Hierarchical SAPO-11 molecular sieves were synthesized with three different mesopore structure directing agents (meso-SDAs): cetyltrimethylammonium bromide (CTAB), polyvinyl alcohol (PVA) and [3-(trimethoxysilyl) propyl] dimethyloctadecylammonium chloride (TPOAC).
Microporous and Mesoporous Materials 329 (2022) 111550 Contents lists available at ScienceDirect Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso Evaluating pore characteristics and acid site locations in hierarchical SAPO-11 by catalytic model reactions Daniel Ali a, Zhihui Li a, Muhammad Mohsin Azim a, Hilde Lea Lein b, Karina Mathisen a, * a b Department of Chemistry, Norwegian University of Science and Technology (NTNU), N-7491, Trondheim, Norway Department of Materials Science and Engineering, Norwegian University of Science and Technology (NTNU), N-7491, Trondheim, Norway A R T I C L E I N F O A B S T R A C T Keywords: Hierarchical SAPO-11 Zeotype Hydrothermal synthesis Model reactions Hierarchical SAPO-11 molecular sieves were synthesized with three different mesopore structure directing agents (meso-SDAs): cetyltrimethylammonium bromide (CTAB), polyvinyl alcohol (PVA) and [3-(trimethoxysilyl) propyl] dimethyloctadecylammonium chloride (TPOAC) Two model reactions, methanol-to-hydrocarbons (MTH) and the Beckmann rearrangement (BMR) of cyclohexanone oxime, were employed to evaluate the pore topology and acid site locations of the hydrothermally synthesized hierarchical SAPO-11s Initially, the modified porosity of the hierarchical SAPO-11s was thoroughly probed by employing a set of general characterization methods and by comparing the results to the conventional microporous C-SAPO-11 The nitrogen physisorption results revealed that CTAB-11 had a uniform distribution of mesopores centered at 2.8 nm, whereas the presence of mesopores in PVA-11 could not be convincingly resolved through conventional methods Instead, the pore topology of PVA-11 was determined by utilizing model reactions, where the shape selective MTH model reaction revealed that the sample had mesopores present through an increased production of large products compared to the conventional C-SAPO-11 Additionally, the MTH model reaction showed that while PVA shifted the location of the Brønsted acid sites (BAS) towards the mesopores, CTAB did not affect the BAS location of SAPO-11 Finally, the BMR model reaction elucidated the excellent intrapore connectivity of the hierarchical SAPO-11s through an increased lifetime compared to the conventional C-SAPO-11 Introduction The silicoaluminophosphate-11 (SAPO-11) is a microporous, acidic SAPO with the AEL framework which consists of 10-membered elliptical rings with 0.40 × 0.65 nm pore openings aligned in a one-dimensional array [1] Compared to other SAPOs such as SAPO-34 (CHA) and SAPO-5 (AFI), SAPO-11 is known to be mildly acidic, i.e it has a low density of Brønsted acid sites [2,3] Still, SAPO-11 is a well-studied material and is catalytically active for many reactions, including hydroisomerization of n-alkanes [4–6], methanol-to-hydrocarbons [2,3, 7] and the vapor phase isomerization of cyclohexanone oxime (Beck mann rearrangement) [8] Due to its narrow micropores however, the SAPO suffers from deactivation over time, largely due to diffusion and mass transfer limitations which result in coke formation due to trapped hydrocarbons [3,5,8] In order to mitigate this, the introduction of an auxiliary large-pore system (most often mesopores) to make what is known as hierarchical SAPO-11 has recently gained attention [4,5, 9–11] Here, the larger pores are thought to function as super-highways that transport molecules to the micropores and improve the accessibility to active sites This would simultaneously reduce the diffusion limita tions and enhance the lifetime of the catalyst Several approaches for synthesizing hierarchical SAPO-11 have been reported [6,11,12], where hydrothermal synthesis [4,10,13,14] is possibly the most frequently employed synthesis method For hydro thermal synthesis, the auxiliary pore system is introduced during the initial synthesis process either by hard-templating [15], or by soft-templating with mesopore structure directing agents (meso-SDAs) such as surfactants or polymers [9,10] Indeed, quaternary ammonium surfactants, organosilane surfactants and polymers have all previously been successfully applied as meso-SDAs for the hydrothermal synthesis of hierarchical SAPO-11 [10,15,16] However, there has yet to be direct comparison of how these soft-template meso-SDAs may produce different types of hierarchy in hierarchical SAPO-11 Two model reactions which have already shown promise for the evaluation of hierarchy in SAPOs, are the methanol-to-hydrocarbons reaction (MTH) [17] and the isomerization of cyclohexanone oxime * Corresponding author E-mail address: karina.mathisen@ntnu.no (K Mathisen) https://doi.org/10.1016/j.micromeso.2021.111550 Received June 2021; Received in revised form 13 September 2021; Accepted November 2021 Available online November 2021 1387-1811/© 2021 The Authors Published by Elsevier Inc This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) D Ali et al Microporous and Mesoporous Materials 329 (2022) 111550 (Beckmann rearrangement) [18] While SAPO-11 typically has low hy drocarbon yields during the MTH reaction due to its mild acid properties (vide supra) [3], it is known to be a highly active catalyst for the Beck mann rearrangement [8] The Beckmann rearrangement (BMR) of cyclohexanone oxime rearranges the oxime into its lactam oligomer, the industrially relevant nylon-6 precursor, ε-caprolactam (CPL) [19] Pre vious reports have already shown that the introduction of auxiliary mesopores into microporous SAPOs leads to a longer lifetime for the reaction due to alleviation of diffusion limitations [18,20] Furthermore, the BMR also shows promise for demonstrating the presence of con nected micro- and mesopores in both one- and three-dimensional SAPOs, such as the SAPO-5 and SAPO-34, respectively [18] In this study, one-step hydrothermal syntheses of hierarchical SAPO11 molecular sieves were attempted with three different types of mesoSDAs (soft-templates) in order to study the effects of template type on the resulting hierarchical SAPO-11 Specifically, a quaternary ammo nium surfactant (cetyltrimethylammonium bromide, CTAB), a polymer (polyvinyl alcohol, PVA) and an organosilane surfactant ([3-(trime thoxysilyl) propyl] dimethyloctadecylammonium chloride, TPOAC) were employed as meso-SDAs After a comprehensive characterization, the MTH and BMR model reactions were utilized respectively to eval uate the acid site locations and pore topologies of the hierarchical SAPO11s compared to the conventional, microporous SAPO-11 Table An overview of the mesopore structure directing agents (meso-SDAs) employed in this study Parameter Meso-SDA property Short name CTAB PVA TPOAC Name Cetyltrimethylammonium bromide Polyvinyl alcohol [3(trimethoxysilyl) propyl] dimethyloctadecylammonium chloride Quaternary ammonium surfactant Polymer Organosilane surfactant Structure Experimental 2.1 Synthesis of samples Meso-SDA type 2.1.1 Conventional SAPO-11 The conventional SAPO-11 was hydrothermally synthesized using a single-SDA modification of the procedure described by Zhao et al [10] in order to obtain phase pure SAPO-11 An initial solution of aluminium isopropoxide (Al(O-i-Pr)3, 11.03 g, Sigma Aldrich, ≥98%) in deionized water (H2O, 57.39 g) was stirred until homogeneous, after which phosphoric acid (H3PO4, 6.22 g, Merck, 85%) was added dropwise and the resulting mixture was stirred for h Subsequently, tetraethyl orthosilicate (TEOS, 1.13 g, Sigma Aldrich, 98%) was added and the mixture was stirred for h before dropwise addition of the micropore SDA (micro-SDA), dipropylamine (DPA, 3.28 g, Fluka, ≥99%) After stirring for an additional h, the final mixture, with a theoretical composition of 1.0Al: 1.0P: 0.1Si: 0.6DPA: 60H2O, was adjusted to pH 6.0 using phosphoric acid before being poured into a 60 mL Teflon-lined stainless-steel autoclave for crystallization at 170 ◦ C for 48 h After quenching, the resulting powder was washed three times with deionized water and once with ethanol The final product, C-SAPO-11, was ob tained after drying for h at 110 ◦ C and finally calcining for h at 600 ◦ C in air acronym of the meso-SDA used for the synthesis, resulting in the ma terials CTAB-11, PVA-11 and TPOAC-11 To optimize the crystallinity and phase purity of the hierarchical SAPO-11s, a parameter study of the crystallization times and temperatures was carried out and has been detailed in the supplementary information, Tables S1–1 Accordingly, the crystallization temperature for CTAB-11 and PVA-11 was set to 170 ◦ C, whereas the crystallization times were 84 h and 48 h, respec tively For TPOAC-11, the crystallization temperature and time was 200 ◦ C and 48 h, respectively 2.2 Characterization The experimental information regarding the characterization tech niques is based on previous reports [17] and has been detailed below X-ray powder diffraction (XRD) was performed on a Bruker D8 Focus X-ray Diffractometer with a CuKα radiation source (1.5406 Å) and LynxEye™ SuperSpeed Detector The diffractograms were recorded from to 60◦ with a step size of ~0.01◦ A fixed 0.2 mm divergence slit was used throughout the run Relative crystallinities were calculated according to previously reported methods [21,22] using the sum of the following reflections of 2θ: 8.1◦ , 9.4◦ , 13.1◦ , 15.6◦ , 20.3◦ and 21.0◦ Nitrogen physisorption analyses were carried out on a Micromeritics Tristar 3000 Surface Area and Porosity Analyzer at − 196 ◦ C Prior to measurements, the materials were degassed under vacuum at 250 ◦ C using a Micromeritics VacPrep 061 Sample Degas System in order to remove water and other volatile adsorbates The specific surface area was determined by the BET (Brunauer-Emmett-Teller) method while the micropore and external area were estimated using the t-plot method Finally, the specific pore volumes were obtained by BJH (Barrett-Joy ner-Halenda) analysis Scanning electron microscopy (SEM) was performed on a Hitachi S–3400 N where the samples were gold coated by sputtering using an Edwards Sputter Coater (S150B) prior to imaging Images were captured in secondary electron (SE) mode while particle sizes were determined 2.1.2 Hierarchical SAPO-11 Hierarchical SAPO-11 was synthesized by adding a certain equiva lent of mesopore SDA (meso-SDA) to the synthesis procedure of the conventional material (vide supra) Specifically, the meso-SDA was either the polymer polyvinyl alcohol (PVA, Alfa Aesar, 86–89%), the organosilane surfactant [3-(trimethoxysilyl) propyl] dimethyloctadecy lammonium chloride (TPOAC, Sigma Aldrich, 42 wt%), or the quater nary ammonium surfactant cetyltrimethylammonium bromide (CTAB, Sigma Aldrich, >99%) (see also Table 1) The meso-SDA was added to the initial solution of Al(O-i-Pr)3 and H2O In accordance to previous reports [10], an additional micro-SDA, diisopropylamine (DIPA, 0.82 g, Sigma Aldrich, ≥99.5%), was added in conjunction with DPA in order to produce phase pure hierarchical samples The final mixtures for poly mer- and surfactant-based syntheses had theoretical compositions of 1.0Al: 1.0P: 0.1Si: 0.3DPA: 0.3DIPA: 60H2O: 15 gPVA LH2O− PVA and 1.0Al: 1.0P: 0.1Si: 0.3DPA: 0.3DIPA: 60H2O: 0.025 surfactant, respec tively The washing, drying and calcination procedures were identical to that of the conventional C-SAPO-11 and labelling was done by using the D Ali et al Microporous and Mesoporous Materials 329 (2022) 111550 from single images constituting at least 100 particles per sample using the software ImageJ (version 1.52a) [23] Thermogravimetric analyses coupled with mass spectrometry (TGAMS) were carried out with 10–15 mg of filtered particle size (212–425 μm) on a Netzsch Jupiter STA 449 equipped with a QMS 403 Aăelos quadrupole mass spectrometer The flow consisted of 45 mL min− air and 25 mL min− argon while the temperature program started at 35 ◦ C, subsequently heated to 850 ◦ C at a rate of ◦ C min− 1, and held for h before finally cooling down to room temperature at a rate of ◦ C min− Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) was conducted using an Agilent 8800 Triple Quadropole ICP-MS (ICP-QQQ) with a SPS Autosampler The samples (20–40 mg) were decomposed with concentrated nitric acid (HNO3, 1.5 mL, 65%) and concentrated hydrofluoric acid (HF, 0.5 mL, 40%) The final solution was diluted with deionized water and filled into a 16 mL sample tube Before analysis, the samples were re-diluted in 5% HNO3 and 115In was added as an internal standard Standards from Inorganic Ventures were used for quantification Carbon monoxide (CO) adsorption was performed with a Bruker Vertex 80 FTIR spectrometer equipped with an LN-MCT detector from Kolmar Technologies and a custom-built transmission cell Measure ments were conducted at an aperture setting of mm, a scanner velocity of 20 kHz and a resolution of cm− Samples were pressed into selfsupported wafers (5–7 mg) and were pre-treated for h at 500 ◦ C under vacuum to remove adsorbed water and impurities Subsequently, the cell was cooled to − 196 ◦ C before slowly introducing CO (AGA) Finally, stepwise desorption of CO was conducted by gradually lowering the pressure in the system until the initial spectrum was recovered Results & discussion 3.1 General characterization To investigate the effects of utilizing different meso-SDAs for syn thesizing hierarchical SAPO-11, a thorough general characterization was conducted Initially, XRD was employed to evaluate the phase pu rity as well as the relative crystallinity of the samples Following this, ICP-MS was conducted in order to determine the elemental composition of the samples, while SEM was carried out to assess morphologies and particle sizes Finally, nitrogen physisorption was performed in order to evaluate the presence of incorporated mesopores in the hierarchical SAPO-11 systems The X-ray diffractograms of the calcined conventional and hierar chical SAPO-11s are stacked together with the simulated AEL pattern in Fig [1] The major crystalline phase for C-SAPO-11, CTAB-11 and PVA-11 was the AEL phase, indicating that the SAPO-11 structure was successfully obtained for these samples For TPOAC-11 however, the AEL reflection at 21.2◦ (denoted with a circle in Fig 1) was missing and instead, several impurities (marked with asterisks in Fig 1) had appeared These impurities were also present in the as-synthesized sample and due to the low crystallinity (