alkylation reaction underlined the value of acid and base treatments.

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ASSOCIATED CONTENT

S Supporting Information *

MAS NMR conditions, additional MAS NMR spectra, additional XRD patterns, and porous properties of treated AlPO-5, SAPO-5, SAPO-34, and silicalite-1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address †

Deparment M2S, K.U. Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS ETH Zurich and the Swiss National Science Foundation (project number 200021-134572) are acknowledged for ́ financial support. Dr. M. Hernández-Rodriguez is acknowledged for the synthesis of SAPO-5. We thank Dr. S. Mitchell and Dr. I Czekaj for microscopy and XPS analyses, respectively. Micromeritics Instrument Corporation is acknowledged for collaboration with respect to porosity analysis.

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ABBREVIATIONS AlPO, aluminophosphate; SAPO, silicoaluminophosphate; DEA, diethylamine; TPA, tetrapropylammonium; EDTA, ethylenediaminetetraacetic acid; MR, membered ring; PDA, pore-directing agent; CBCC, charge-balancing countercation

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REFERENCES

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Hierarchical Silicoaluminophosphates by Postsynthetic Modification

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