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Article

Ionothermal Crystallization of SAPO-11 Using Novel Pyridinium Ionic Liquid and Its Catalytic Activity in Esterification of Levulinic Acid into Ethyl Levulinate

by
Al Issa Jehad Moh’dFathi Mohammad
1,
Vinithaa Saminathan
1,
Zeinhom M. El-Bahy
2,
Laure Michelin
3,4,
Tau Chuan Ling
5 and
Eng-Poh Ng
1,*
1
School of Chemical Sciences, Universiti Sains Malaysia, Penang 11800, Malaysia
2
Department of Chemistry, Faculty of Science, Al-Azhar University, Nasr City, Cairo 11884, Egypt
3
Université de Haute-Alsace, Axe Matériaux à Porosités Contrôlées, Institut de Science de Matériaux de Mulhouse UMR 7361, ENSCMu, 3b rue Alfred Werner, 68093 Mulhouse, France
4
Université de Strasbourg, 67000 Strasbourg, France
5
Institute of Biological Sciences, Faculty of Science, University of Malaya, Kuala Lumpur 50603, Malaysia
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(2), 433; https://doi.org/10.3390/catal13020433
Submission received: 14 January 2023 / Revised: 1 February 2023 / Accepted: 13 February 2023 / Published: 17 February 2023
(This article belongs to the Special Issue Ionic Liquids for Green Catalysis and Separation)
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Abstract

:
A study using a novel pyridinium ionic liquid, namely 1-propylpyridinium bromide ([PPy]Br), to crystallize SAPO-11 under ionothermal conditions is reported. By carefully following the crystallization process, SAPO-11 can readily be crystallized in the presence of [PPy]Br, which serves as a synthesis solvent and structure-directing agent, at 150 °C after 133 h of heating. The study also focuses on manipulating other synthesis parameters (e.g., crystallization temperature, phosphorous content, silicon amount and [PPy]Br concentration) and investigating their respective effects on the formation of SAPO-11. The crystallized SAPO-11 has an acidic nature and a high surface area. Under conductive instant heating conditions, the SAPO-11 catalyst is very active in the conversion of levulinic acid into ethyl levulinate; 93.4% conversion and 100% selectivity of ethyl levulinate are recorded at 180 °C after 30 min of reaction. This result is comparable to or even better than those of conventional homogeneous catalysts.

1. Introduction

Zeolites are hydrated inorganic porous crystalline materials made up of interlinked tetrahedra of metal or nonmetal oxides (Si, Al or P). Among the zeolite family, aluminophosphates (AlPO-n) and silicoaluminophosphates (SAPO-n) are of particular interest, especially in petrochemical industries, due to their fascinating catalytic, ion exchange and separation properties [1,2]. In particular, SAPO-11 has well-aligned unidimensional elliptical pores of 4.0 × 6.5 Å2 and is well suited for various chemical reactions, for instance, the hydroisomerization of n-paraffins [3], dewaxing [4], hydrodeoxygenation (HDO) of vegetable oils [5], n-alkane cracking [6] and oxidation of cyclohexane [7], thanks to its unique pore shape that leads to high product selectivity.
Classically, SAPO-11 can be crystallized via the hydrothermal route at 150–300 °C from reactive precursors containing amine or quaternary ammonium salts as organic structure-directing agents [8]. The presence of these organic species and the adjustment of synthesis conditions are important to ensure the formation of pure SAPO-11 with distinct morphological structures for task-specific applications [9,10,11]. Recently, an alternative route, the so-called ionothermal technique, for synthesizing zeolite materials has been reported [12,13,14]. Unlike the hydrothermal technique, this method uses a very polar ionic liquid instead of water as a nonaqueous solvent for transporting the inorganic reactants without interacting with the components in the ionogel, while concurrently also serving as a structure-directing agent [15]. Due to using these nonvolatile solvents, the attributes of the ionothermal approach include the following distinct features: low or virtually no volatility, negligible vapor pressure, better thermal stability, tunable hydrophilibility and high reusability [16].
So far, imidazolium ionic liquids are the most commonly used in ionothermal synthesis for preparing SAPO-5 [17], MnAlPO-5 [18], SAPO34 [19], SAPO-LTA [20] and GaPO4-LTA [21], while SAPO-11 can only be crystallized with the assistance of microwave heating [22]. Although various classes of ionic liquids (morpholinium, pyridinium, pyrrolidinium, piperidinium, imidazolium, etc.) have been synthesized [23,24,25], the ionothermal synthesis of SAPO-n zeolites is focused only on imidazolium-based ionic liquids. Hence, searching for ionic liquids other than imidazolium salts for crystallizing SAPO-n zeolites (e.g., SAPO-11) with unique surface and structural properties is of the utmost importance and worthy of further investigation.
Pyridinium ionic liquid is an ionic liquid containing a pyridinium cation. Compared with the imidazolium cation, the pyridinium cation is less polar due to the presence of only one N atom in its six-membered cyclic ring. In addition, the charge on the ring is fully localized, and hence, the coulomb and chemical interactions between the pyridinium cation and the counter anion are weaker than in the imidazolium based ionic liquid [26]. As a result, both features are expected to alter the crystallization profile of zeolites which indirectly affect the physicochemical properties of zeolite solids. To the best of our knowledge, no studies on the use of pyridinium ionic liquid for crystallizing zeolite materials under ionothermal conditions have been reported thus far.
In this study, SAPO-11 is crystallized with a new N-heterocyclic class of ionic liquid, namely 1-propylpyridinium bromide ([PPy]Br), in an ionothermal environment where the time-dependent study and effects of synthesis parameters (crystallization temperature, phosphorous content, silicon amount and [PPy]Br concentration) on the formation of SAPO-11 are investigated. Then, the prepared SAPO-11 is characterized before its catalytic behavior is studied in the esterification of levulinic acid and ethanol under conductive instant heating conditions that mimic rapid microwave heating.

2. Results and Discussion

2.1. Time-Dependent Study of Ionothermal Crystallization of SAPO-11

In this work, SAPO-11 is crystallized under ionothermal conditions (150 °C) using 1-propylpyridinium bromide ([PPy]Br) as both structure-directing agent and solvent, which is different from the hydrothermal technique applied in the classical zeolite synthesis system. At 50 h, no solid product was recovered upon centrifugation, indicating the complete dissolution and participation of inorganic reactants in the ionic liquid. At 110 h, a small amount of amorphous nanoparticles (140 nm) was formed and sedimented, indicating that the polymerization of inorganic Si, Al and P oligomers into a heavier solid had taken place (E-1, Figure 1a) [27]. A semicrystalline SAPO-11 product, consisting of spherical amorphous nanoparticles (ca. 270 nm) and broomstick-like particles (ca. 1.9 μm), starts to form at 120 h revealing the occurrence of nucleation (E-2, Figure 1b).
The XRD amorphous hump completely disappears after 133 h of ionothermal heating, and the XRD pattern of the E-3 solid displays intense peaks which can perfectly match with the simulated XRD pattern of AEL-type zeolite (E-3, Figure 1c) [28]. Meanwhile, the Fe-SEM confirms that all the amorphous nanoparticles were consumed as nutrients for the growth of SAPO-11 (Figure 1c) [29]. As a result, large and well-defined broomstick crystals (ca. 2.3 μm) are formed, completely different from those crystallized in other systems (screw-like [9], nanoaggregate [30], and columnar [31]). However, after 200 h of heating, the XRD analysis shows cocrystallization of SAPO-11 with SAPO-5 (AFI topology) (2θ = 7.43°, 20.97° and 22.38°) as a minor phase in the E-4 sample [28], thus revealing the metastability of SAPO-11 zeolite.

2.2. Effect of Synthesis Conditions

2.2.1. Effect of Heating Temperature

The heating temperature is very important for chemical processes as it provides energy to initiate the reactions [32]. Hence, the ionothermal synthesis of SAPO-11 was performed by heating the ionogel of 1.0Al: 1.8P: 0.3Si: 38[PPy]Br at 130 °C, 140 °C, 150 °C and 160 °C (denoted as E-5, E-6, E-3 and E-7, respectively) for 133 h to study its influence on the crystallization profile of SAPO-11. A low temperature seems to be insufficient to promote crystallization: the E-5 is produced as an amorphous nanospherical solid (ca. 20 nm) while E-6 contains rarely stick-like SAPO-11 crystals (ca. 940 nm) in the amorphous solids (Figure 2a,b). Only several XRD peaks due to SAPO-11 are shown (2θ = 9.36° [020], 20.41° [310], 21.11° [002], 22.23° [231], 22.68° [141] and 23.13° [240]). The XRD peaks are broadened, revealing small SAPO-11 crystallites, as also confirmed by the FE-SEM data [33]. By providing more energy through increasing the temperature to 150 °C, it is possible to see the full crystallization of SAPO-11 zeolite which results in the emergence of typical AEL X-ray diffraction peaks (E-3, Figure 2c). As shown, the SAPO-11 exhibits broomstick-like features, while the morphological structure strongly depends on the synthesis technique (e.g., hydrothermal, ionothermal, dry gel conversion, microwave heating, etc.), hydrophilicity and type of ionic liquid used [34]. Supplying excessive energy by raising the temperature to 160 °C leads to the activation and crystallization of a more stable crystalline phase, where a partial transformation of SAPO-11 into berlinite metasomatic phosphate (a nonporous crystalline dense phase) occurs (E-7, Figure 2d). Hence, 150 °C is the optimum temperature for crystallizing SAPO-11 in an ionothermal environment.

2.2.2. Effect of P/Al Molar Ratio

The amount of phosphorus added (in H3PO4 form) to the ionogel plays a very important role in the crystallization process, because phosphorus is one of the main components of SAPO-11 zeolite besides aluminum and silicon. In addition, H3PO4 also modulates the pH of the ionogel, which can alter the entire crystallization environment of zeolites [35]. Thus, the amount of H3PO4 (presented in P/Al molar ratio) is studied by heating the ionogel of 1.0Al: xP: 0.3Si: 38[PPy]Br (x = 1.2: E-8, x = 1.8: E-3, x = 2.1: E-9, x = 2.4: E-10) at 150 °C for 133 h. When x = 1.2, an amorphous solid with irregular shape, as proven by the broad XRD hump at 2θ = 20°–33°, is observed (Figure 3a). Increasing P content to x = 1.8, we witness the complete conversion of the amorphous solid into SAPO-11. This reveals the importance of H3PO4 in providing an adequate pH environment for the promotion of nucleation and crystal growth [36]. However, further increasing the x values to 2.1 and 2.4 produces a more acidic ionogel that favorably crystallizes berlinite dense phase and AFI-type zeolite, and this observation is in line with the work reported by Khoo et al. [37].

2.2.3. Effect of Si Content

As mentioned, the element Si is another primary building unit component of the SAPO-11 zeolite framework that controls the crystallization process [38]. In addition, the isomorphous substitution of Si4+ into the SAPO-11 framework may result in variable physiochemical attributes (e.g., creating acid sites) which are highly demanding in heterogeneous catalysis [39]. Hence, the amount of Si content in ionogels of 1.0Al: 1.8P: ySi: 38[PPy]Br was varied (y = 0, 0.4, 0.9 and 1.3) to study its effects on the crystallization kinetics of SAPO-11. When no Si (y = 0) is added, tridymite (2θ = 21.6° [112] and 21.8° [−404]) as a competing phase is formed together with SAPO-11 (E-11, Figure 4a). The results from the FE-SEM micrograph also agree with the XRD data showing that bulk SAPO-11 crystals with stick-like shape (14 μm) are cocrystallized with hexagonal tridymite crystals. When y = 0.4, both phases remain dominant, with a small amount of SAPO-5 (AFI topology) phase (2θ = 7.42° [100]) detected (E-12, Figure 4b). In SEM, the SAPO-5 zeolite can be characterized by its hexagonal rod morphology, which is commonly reported for AFI-type molecular sieves [40]. SAPO-11 is formed at the expense of tridymite as the Si content increases until fully pure SAPO-11 is obtained at y = 1.3 (E-3, Figure 4d). Hence, the results show that varying the Si content has a considerable effect on phase selectivity during the ionothermal process [41].

2.2.4. Effect of [PPy]Br Content

An ionic liquid is essential for controlling the concentration of raw chemicals besides serving as a solvent for the ionothermal synthesis of zeolite [42]. Simultaneously, it also acts as a potent structure-directing agent in the formation of zeolites. Hence, the precursor mixture of 1.0Al: 1.8P: 0.3Si: z[PPy]Br (z = 14, 22, 30 and 38) was heated at 150 °C for 133 h in order to investigate the effects of [PPy]Br on the crystal evolution of SAPO-11. When z = 14, the tridymite dense phase tends to be crystallized as the major crystalline product with SAPO-5 (AFI) and SAPO-11 (AEL) as minor phases (E-14, Figure 5a). According to Basina et al., an insufficient amount of ionic liquid may cause slow and inefficient dissolution of inorganic reactants, affecting the equilibrium concentration for initiating the nucleation of zeolite crystals [43]. If the [PPy]Br content is increased to z = 22 or 30, it allows better dissolution of reactants, resulting in the favored crystallization of SAPO-11 over tridymite (E-14, Figure 5b,c). The optimum [PPy]Br content is found to be z = 38, when pure SAPO-11 solid is obtained. Hence, besides serving as solvent, [PPy]Br also plays a structure-directing role and a charge-balancing role in stabilizing the micropore channel system of SAPO-11 [44].

2.3. Pore Stabilization and Confinement Roles of PPy]Br in SAPO-11

Thermogravimetry analysis can reveal the roles of [PPy]Br in stabilizing the AEL framework and the strength of this supramolecule’s interaction with the SAPO-11 pore channel system.
In this regard, TGA/DTG data of pure [PPy]Br ionic salt and as-synthesized SAPO-11 (E-3) that contains occluded [PPy]Br were recorded. The free [PPy]Br exhibits one-step weight loss (96.5%) at 190–300 °C due to the decomposition of ionic liquid (Figure 6a) [45]. In contrast, the noncalcined SAPO-11 displays three steps of weight loss. The first weight-loss step (0.6%), at <170 °C, is due to the desorption of water from the surface of SAPO-11 [46], whereas the second (3.0%, 170–320 °C) and third (3.8%, 320–450 °C) steps of weight loss are attributed to the decomposition of the ionic liquid molecules [47]. As calculated, the [PPy]+/TO2 (T = Si, Al or P) molar ratio of as-synthesized SAPO-11 is 0.036, which corresponds to ca. 28 TO2 primary building units enfolding one [PPy]+ organic cation in a three-dimensional manner. In addition, the weight loss of organic moiety occurs at a higher temperature due to strong interaction and confinement effects of the [PPy]Br molecule in the micropores. As a result, the trapped [PPy]Br requires a longer time and higher temperature to escape from the SAPO-11 pore channels [18]. Hence, it is revealed that the encapsulation of [PPy]+ follows the “ship in a bottle” approach, whereby TO2 primary building units form oligomers and then polymers around the [PPy]+ (4.33 Å × 7.93 Å × 2.54 Å) [48] before the organic cation is tightly wrapped and confined in the pores forming a pore size diameter of 4.63 Å.
The 13C NMR spectroscopy also shows that the [PPy]Br is intact after being trapped inside the SAPO-11 micropores; all NMR peaks due to [PPy]Br molecules are shown in the as-synthesized SAPO-11 (Figure 7b). In this sample, the ionic liquid molecules (especially C1, C3 and C6) also experience chemical shifts compared with their free counterpart (Figure 7a). Furthermore, their resonance bands are also broadened, indicating strong molecule-wall interaction and geometric constraints in confined space that disallow molecular movement inside SAPO-11 micropores [49]. Thus, the thermogravimetry and NMR data further confirm the stabilizing and pore-filling effect roles of [PPy]Br during the crystallization of SAPO-11.

2.4. Porous and Acid Properties of [PPy]Br Synthesized SAPO-11

The purity, crystallinity and catalytic performance of SAPO-11 are directly related to its porous properties, and hence, nitrogen adsorption–desorption analysis was performed on the calcined SAPO-11 at −196 °C. The N2 adsorption–desorption curves of SAPO-11 display type I adsorption isotherm due to its high microporous characteristics (SMic = 110 m2 g−1) with some N2 uptake at high P/Po, confirming its secondary textural mesoporosity (Table 1, Figure 8 and inset) [50]. The sample also shows a considerably high surface area (SBET = 149 m2 g−1, SMic = 110 m2 g−1), high total pore volume (Vtot = 0.24 cm3 g−1) and large average pore diameter (dp = 29.5 nm where primary micropores and secondary textural mesopores are considered), whereby its porosity is comparable with the SAPO-11 synthesized using dipropylamine under hydrothermal conditions (SBET = 100 m2 g−1, SMic = 50 m2 g−1, Vtot = 0.15 cm3 g−1 [51]).
FTIR spectroscopy investigation of pyridine adsorption is one of the most popular methods for characterizing the surface acidity. The study of the framework acidity (type, amount and strength) is based on the bonding strength of the N nucleophile group of pyridine to the acid sites of SAPO-11 (Si species) [52]. The FTIR spectra of pyridine adsorbed SAPO-11 after desorption at 150 and 300 °C are shown (Figure 9). In general, the SAPO-11 contains both Lewis and Brønsted acidities, based on the IR bands at 1455 and 1545 cm−1, respectively. The Lewis acid sites (158 µmol g−1), which come from structural defects after calcination, are higher than the Brønsted acid sites (136 µmol g−1) (Table 2, Figure 9) [53]. The quantity of acidity of SAPO-11 at 300 °C is also determined where it can distinguish the strength of acid sites. It can be seen that some IR bands are still retained, indicating the presence of medium-to-strong Lewis (40 µmol g−1) and Brønsted (61 µmol g−1) acid sites in the solid. Hence, the results show that the synthesized SAPO-11 is a mild-to-strong acid solid catalyst with a total acidity slightly higher than that of the conventional SAPO-11 [54].

2.5. Catalytic Study

Conversion of levulinic acid into ethyl levulinate via esterification was used as a probe reaction (Scheme 1) to study the catalytic behavior of SAPO-11 under conductive instant heating conditions. Before use, the calcined SAPO-11 was checked with TGA/DTG to ensure that all ionic liquid had been removed. Without adding any catalyst, the rate of levulinic acid conversion is very low (6.6%) after 60 min of heating at 150 °C. This indicates that the esterification of levulinic acid is an activated reaction. Interestingly, the conversion is enhanced tremendously when SAPO-11 is added, after which 75.5% of conversion is recorded after heating at 150 °C for 20 min, revealing that SAPO-11 is a potential active catalyst for the conversion of levulinic acid into ethyl levulinate (Figure 10A(a)).
The esterification reaction was also investigated at various temperatures (150–180 °C) from 0 to 60 min. The reaction conversion is enhanced by increasing the heating temperature, as shown in Figure 10A. This is due to the fact that when more energy is provided to the system, the reaction rate increases, as stated by the Arrhenius equation [55]. For instance, at 30 min of reaction, 82.6% of conversion is recorded at 150 °C, and this increases to 87.0% (160 °C) and 90.6% (170 °C) before reaching 93.4% at 180 °C (Figure 10A(a–d)). Based on the reaction study, the optimum reaction time and temperature are 30 min of heating at 180 °C, because the conversion rate nearly plateaus after 30 min.
The effect of catalyst amount was also studied at 180 °C for 30 min with a molar ratio of 1:11 (levulinic acid:ethanol) using 0–0.125 g of SAPO-11. Without catalyst, the conversion rate is merely 6.6%, and the levulinic acid conversion rate rises significantly to 52.7% when 0.025 g catalyst is added (Figure 10B). The conversion rate keeps increasing to 93.4% with the catalyst load increased to 0.100 g. Such a phenomenon can be explained by the high availability of Brønsted acid sites when more solid acid catalyst is added, resulting in a high reaction conversion. However, the levulinic acid conversion slightly decreases when the catalyst loading is further increased to 0.125 g (91.7%), which may be due to the excessive catalyst loading causing inefficient stirring [56]. Furthermore, the water molecules produced during the esterification reaction can also deactivate the SAPO-11 catalyst’s active sites, and this compromises the effectiveness of the catalyst [57]. Therefore, 0.100 g of SAPO-11 catalyst is the ideal loading.
The catalytic performance of SAPO-11 is also compared with conventional homogeneous catalysts (H2SO4, HCl, acetic acid, p-toluenesulfonic acid) at 180 °C for 30 min of instant heating (Figure 11a). It is shown that both H2SO4 and HCl achieve 100% conversion, but their selectivity towards ethyl levulinate are merely 86.3% and 90.1%, respectively; 4-ethoxy γ-valerolactone and alpha-Angelica lactone were produced as by-products in both catalyzed reactions. For acetic acid, a low conversion rate (74.2%) with 100% selectivity of ethyl levulinate are recorded; this can be explained by its weak acidity. On the other hand, SAPO-11 shows comparable catalytic performance to that of p-toluenesulfonic acid, achieving nearly 95% conversion and excellent 100% ethyl levulinate selectivity. The superior performance of SAPO-11 can hence be explained by its mild-to-strong acidity and unidimensional 10-membered ring pore channel (4.63 × 2.01 × 2.01 Å3) that exhibits good molecular size-sieving ability for selectively producing only ethyl levulinate as a single product.
Although some homogeneous catalysts achieve 100% conversion, they are not reusable. On the other hand, the SAPO-11 catalyst is recyclable, and the reaction performance is nearly maintained even after five consecutive cycles of reaction (Figure 11b). Thus, being reusable, ecofriendly and not corrosive, SAPO-11 is preferable to its homogeneous counterparts as an acid catalyst in the esterification of levulinic acid into ethyl levulinate. Comprehensive works on synthesizing various zeolite materials using other pyridinium ionic liquids for advanced applications (e.g., membrane, catalyst) are in progress.

3. Methodology

3.1. Synthesis of 1-Propylpyridinium Bromide, [PPy]Br

Scheme 2 illustrates the synthesis procedure of a novel [PPy]Br ionic liquid. Typically, pyridine (94.846 g, 99%, Acros Organics, Geel, Belgium) and 1-bromopropane (221.20 g, 99%, Acros Organics, Geel, Belgium) were mixed together in a 500-mL round bottom flask. Then, the mixture was magnetically stirred and refluxed at 70 °C for 20 h. The mixture was subjected to rotary evaporation (100 rpm) at 60 °C under a vacuum (50 mbar) to remove the nonreacted 1-bromopropane. The obtained yellowish solid was then soaked (4 h) and washed with acetone (200 mL) 4 times before it was dried in an oven at 105 °C overnight to obtain the 1-propylpyridinium bromide ([PPy]Br) ionic salt. Yield: 86.2%. 1H NMR (500 MHz, D2O): δ = 1.01 (3H, t, J = 7.2 Hz, pyridine N-CH2-CH2-CH3), 2.11 (2H, m, J = 7.25, pyridine N-CH2-CH2-CH3), 4.71 (2H, t, J = 7.23 Hz, pyridine N-CH2), 8.17 (2H, t, J = 6.3 Hz, pyridine N-CH=CH), 8.64 (1H, t, J = 7.77 Hz, pyridine N-CH=CH-CH), 8.96 (2H, d, J = 5.7 Hz, pyridine N=CH). 13C NMR (500 MHz, D2O): δ = 10.26, 24.05, 63.71, 127.84, 144.01, 145.55. FTIR (KBr disk) cm−1: 1167 (C–N), 1486 (C=N), 1501 and 1634 (aromatic C=C), 2879 (Csp3–H), 3055 (Csp2–H), 3407 cm−1 (broad, O–H). Anal. Cacld. for C8H12NBr: C, 47.54%; H, 5.95%; N, 6.92%; found: C, 46.83%; H, 5.85%; N, 6.86%.

3.2. Ionothermal Synthesis of Silicoaluminophosphate Number 11 (SAPO-11)

Initially, aluminum isopropoxide (0.192 g, 98%, Sigma-Aldrich, Darmstadt, Germany) was placed into a Teflon-lined stainless-steel autoclave. Next, the [PPy]Br ionic liquid (7.000 g), phosphoric acid (0.192 g, 85%, Sigma-Aldrich, Darmstadt, Germany) and tetraethyl orthosilicate (0.056 g, 98%, Sigma Aldrich, Darmstadt, Germany) were subsequently added. The final chemical composition had a molar ratio of 1.0Al: 1.8P: 0.3Si: 38[PPy]Br. The autoclave was capped and placed in an oven at 150 °C for various crystallization times (110–200 h). The mixture was dispersed in distilled water and the sedimented solid product was recovered using high-speed centrifugation (10,000 rpm, 5 min, Drawell, Chongqing, China). The process of dispersion and purification with distilled water was repeated another three times before the final solid product was dried at 90 °C overnight. The resulting as-synthesized SAPO-11 was calcined at 580 °C for 6 h at a ramping rate of 1 °C min−1 under air atmosphere.
For the study of the effects of synthesis variables (heating temperature, phosphorous content, silicon amount and [PPy]Br concentration), a similar synthesis procedure was applied wherein the resulting ionogel with a chemical composition of 1.0Al: xP: ySi: z[PPy]Br (x = 1.2–2.4, y = 0–0.3, z = 30–54) was heated at 130–160 °C for 133 h. The crystallization conditions of the solids are summarized in Table 2.

3.3. Characterization

The functional groups of the ionic liquid were confirmed using a Perkin Elmer’s System 2000 infrared spectrometer (resolution 4 cm−1, 100 scans, Waltham, MA, USA) where the KBr pellet technique was applied (KBr:sample mass ratio = 60:1). The liquid’s 1H and 13C NMR spectra were recorded with a Bruker Advance 500 MHz spectrometer (Waltham, MA, USA) operated at 500 and 126 MHz for 1H and 13C, respectively, using a single pulse excitation with π /2 (3 µs) pulses. In both analyses, deuterium oxide (D2O) and tetramethylsilane (TMS) were used as the solvent and internal standard for chemical shift, respectively. The elemental constituents of the [PPy]Br ionic liquid were determined using a Perkin Elmer 2400 Series II CHNS/O analyzer (Waltham, MA, USA). The crystallization process and crystalline phase of solid samples were studied using a Bruker D8 AVANCE diffractometer (Waltham, MA, USA) operated at a scanning rate of 0.2° min−1 and 40 kV/10 mA with the copper Kα radiation source (λ = 1.5406 Å). The crystal size and morphology of solids were analyzed using a HITACHI Regulus 8220 FE-SEM (Tokyo, Japan) at low acceleration voltage (2 kV). The porous properties of SAPO-11 were studied using a Micromeritics ASAP 2010 analyzer (Norcross, GA, USA) where the sample (ca. 80 mg) was first degassed at 250 °C for 10 h and then subjected to N2 adsorption-desorption at −196 °C. The specific surface area, micropore surface area and pore size distribution were determined using the Brunauer–Emmett–Teller (BET), t-plot and nonlocalized density functional theory (NLDFT) models, respectively, while the total pore volume was calculated using the total N2 uptake at P/Po = 0.998. The surface acidity of the synthesized SAPO-11 was investigated with the in situ pyridine-FTIR spectroscopy technique. First, a self-supporting wafer of sample (ca. 12 mg) was degassed at 300 °C for 4 h under vacuum (10−3 mbar). After cooling down to room temperature, the reference spectrum was collected (8 cm−1 resolution, 200 scans). Pyridine was adsorbed onto the sample for 3 min prior to the evacuation of non-adsorbed pyridine and heating at 150 °C for 1 h to remove physisorbed pyridine. The spectrum was recorded after cooling. The wafer was heated again at 300 °C for 1 h before an IR spectrum was collected. The number of Brønsted and Lewis acid sites was calculated using molar integral extinction coefficients of εBrønsted = 0.73 cm µmol−1 and εLewis = 0.96 cm µmol−1 [58].

3.4. Catalytic Reaction Study

The esterification of levulinic acid and ethanol into ethyl levulinate over an E-3 sample (hereafter referred to as SAPO-11) was performed under conductive instant heating conditions that mimic rapid microwave heating. Initially, the calcined SAPO-11 (0.100 g) was activated at 300 °C for 3 h before it was mixed with levulinic acid (0.04 g, 0.34 mmol, 98%, Acros Organic) and ethanol (0.202 g, 4.38 mmol, 99.7%, Qrec) in a 10-mL glass vial. The mixture was capped and heated in an Anton Paar’s Monowave 50 conductive instant heating reactor (Graz, Austria) at 150–180 °C for 0–60 min under magnetic stirring (800 rpm). The catalyst was separated using centrifugation (10,000 rpm, 10 min, Drawell, Chongqing, China) and the reaction liquid (1 µL) was withdrawn and injected into a GC-MS (Agilent 7000 Series Triple Quad, Santa Clara, CA, USA) and a GC-FID (Agilent’s HP6890 GC, Santa Clara, CA, USA) for qualitative and quantitative analyses, respectively; toluene (98%, Merck) was used as an internal standard. Each experiment was repeated twice, and error bars were plotted based on the two replicate experiments.

4. Conclusion

In conclusion, SAPO-11 has successfully been crystallized under ionothermal conditions using a novel 1-propylpyridinium bromide ([PPy]Br) ionic liquid as both solvent and structure-directing agent. By carefully inspecting the crystallization process, pure SAPO-11 can be crystallized at 150 °C after 133 h of ionothermal heating. In addition, the effects of synthesis parameters (viz. crystallization temperature, phosphorous content, silicon amount and [PPy]Br concentration) were investigated and we showed that moderate temperature, low P amount, and high Si and [PPy]Br contents are essential for minimizing the cocrystallization of other crystalline phases (e.g., AFI, berlinite and tridymite). The synthesized SAPO-11 has a high surface area (149 m2 g−1) and exhibits Brønsted and Lewis acidities which are beneficial for the esterification of levulinic acid and ethanol. Under conductive instant heating conditions, the SAPO-11 catalyst gives excellent performance; 93.4% conversion and 100% selectivity of ethyl levulinate are achieved within 30 min at 180 °C, and this result is comparable to or even better than those of conventional homogeneous catalysts. From the viewpoint of materials chemistry, the use of a new class of pyridinium ionic liquids in ionothermal synthesis opens up enormous possibilities for crystallizing various zeolite molecular sieves with unique characteristics suitable for engineering high-performance materials in catalysis and separation.

Author Contributions

Investigation and writing—original draft preparation, A.I.J.M.M. Mohammad; investigation, V.S.; resources, Z.M.E.-B.; data curation, L.M.; writing—review and editing and data curation, T.C.L., writing—review and editing, supervision, project administration, funding acquisition and conceptualization, E.-P.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

The financial support from Ministry of Higher Education Malaysia for Fundamental Research Grant Scheme with project code: FRGS/1/2022/STG05/USM/02/12 is gratefully acknowledged. The authors also extend their appreciation to Mohamed E. El Sayed, Mohammad N. Murshed and Esmail Hassan Abdullatif Al-Sabri for assisting in samples measurement.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. XRD patterns and FESEM images of (a) E-1, (b) E-2, (c) E-3 and (d) E-4 samples ionothermally heated at 150 °C for 110 h, 120 h, 133 h and 200 h, respectively. F denotes the presence of AFI crystalline phase. The theoretical XRD peak positions of the AEL phase are shown as vertical blue lines.
Figure 1. XRD patterns and FESEM images of (a) E-1, (b) E-2, (c) E-3 and (d) E-4 samples ionothermally heated at 150 °C for 110 h, 120 h, 133 h and 200 h, respectively. F denotes the presence of AFI crystalline phase. The theoretical XRD peak positions of the AEL phase are shown as vertical blue lines.
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Figure 2. XRD patterns and FESEM micrographs of (a) E-5, (b) E-6, (c) E-3 and (d) E-7 samples prepared using the ionogel of 1.0Al: 1.8P: 0.3Si: 38[PPy]Br and heated at 130 °C, 140 °C, 150 °C and 160 °C for 133 h, respectively.
Figure 2. XRD patterns and FESEM micrographs of (a) E-5, (b) E-6, (c) E-3 and (d) E-7 samples prepared using the ionogel of 1.0Al: 1.8P: 0.3Si: 38[PPy]Br and heated at 130 °C, 140 °C, 150 °C and 160 °C for 133 h, respectively.
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Figure 3. XRD patterns and FESEM images of (a) E-8 (x = 1.2), (b) E-3 (x = 1.8), (c) E-9 (x = 2.1) and (d) E-10 (x = 2.4) samples. The samples were prepared using the ionogel of 1.0Al: xP: 0.3Si: 38[PPy]Br heated at 150 °C for 133 h. F and B denote the presence of AFI and berlinite crystalline phases, respectively.
Figure 3. XRD patterns and FESEM images of (a) E-8 (x = 1.2), (b) E-3 (x = 1.8), (c) E-9 (x = 2.1) and (d) E-10 (x = 2.4) samples. The samples were prepared using the ionogel of 1.0Al: xP: 0.3Si: 38[PPy]Br heated at 150 °C for 133 h. F and B denote the presence of AFI and berlinite crystalline phases, respectively.
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Figure 4. XRD patterns and FESEM micrographs of (a) E-11 (y = 0), (b) E-12 (y = 0.4), (c) E-3 (y = 0.9) and (d) E-13 (y = 1.3) samples. The samples were prepared using the ionogel of 1.0Al: 1.8P: ySi: 38[PPy]Br heated at 150 °C for 133 h. F and T denote the presence of AFI and tridymite crystalline phases, respectively.
Figure 4. XRD patterns and FESEM micrographs of (a) E-11 (y = 0), (b) E-12 (y = 0.4), (c) E-3 (y = 0.9) and (d) E-13 (y = 1.3) samples. The samples were prepared using the ionogel of 1.0Al: 1.8P: ySi: 38[PPy]Br heated at 150 °C for 133 h. F and T denote the presence of AFI and tridymite crystalline phases, respectively.
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Figure 5. XRD patterns and FESEM micrographs of (a) E-14 (z = 14), (b) E-15 (z = 22), (c) E-16 (z = 30) and (d) E-3 (z = 38) samples. The samples were prepared using the ionogel of 1.0Al: 1.8P: 0.3Si: z[PPy]Br heated at 150 °C for 133 h. F and B denote the presence of AFI and berlinite crystalline phases, respectively.
Figure 5. XRD patterns and FESEM micrographs of (a) E-14 (z = 14), (b) E-15 (z = 22), (c) E-16 (z = 30) and (d) E-3 (z = 38) samples. The samples were prepared using the ionogel of 1.0Al: 1.8P: 0.3Si: z[PPy]Br heated at 150 °C for 133 h. F and B denote the presence of AFI and berlinite crystalline phases, respectively.
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Figure 6. TGA/DTG curves of (a) free [PPy]Br ionic salt and (b) as–synthesized SAPO-11 (E-3).
Figure 6. TGA/DTG curves of (a) free [PPy]Br ionic salt and (b) as–synthesized SAPO-11 (E-3).
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Figure 7. (a) Liquid state NMR spectrum of [PPy]Br ionic molten salt, and (b) solid state MAS NMR spectrum of as-synthesized SAPO-11 (E-3).
Figure 7. (a) Liquid state NMR spectrum of [PPy]Br ionic molten salt, and (b) solid state MAS NMR spectrum of as-synthesized SAPO-11 (E-3).
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Figure 8. Nitrogen adsorption (close symbols) and desorption (open symbols) isotherms and (inset) pore size distribution of calcined SAPO-11 (E-3) sample at −196 °C.
Figure 8. Nitrogen adsorption (close symbols) and desorption (open symbols) isotherms and (inset) pore size distribution of calcined SAPO-11 (E-3) sample at −196 °C.
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Figure 9. IR spectra of pyridine adsorption after desorption at (a) 150 °C and (b) 300 °C.
Figure 9. IR spectra of pyridine adsorption after desorption at (a) 150 °C and (b) 300 °C.
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Scheme 1. Esterification of levulinic acid and ethanol for producing ethyl levulinate.
Scheme 1. Esterification of levulinic acid and ethanol for producing ethyl levulinate.
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Figure 10. (A) Conversion of levulinic acid via esterification enhanced by SAPO-11 (E-3) catalyst at (a) 150 °C, (b) 160 °C, (c) 170 °C and (d) 180 °C under conductive instant heating using reaction conditions: mSAPO-11 = 0.100 g, levulinic acid:ethanol ratio = 1:11, and (B) effect of catalyst loading on the conversion of levulinic acid using reaction conditions: temperature = 180 °C, time = 30 min, levulinic acid:ethanol ratio = 1:11.
Figure 10. (A) Conversion of levulinic acid via esterification enhanced by SAPO-11 (E-3) catalyst at (a) 150 °C, (b) 160 °C, (c) 170 °C and (d) 180 °C under conductive instant heating using reaction conditions: mSAPO-11 = 0.100 g, levulinic acid:ethanol ratio = 1:11, and (B) effect of catalyst loading on the conversion of levulinic acid using reaction conditions: temperature = 180 °C, time = 30 min, levulinic acid:ethanol ratio = 1:11.
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Figure 11. (a) Comparison of catalytic activity between synthesized SAPO-11 and conventional homogeneous acids and (b) SAPO-11 catalyst reusability test. Reaction conditions: T = 180 °C, t = 30 min, levulinic acid:ethanol ratio = 1:11, mSAPO-11 = 0.100 g (17.98 μmol) equivalent to 0.0017 g of HCl, 0.0018 g of H2SO4, 0.0011 g of CH3COOH and 0.0035 g of p-toluenesulfonic acid (PTSA).
Figure 11. (a) Comparison of catalytic activity between synthesized SAPO-11 and conventional homogeneous acids and (b) SAPO-11 catalyst reusability test. Reaction conditions: T = 180 °C, t = 30 min, levulinic acid:ethanol ratio = 1:11, mSAPO-11 = 0.100 g (17.98 μmol) equivalent to 0.0017 g of HCl, 0.0018 g of H2SO4, 0.0011 g of CH3COOH and 0.0035 g of p-toluenesulfonic acid (PTSA).
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Scheme 2. The synthesis route of 1-propylpyridinium bromide ([PPy]Br) ionic liquid.
Scheme 2. The synthesis route of 1-propylpyridinium bromide ([PPy]Br) ionic liquid.
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Table
Table 1. Surface acidity of SAPO-11 calculated using pyridine-IR spectroscopy method. B = Brønsted acid sites and L = Lewis acid sites.
Table 1. Surface acidity of SAPO-11 calculated using pyridine-IR spectroscopy method. B = Brønsted acid sites and L = Lewis acid sites.
SampleTotal Acidity (150 °C)
(μmol g−1)		Medium-to-Strong Acidity (300 °C) (μmol g−1)
BLB+LBLB+L
SAPO-111361582946140101
Table
Table 2. The chemical composition of ionogel, crystallization time, heating temperature and phase products obtained for the synthesis parameters effect study.
Table 2. The chemical composition of ionogel, crystallization time, heating temperature and phase products obtained for the synthesis parameters effect study.
SampleVariable ParameterIonogel Molar CompositionT (°C)t (h)Products *
AlPSi[PPy]Br
E-1Crystallization
time, t		1.01.80.338150110Amorphous
E-2120Amorphous > AEL
E-3133AEL
E-4200AEL > AFI
E-5Heating temperature, T1.01.80.338130133Amorphous
E-6140Amorphous > AEL
E-3150AEL
E-7160Berlinite > AEL
E-8P1.01.20.338150133Amorphous
E-31.8AEL
E-92.1AEL > Berlinite
E-102.4AEL = Berlinite > AFI
E-11Si1.01.8038150133AEL = Berlinite
E-120.1AEL = Tridymite > AFI
E-30.2AEL > Tridymite
E-130.3AEL
E-14[PPy]Br1.01.80.314150133Tridymite >> AEL > AFI
E-1522Tridymite = AEL
E-1630AEL >> Tridymite
E-338AEL
* AEL = SAPO-11, AFI = SAPO-5.
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Mohammad, A.I.J.M.; Saminathan, V.; El-Bahy, Z.M.; Michelin, L.; Ling, T.C.; Ng, E.-P. Ionothermal Crystallization of SAPO-11 Using Novel Pyridinium Ionic Liquid and Its Catalytic Activity in Esterification of Levulinic Acid into Ethyl Levulinate. Catalysts 2023, 13, 433. https://doi.org/10.3390/catal13020433

AMA Style

Mohammad AIJM, Saminathan V, El-Bahy ZM, Michelin L, Ling TC, Ng E-P. Ionothermal Crystallization of SAPO-11 Using Novel Pyridinium Ionic Liquid and Its Catalytic Activity in Esterification of Levulinic Acid into Ethyl Levulinate. Catalysts. 2023; 13(2):433. https://doi.org/10.3390/catal13020433

Chicago/Turabian Style

Mohammad, Al Issa Jehad Moh’dFathi, Vinithaa Saminathan, Zeinhom M. El-Bahy, Laure Michelin, Tau Chuan Ling, and Eng-Poh Ng. 2023. "Ionothermal Crystallization of SAPO-11 Using Novel Pyridinium Ionic Liquid and Its Catalytic Activity in Esterification of Levulinic Acid into Ethyl Levulinate" Catalysts 13, no. 2: 433. https://doi.org/10.3390/catal13020433

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