Zeolitic Imidazolate Frameworks ZIF-8 and MAF-5 as Highly

Jun 11, 2019 - For the past 40 years, low molecular weight E-series ethers have ... of the immobilized ionic liquid (IL) 1,1,3,3-tetramethylguanidium ...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 10750−10758

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Zeolitic Imidazolate Frameworks ZIF‑8 and MAF‑5 as Highly Efficient Heterogeneous Catalysts for Synthesis of 1‑Methoxy-2-propanol from Methanol and Propylene Oxide Maria N. Timofeeva,*,†,‡ Ivan A. Lykoyanov,†,‡ Valentina N. Panchenko,†,‡ Kristina I. Shefer,† Biswa Nath Bhadra,§ and Sung Hwa Jhung*,§ †

Boreskov Institute of Catalysis SB RAS, Pr. Akad. Lavrentieva 5, 630090, Novosibirsk, Russian Federation Novosibirsk State Technical University, Pr. K. Marksa 20, 630067, Novosibirsk, Russian Federation § Department of Chemistry and Green-Nano Materials Research Center, Kyungpook National University, Sankyuck-Dong, Buk-Ku, Daegu 702-701, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: Herein, we demonstrated that zinc zeolitic imidazolate frameworks (ZIFs) based on 2-methylimidazole (ZIF-8) and 2ethylimidazole (MAF-5 or ZIF-14) linkers could be used as effective heterogeneous catalysts for catalytic synthesis of 1-methoxy-2propanol (1-MP) that is widely used in industry due to its negligible toxicity as an ecofriendly solvent and as an intermediate for the synthesis of different chemicals. Application of these materials decreased the reaction temperature to 110 °C. 1-MP was found to be the major product, with 92.1−93.8% selectivity. The activity of MAF-5 was higher than that of ZIF-8. This difference can be explained by the high basicity of MAF-5. MAF-5 and ZIF-8 materials showed good reusability for 5 cycles in catalysis, which indicates their high potential for catalytic applications. Compared with reported catalysts, the studied ZIFs, especially MAF-5, showed not only the highest conversion of PO but also the highest selectivity for 1-MP under similar reaction conditions

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INTRODUCTION The glycol ethers are a group of chemicals based on alkyl ethers of ethylene oxide (E-series ethers) or propylene oxide (P-series ethers), which are widely used as solvents and intermediates for other industrial applications. For the past 40 years, low molecular weight E-series ethers have been replaced by low molecular weight P-series ethers due to their lower toxicity. One of the P-series ethers is propylene glycol methyl ether (1-methoxy-2-propanol, 1-MP). 1-MP is widely used as an ecofriendly solvent in the manufacture of lacquers and paints and as a solvent for celluloses, acrylics, dyes, inks, and stains. It is also used in cleaning products such as glass and rug cleaners and carbon and grease removers. Other applications of 1-MP include its use as an antifreeze in industrial engines and as an intermediate for the synthesis of the pesticide (S)metolachlor and the green solvent propylene glycol monomethyl ether acetate.1,2 The reaction of propylene oxide (PO) with methanol (MeOH) in the presence of acidic or alkaline catalysts is one of the synthesis methods of 1-MP (Scheme 1).3 It is well-known that product distribution is regulated by the acid−base nature of the catalyst. In the presence of catalysts with Brønsted acid sites (BAS) (BF3, H2SO4), a primary alcohol (2-methoxy-1propanol, 2-MP) is mainly formed, whereas the presence of catalysts with Lewis acid sites (LAS) and basic sites affects the formation of the secondary alcohol (1-PM).4 Industrial production of 1-MP is based the application of homogeneous catalysts (NaOH and amines). Even though the heterogeneous © 2019 American Chemical Society

Scheme 1. Reaction between Propylene Oxide and Methanol

catalysts for the synthesis of 1-MP suggested in literature allow for simplification of the separation of catalysts and decrease amount of liquid waste, they have certain deficiencies and problems, e.g., high reaction temperature, low activity or selectivity to 1-MP, and so on. Thus, systems based on pillared clays, such as Al-, Ga-, Zr-, and Zr-,Al-pillared clays (PILCs),5,6 were demonstrated to be highly active at low temperatures of 50−70 °C. However, selectivity to 1-MP was less than 68− 72%. The high selectivity to 1-MP (90−97%) was registered in the presence of MxOy (M: Cu, Fe, Zn, Sn, and Ni),7 MgO,8,9 metal phosphate molecular sieves,10 natural layered double hydroxides (Brucite),11 and synthetic layered double hydroxides (LDHs),1,12 but this selectivity was observed at a high temperature (equal to or higher than 130−150 °C). Received: Revised: Accepted: Published: 10750

February 1, 2019 May 28, 2019 May 29, 2019 June 11, 2019 DOI: 10.1021/acs.iecr.9b00655 Ind. Eng. Chem. Res. 2019, 58, 10750−10758

Industrial & Engineering Chemistry Research

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Several recent studies have shown that the reaction temperature could be reduced by the application of nitrogen containing compounds. Liang et al.13 showed that the reaction temperature can be decreased to 110−120 °C in the presence of the immobilized ionic liquid (IL) 1,1,3,3-tetramethylguanidium lactate on bentonite and SBA-15 (IL/support). The yields of 1-MP were 95−96% and 96−97% in the presence of IL/bentonite and IL/SBA-5, respectively.13 The use of mesoporous silica MCM-41 as a support for ILs was proposed.14 Thus, application of cetyltrimethylammonium hydroxide ([CTA]OH) encapsulated in MCM-41 has led to a yield of 92% with 92−93.5% selectivity toward 1-MP at 110 °C. Recently, Tehrani et al.15 demonstrated that the metal− organic frameworks TMU-18 and TMU-19, which are formed by zinc ions, the urea-containing dicarboxylate ligand, and 4,4′bipyridine/1,2-bis(4-pyridyl)ethane as the pillar ligand can be used as catalysts for the ring-opening reaction of epoxides (styrene oxide, cyclohexene oxide, γ-phenoxypropylene oxide, and allyl(2,3-epoxypropyl)oxide) in the presence of MeOH. On that basis, it can be assumed that zeolitic imidazolate frameworks (ZIFs) formed by nitrogen-containing compounds are good candidates for the synthesis of 1-MP from PO and MeOH. ZIFs based on 2-methylimidazole (MIM) (ZIF-8, [Zn(MIM)2]) and 2-ethylimidazolate (EIM) (ZIF-14 or MAF5 (metal-azolate frameworks) as well as [Zn(EIM)2]) linkers have been already used as adsorbents because of their unique structural, textural, and physicochemical properties.16−18 As zeotype material, ZIF-8 has a sodalite (SOD) topology (Table S1).19 The framework of ZIF-8 is formed by truncated octahedral (4668) cages fused on the Zn4(MIM)4 and Zn6(MIM)6 rings, which generate a three-dimensional pore system with large cavities (11.6 Å) interconnected by narrow pore apertures (3.4 Å).19,20 MAF-5, in contrast to ZIF-8, has a zeolite ANA (Analcime) framework topology (Table S1).20 The framework of MAF-5 is composed of Zn4(EIM)4, Zn6(EIM)6, and Zn8(EIM)8 rings, which form an elongated, ellipsoidal cavity (d = 7.0 Å, l = 10.0 Å) enclosed by the D3 symmetric (6283) cage.20 In addition, these materials possess high specific surface area, high microporosity, and uniform pore distribution. Especially noticeable features of ZIF-8 and MAF-5 are high hydrophobicity and hydrothermal and chemical stability. ZIF-8 and MAF-5 is of interest for catalysis due to the availability of Zn2+ (as LAS) and imidazolate linker (as basic sites) in the structure.21−24 It is precisely through these sites that ZIF-8 was already used as a catalyst for the conversion of CO2 to cyclic carbonates,22,25 for Knoevenagel reactions,26,27 for transesterification of vegetable oil with alcohols,21 and for the esterification reaction of oleic acid with glycerol.28 All these characterizations make ZIF-8 and MAF-5 good candidates for the synthesis of 1-MP from PO and methanol. Herein, we demonstrate the catalytic properties of MAF-5 and ZIF-8 in this reaction. We can assume that differences in chemical composition and structure of the studied ZIFs will allow us to both increase and control the activity and selectivity of the reaction. We used a combination of catalytic and physicochemical methods to support such claim. Thus, the nature of the basic sites of the ZIFs was analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy using CDCl3 as a probe molecule. In addition, we tried to estimate the catalytic potential of the studied ZIFs as heterogeneous catalysts for this reaction

Article

EXPERIMENTAL SECTION

Materials. PO (99.5%), Zn(NO3)2·6H2O, MIM, and EIM were purchased from Acros Organic. Methanol (>99%) was purchased from J. T. Baker. ZIF-8 was prepared according to the method described in literature.29 MAF-5 was synthesized from EIM and Zn(NO3)2· 6H2O according to the method described in ref.16 Instrumental Measurements. X-ray diffraction (XRD) patterns were obtained with the use of a Thermo ARL X’tra device, with the CuKα-radiation (λ = 1.5418 Å), focusing geometry θ−2θ in the scanning mode within the range of angles of 2θ from 3 to 75° with the step of 0.05°. The porosity of the ZIFs was determined from the adsorption isotherm of N2 at −196 °C using a Micromeritics ASAP 2400. The specific surface area (SBET) was calculated from the adsorption data over the relative pressure range between 0.05 and 0.20. The total pore volume (VΣ) was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99. Thermogravimetric analysis (TGA) was performed on a NETZSCH STA449 C Jupiter instrument under argon flow in the range of 50−800 °C at a heating rate of 10 °C/min. DRIFT spectra were recorded using KBr pellets containing 3 wt % of sample on a FTIR-8300S spectrometer (Shimadzu) in the 300−4000 cm−1 range. The basicity of ZIFs was determined by DRIFT spectroscopy with CDCl3 as probe molecule (Supporting Information) to follow the technique that was explained in ref.30 Adsorption of MeOH and PO on the surface ZIFs was investigated by DRIFT spectroscopy (Supporting Information). Catalytic Tests. Catalytic properties of ZIFs were tested in the synthesis of 1-methoxy-2-propanol from propylene oxide (PO) and methanol at 110 or 120 °C. The reaction was performed with in an autoclave (Tynyclave steel, Buchiglasuster, inner volume of 25 cm3). The typical procedure was as follows: 7.4 mmol PO, MeOH (MeOH/PO = 8 mol/mol), and catalyst (1.85 wt %) were added into the autoclave. After reaction at 110 °C for 5 or 7.5 h under magnetic stirring, the reactor was cooled down to room temperature. The products, after separation by centrifugation of the catalysts, were analyzed with a GC (Agilent 7820) equipped with a flame ionization detector and HP-5 capillary column. As an internal standard, n-decane was used (Supporting Information). The error of measurement was less than 10%

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RESULTS AND DISCUSSION Synthesis and Characterization of ZIF-8 and MAF-5. Structural Analysis of ZIF-8 and MAF-5. ZIF-8 and MAF-5 were synthesized by the solvothermal method with MIM/Zn2+ and EIM/Zn2+ molar ratios of 8.3 and 2.0, respectively, in methanol solution. Such variants of the synthesis favored the formation of ZIF-8 and MAF-5 phases with high crystallinity, which was confirmed by physicochemical investigations. Thus, structures of ZIF-8 and MAF-5 materials were evident by powder X-ray diffraction. Diffraction peaks observed in XRD patterns for these samples are consistent with XRD patterns of ZIF-831 and MAF-520 (Figure 1). The high crystallinity of the materials is suggested by a very sharp peak below 10° in the XRD diffractograms of the samples. The ZIF-8 phase has a cubic unit cell parameter of a = 17.002(5) Å and space group of I4̅3m. The MAF-5 phase has a cubic cell parameter of a = 26.55(1) Å and space group of Ia3̅d. 10751

DOI: 10.1021/acs.iecr.9b00655 Ind. Eng. Chem. Res. 2019, 58, 10750−10758

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Industrial & Engineering Chemistry Research

In line with the different structures, these materials also showed significantly different textural properties (Table 1). Table 1. Textural Properties of ZIF-8 and MAF-5

ZIF-8 MAF-5 (ZIF-14)

crystal size (nm)

SBET (m2/g)

VΣ (cm3/g)

Vμ (cm3/g)

100−150 190−260

1439 464

0.58 0.32

0.41 0.17

ZIF-8 had a higher specific surface area (1439 m2/g) than that of MAF-5 (464 m2/g). The textural properties are in agreement with the reported values of ZIF-834 and MAF5,16,17 further supporting the successful syntheses of ZIF-8 and MAF-5. The thermal stabilities of the systems were investigated by thermogravimetric analysis. TGA thermograms for as-synthesized MAF-5 and ZIF-8 samples are shown in Figure S2. The MAF-5 sample has three weight losses in the TGA plot. It can be assumed that the weight losses at 206 and 523 °C are due to the evaporation of trapped solvent and the carbonization of unreacted EIM, respectively, and the mass loss at 669 °C indicates the collapse of the framework. A similar profile is observed in the thermogram of ZIF-8. Three weight losses are observed at 187, 256, and 608 °C (Figure S1), fully consistent with the literature.20 Analysis of Basicity of ZIF-8 and MAF-5. In this study, attention was focused on the investigation of basic properties because basic sites can favor the formation of the target product 1-MP. Analysis of basic properties of ZIF-8 and MAF5 samples was carried out by DRIFT spectroscopy using deuterated chloroform (CDCl3) as a C−H acid probe. We previously used this method to analyze the basicity of MOFs, such as MIL-100(Al), Cu(BTC), UiO-66, and NH2−UiO66.35 DRIFT spectra (with deconvolution) of CDCl3 adsorbed on ZIF-8 and MAF-5 samples are shown in Figure 3. Two

Figure 1. XRD patterns of ZIF-8 and MAF-5 samples.

The morphological features of MAF-5 and ZIF-8 samples were studied by high-resolution transmission electron microscopy (HR-TEM). According to experimental data (Figure S1), particles of ZIF-8 and MAF-5 are of a cubic shape. The particle sizes of ZIF-8 and MAF-5 are 100−150 nm and 190−260 nm, respectively. IR spectral investigations also point toward the formation of ZIF-8 and MAF-5. Thus, in the spectrum of ZIF-8 the formation of bonds between Zn2+ and the nitrogen atom of the linker are supported by the appearance of the characteristic band at 419 cm−1, which is attributed to a Zn−N stretching mode (Figure 2). In the spectrum of ZIF-8 there are is the

Figure 2. IR spectra of ZIF-8 and MAF-5 in KBr before and after recycling tests in reaction between propylene oxide and methanol.

band at 1313 cm−1 assigned to the C−H stretching bands of the methyl groups, as well as bands at 1455, 1429, and 1143 cm−1 corresponding to C−N and CN, and the band at 1582 cm−1 attributed to an axial deformation is visible in the C N.32,33 In the spectrum of MAF-5, we also can see the bands in the regions of 1350−1500 cm−1, 900−1350 cm−1, and below 800 cm−1, which are attributed to the stretching vibrations of the entire ring, the in-plane bending, and the out-of-plane bending vibrations, respectively. The Zn−N stretching mode is also observed at 418 cm−1

Figure 3. DRIFT spectra of CDCl3 adsorbed on ZIF-8 and MAF-5.

bands at 2245 and 2255 cm−1 are observed in the spectrum of ZIF-8, which can be related to the interaction of CDCl3 with basic sites. At the same time, we reveal bands at 2250, 2243, and 2235 cm−1 in the spectrum of MAF-5. At present, the available information concerning the structure of the basic sites of ZIFs is rather limited,22−24,27 making it difficult to interpret 10752

DOI: 10.1021/acs.iecr.9b00655 Ind. Eng. Chem. Res. 2019, 58, 10750−10758

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Industrial & Engineering Chemistry Research spectra. On the basis of research findings of solid-state 1H nuclear magnetic resonance (1H MAS NMR) spectroscopy, the CDCl3 molecule interacts with the six-membered ring apertures of ZIF-8.36,37 Ueda et al.36 investigated CDCl3 adsorption on ZIF-8 by theoretical and 1H MAS NM methods and suggested that CDCl3 can absorb in two sites, i.e., (a) sites located in the side of the micropore, where −CH3 groups form a bottleneck, and (b) sites located in the side where C−H moieties form the bottleneck. Both these sites are on one of the four diagonal axes of the cubic lattice of the ZIF-8 unit cell. The nature of the basic sites of ZIF-8 were also studied by temperature-programmed desorption of CO2.22−24 Two types of basic sites were found to be in ZIF-8. There were weak basic sites (desorption temperature, 50−180 °C) and strong basic sites (desorption temperature, 180−400 °C). The amount of basic sites ranged between 0.11 and 0.70 mmol/g and depended on the method of ZIF-8 synthesis. Chizallet et al.21 investigated the acid−base properties of ZIF-8 by combining IR spectroscopy using CO as probe molecule and density functional theory (DFT) for calculations of model structures. It was found that ZIF-8 has both basic sites (Zn−OH groups and N-moieties of nonbridging linkers) and acidic sites, i.e., LAS formed by Zn2+ without one or two linkers) due to the defects on the external surface of ZIF-8. These defects also favor the appearance of BAS formed by −NH groups of nonbridging linkers. The presence of acid sites was also confirmed by temperature-programmed desorption of NH3.22

strong basicity, should lead to the formation of ZIFs materials with strong basic sites. Moreover, we can suggest that this difference is related to the feature of their frameworks. The framework of ZIF-8 consists of a high order of truncated octahedral cages (Table S1). At the same time, the framework of MAF-5 is highly distorted (Table S1), which probably leads to changes in electron density in the basic sites. It is interesting to compare the basicity of ZIF-8 and MAF-5 with that of NH2−UiO-6641 and NH2−(CH2)x−SiO2 (x = 0−3) samples.42 As can be seen from Table 2, PA value decreases in the following order: NH 2−(CH 2)3 −SiO2 > NH 2−(CH 2)2 −SiO2 > MAF‐5 > NH 2−UiO‐66 > ZIF‐8 > NH 2−SiO2

The basicity of MAF-5 is higher than that of NH2−UiO-66, which can be related to the interaction strength between −NH2 groups and carboxylic groups of the linker (2-aminoterephthalic acid). At the same time, the basicity of MAF-5 is lower than that of NH2−(CH2)x−SiO2 (x = 2 and 3) and is higher than that of NH2−SiO2. The low basicity of NH2−SiO2 is a result of the strong interaction between −NH2 groups and surface Si−OH groups. This interaction reduces as the distance between −NH2 groups and the surface of surface of SiO2 increases. Synthesis of 1-MP in the Presence of ZIF-8 and MAF5. Catalytic Properties of ZIF-8 and MAF-5. The catalytic properties of ZIF-8 and MAF-5 were studied in the synthesis of 1-MP from methanol and PO at 110 °C (Scheme 1). The main results are shown in Table 3. 1-MP was found to be the major product, with 92.1−93.8% selectivity. The stability of ZIFs to Zn2+ leaching in the course of the reaction and heterogeneous character of the reaction were confirmed by special experiments. Figure 4A shows the kinetic curve of such experiment in the presence of ZIF-8. In this experiment, the ZIF-8 catalyst was filtered off when the conversion of PO was 37.5%, and then the filtrate, without the catalyst, was again stirred at 110 °C for 3 h in an autoclave. The chromatographic analysis showed that the change of PO conversion was negligible. The amount of catalyst used in the reaction system has a large influence on the reaction. As shown in Figure S3A, conversion of PO linearly rises with increasing weight of MAF5 in the mixture, up to 2.2 wt %. We can assume that this phenomenon is related to the increasing side reaction rate, i.e., polymerization of PO,43 that can block the active sites for activation of MeOH. The reaction order over MAF-5 was estimated using the Van’t Hoff differential method with W = k· Cn (Figure S3B). It was found that the reaction order over MAF-5 was 0.97 ± 0.03, with a correlation coefficient (R2) of 0.977. To determine the activation energy (Ea) of the reaction in the presence of ZIF-8 and MAF-5, experiments were conducted at 100, 110, and 120 °C. Apparent Ea calculated from the Arrhenius plots (Figure S4) were 61.6 ± 0.2 and 64.0 ± 0.1 kJ/mol for MAF-5 and ZIF-8, respectively. Kinetic curves for the conversion of PO in the presence of ZIF-8 and MAF-5 are shown in Figures 4A and S5. Both curves follow the first-order reaction rate due to the excess MeOH in the reaction (8.0 mol/mol MeOH/PO) (Figure 4B). From these data, one can see that the activity of MAF-5 is higher compared with ZIF-8. After 5 h of the reaction, PO conversions over ZIF-8 and MAF-5 were 54.4 and 90.3%, respectively. The difference in activities can be explained by a few reasons. The first reason can be related to the differences

Table 2. Spectral Characteristics (νC‑D) of OH-Groups for Different Catalytic Systems according to Adsorption of CDCl3 νC‑D (cm−1) CDCl3 MIM38 ZIF-8 MAF-5 NH2−UiO-6641 NH2−SiO2 (70 μmol/g)42 NH2−(CH2)2−SiO2 (710 μmol/g)42 NH2−(CH2)3−SiO2 (390 μmol/g)42

PA (kJ/mol)

2268 2255 2245 2250 2243 2235 2253 2245 2255 2215 2200

963.4 812 858 839 864 884 839 867 829 919 938

Table 2 shows the strength of basic sites of ZIF-8 and MAF5 calculated as the proton affinity (PA) using the following equation: log νC‐D = 0.0066PA − 4.36

According to experimental data, the strength of ZIF-8 is lower (858 kJ/mol) than that of MIM (963 kJ/mol).38 This difference can be related to the formation of a Zn−N bond via the interaction between Zn2+ and the nitrogen atom of the organic linker. A strength comparison of ZIF-8 and MAF-5 samples indicates that ZIF-8 (858 kJ/mol) possesses lower basicity than MAF-5 (884 kJ/mol), which can be explained by the difference in pKa of the conjugate acid for MIM (7.85)39,40 and EIM (8.0).39,40 It may be said that 2-alkylimidazole, with 10753

DOI: 10.1021/acs.iecr.9b00655 Ind. Eng. Chem. Res. 2019, 58, 10750−10758

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Industrial & Engineering Chemistry Research Table 3. Reaction of Propylene Oxide with Methanol in the Presence of Basic Catalytic Systems experimental conditions

selectivity (%)

catalyst

catalyst (wt %)

MeOH/PO (mol/mol)

T (°C)

time (h)

conversion of PO (%)

2-MP

1-MP

ZIF-8 ZIF-8 MAF-5 MAF-5 UiO-66 NH2−UiO-66 NH2−UiO-66/HOAca NH2−SiO254 NH(CH2)2NH2/SiO254 TAPM/SiO2b,54 TBD/SiO2b,54 Brucite11 ZnO7 MgO53 MgO−Al2O353

1.85 1.85 1.85 1.85 1.85 1.85 1.85

8 8 8 8 8 8 8 5 5 5 5 12 36 5 5

110 120 110 110 110 110 110 130 130 130 130 120 150 120 120

5 5 5 7 5 5 5 10 10 10 10 7.5 8 5 5

54.4 100 90.3 100 52.4 42.5 52.8 94.1 100 89.0 94.5 36 65.9 71.1 31.4

6.2 9.4 7.4 7.8 54.2 36.6 55.1

93.8 90.1 92.6 92.2 44.2 62.3 42.7 82.8 84.1 68.6 73.7 87.3 59.3 89.2 72.3

2.2 0.7

8.5

HOAc (0.26 μmol)was added to 50 mg of NH2−UiO-66 (HOAc/NH2-groups = 2:1 mol/mol). bTAPM, 2,4,6-triaminopyrimidine; TBD, 1,5,7triazabicyclo[4,4,0]dec-5-ene.

a

their basicities (Table 2), which is important for activation of the reactants. Reaction Mechanism in the Presence of ZIF-8 and MAF-5. It is well-known6,8,44,45 that the catalytic behavior of oxygencontaining systems is determined and influenced by the M-O pairs. The character of MeOH and PO adsorption and interaction of intermediates depend on the strength of LAS and the basic sites. We can assume that the catalytic behavior of ZIF-8 and MAF-5 is also regulated by the LAS−basic sites pairs. For verification of this hypothesis, we investigated the reaction mechanism in the presence of ZIF-8 and MAF-5 by DRIFT spectroscopy. The main results are shown in Figure 5. Adsorption of PO on ZIF-8 and MAF-5. The C−O−C group is the structural feature of propylene oxide, which can be determined by symmetric (νs(C−O−C)) and asymmetric (νas(C−O−C)) stretching vibrations. After adsorption of PO on both ZIF-8 and MAF-5 at 25 °C and further heating for 15 min at 100 °C, the band at 828 cm−1 assigned to the stretches of the epoxide ring46 is observed in the spectra (Figure 5). In the spectrum of MAF-5/PO, the band at 787 cm−1, which can be attributed to the C−H out-of-plane bending vibrations (ωC−H) in EIM,47 is also observed. We call attention to another feature of the spectra in the region of 400−440 cm−1.

Figure 4. (A) Kinetic curve of reaction between PO and MeOH in the presence of ZIF-8 and MAF-5 (experimental conditions: 7.4 mmol of MeOH, MeOH/PO 8.0 mol/mol, 1.85 wt % of catalyst, 110

( ) versus time.

°C). (B) Plot of ln

C0 Ci

in their structural and textural properties. The replacement of MIM with EIM in structure of ZIF leads to an increase in the pore aperture (Table S1) and a reduction in microporosity (Table 1). Another reason for the lower activity of ZIF-8 in comparison with MAF-5 can be related to the difference in

Figure 5. IR spectra of ZIF-8 and MAF-5 before and after PO and MeOH adsorption. 10754

DOI: 10.1021/acs.iecr.9b00655 Ind. Eng. Chem. Res. 2019, 58, 10750−10758

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Industrial & Engineering Chemistry Research

Step-by-Step Adsorption of PO and MeOH on ZIF-8 and MAF-5. Changes in the IR spectra of samples after the step-bystep addition of PO and MeOH and after heating samples at 100 °C for 15 min were used to determine the role of LAS− basic site pairs in the mechanism of the reaction. One can see from the Figure 5 that the band at 1055 cm−1 disappeared in the spectrum of ZIF-8 after such procedure. Moreover, the ratio of Zn−O/Zn−N modes was changing, i.e., the intensity of the band at 420 cm−1 (Zn−N mode) decreased. All these changes can point to the reaction between PO and MeOH, which was confirmed by the appearance of a new band at 1149 cm−1, assigned to ν(C−O−C) in the ether (1-MP).52 Today, several mechanisms are known for this reaction. 6,8,44,45 All these mechanisms are based on the contribution of the active sites (LAS−basic site). According to Zang et al.,44,45 a concerted dissociative mechanism based on the formation of propylene-like and methoxide species was demonstrated on the surface of MgO. At the same time, a stepwise dissociative mechanism based on only the dissociation of MeOH to methoxide ion and a proton was demonstrated in the presence of Al2O3, CaO, and Mg−Al oxide.3,9,50,51 Keeping data from the IR study in mind, we suggest that a stepwise dissociative mechanism takes place in the presence of ZIFs and can be represented by following steps: (1) physical adsorption of PO and dissociative adsorption of MeOH on Zn2+−N active sites (Scheme 3); (2) reaction of PO with the CH3O− ion via

Adsorption of PO leads to splitting of the band assigned to the Zn−N mode.48 The bands at 420/426 and 423/430 cm−1 are observed in the spectra of ZIF-8/PO and MAF-5/PO, respectively. This new band is probably attributed to the Zn−O mode due to the interaction between the Zn atom and oxygen atom of PO. The literature indicates that PO on the surface of Al2O3, CaO, and Mg−Al oxide forms propylene-like species, showing characteristic bands at 1644 and 3080 cm−1.44,45 It can be argued that physical adsorption of PO occurs on the surface of samples because these spectral bands are absent in the spectra of ZIF-8/PO and MAF-5/PO. Our results are in line with those of earlier studies of PO adsorption on the ZnO surface,8 where it was found that the PO epoxide ring did not open on the ZnO surface. Our results also indicate that ZIFs formed by Zn2+ have no ability to break and open the epoxide ring. Adsorption of MeOH on ZIF-8 and MAF-5. Figure 5 shows the IR spectra of MeOH adsorbed on ZIF-8 and MAF-5 at 25 °C and with heating at 50 °C for 15 min. Unfortunately, most of the bands of MeOH overlap with bands of ZIF-8 and MAF5; however, we have made slight progress due to some nonoverlapping bands. Thus, in the spectrum of ZIF-8 a weak band at 1055 cm−1 is attributed to the C−O stretching vibrations in the methoxide species formed due to the dissociation of MeOH to MeO− and H+.8 Moreover, in the spectrum of ZIF-8, a strong broad band at 3430 cm−1 is clearly visible in the region of 3000−3700 cm−1. This band can suggest the formation of a H···N bond due to the H+ adsorption on the nitrogen atom of the organic linker (2MIM).49−51 We could not interpret precisely the band at 1055 cm−1 in the MAF-5 spectrum due to its overlapping with that of MAF-5. However, MAF-5, different from ZIF-8, showed a band at 430 cm−1, assigned to the Zn−O mode, pointing toward the Zn2+−Oδ−−CH3 bond because of the dissociation of MeOH. These findings are highly consistent with the DFT calculations of the interaction of methanol with the surface functional groups of ZIF-8.21 According to these calculations, adsorption of MeOH on the surface of ZIF-8 occurs on active sites located at the external surface (or possibly in bulk defects of ZIF-8) and results in the methoxide ion and proton. The dissociative adsorption of MeOH was suggested to occur on active sites formed by the LAS−basic site pairs, where LAS are preferably formed by Zn2+ ions without three or two organic linkers (ZnIII or ZnII) and basic sites are formed by Zn−OH or the N atom of 2-MIM (Scheme 2). We also believe that these sites play a key role in the dissociative adsorption of MeOH because this is evident by the integral intensity of the band at 1090 cm−1, attributed to the C−H scissoring (in-plane bending) of the −CH3 group in the organic linker.48 This change may be due to changes in the spatial arrangement of the −CH3 group in the structure.

Scheme 3. Possible Mechanism of MeOH and PO Adsorption on ZIFs

the C(1) atom in the epoxide ring; (3) intermediate picking up the proton, leading to the formation of 1-MP. On the basis of the reaction mechanism and very close Zn−N lengths (Table S1), it is reasonable to assume that the strength of basic sites of ZIFs affects the MeOH activation and, therefore, the reaction rate. Thus, samples with high basicity will be very active in this process. This is why the conversion of PO was lower with ZIF8 as compared with MAF-5 (Table 3, Figure 4A). Efficiency and Stability of ZIF-8 and MAF-5. We also compared the efficiency of ZIF-8 and MAF-5 materials with those of Brucite and Mg,Al-LDHs1,11 metal oxides7,53 and amino-containing systems, such as amino-functionalized SiO254 and NH2-UiO-66. As can be seen in Table 3, activities of metal−organic frameworks (ZIF-8, MAF-5, and NH2−UiO66) were higher compared to those of catalysts reported in the literature. The high activity of these catalysts was observed at 120−150 °C, whereas metal−organic frameworks are active at 110 °C. The activity of metal−organic frameworks decreases in the following order:

Scheme 2. Possible Mechanism of MeOH Adsorption on ZIFs

MAF‐5 (90.3%) > ZIF‐8 (54.4%) > NH 2−UiO‐66 (42.5%)

This order is inconsistent with their basicity (Table 2). We can suggest that the low activity of NH2−UiO-66 is the result of the interaction strength between −NH2 groups and 10755

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Industrial & Engineering Chemistry Research carboxylic groups of the linker. The effect of −COOH groups is clear from the low selectivity toward 1-MP. Moreover, after poisoning of −NH2 groups with HOAc, selectivity toward 1MP decreases from 62.3 to 42.7%, whereas conversion of PO rises from 42.5 to 52.8%. An unusually high selectivity toward 1-MP in the presence of ZIF-8 and MAF-5 (Table 3) can be explained by several reasons. The high selectivity is likely related to exceptional hydrophobicity on both the internal pore and external surfaces of ZIF crystals.55,56 The adsorption capacity of ZIFs toward water is lower, whereas ZIFs readily adsorb large amounts of organic molecules, including methanol. Moreover, the high selectivity in the presence of ZIFs can account for the difference in nature of LAS−basic site pairs. We can suggest that the Zn2+ ion involves d-orbitals for the interaction with N atoms, which favors the improvement in characteristics of the Zn2+−N pair. Another important point was to investigate the reusability and recyclability of ZIF-8 and MAF-5 catalysts. After each catalytic test, ZIFs were separated from the reaction mixture via conventional filtration, washed using methanol, dried (in air), reactivated at 100 °C for 1 h, and then used in the next cycle. As shown in Figure 6, both ZIF-8 and MAF-5 can be

ZIFs was studied by DRIFT spectroscopy using CDCl3 as the probe molecule. It was shown that ZIF-8 possesses lower basicity than MAF-5 due to the difference in pKa of conjugate acids of 2-methylimidazole and 2-ethylimidazole. ZIF-8 and MAF-5 were first used as catalysts for the synthesis of propylene glycol methyl ether from MeOH and PO (MeOH/PO = 8 mol/mol) at 110 °C with 0.72−3.0 wt % of the catalyst. The major product was found to be 1-MP, with 92.1−93.8% selectivity. The reaction rate was dependent on the type of ZIFs. The activity of MAF-5 was higher than that of ZIF-8 due to the high basicity and large pore aperture of MAF5. Importantly, MAF-5 showed the highest PO conversion and the highest 1-MP selectivity compared with any reported catalysts and ZIF-8. It was shown that ZIF-8 and MAF-5 can be reused without significant loss of catalytic activity during at least five catalytic cycles.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b00655. Structure of ZIF-8 and MAF-5 (data are from the literature); HR-TEM images of ZIF-8 and MAF-5; TGA patterns of ZIF-8 and MAF-5; analysis of reaction mixture by gas chromatographic method; method for basicity measurement by DRIFT spectroscopy using CDCl 3 as probe molecule; method of reaction mechanism investigation by DRIFT spectroscopy; kinetic curves of reaction between PO and MeOH (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.N.T.). *E-mail: [email protected] (S.H.J.). ORCID

Maria N. Timofeeva: 0000-0003-2848-875X Sung Hwa Jhung: 0000-0002-6941-1583 Notes

Figure 6. Recycling test in reaction between propylene oxide and methanol in the presence of ZIF-8 and MAF-5 (experimental conditions: 7.4 mmol of MeOH, MeOH/PO 8.0 mol/mol, 1.85 wt % of catalyst, 110 °C, 5 h). The amount of the reactants was corrected based on reaction conditions.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted within the framework of the budget project AAAA-A17-117041710082-8 for Boreskov Institute of Catalysis. This work was also supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (grant number: 2017R1A2B2008774).

used again and again with no noticeable loss in catalytic activity during at least 5 catalytic cycles. After 6 cycles, ZIF-8 showed a slight decrease in PO conversion and selectivity toward 1-MP. The XRD pattern of ZIF-8 after 4 catalytic cycles points to the stability of structure. Recycled (6 times) ZIF-8 does not show an appreciable difference in the DRIFT spectrum compared with that of fresh ZIF-8; however, some new bands also appeared (Figure 2) because of reaction products or intermediates remaining in the used ZIF-8. This observation agrees with the decrease in specific surface area from 1439 m2/g (fresh ZIF-8) to 1002 m2/g (after 6 cycles).

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ABBREVIATIONS ZIF = Zeolite imidazolate frameworks MAF = Metal-azolate frameworks 1-MP, MIM = 2-Methylimidazole EIM = 2-Ethylimidazolate 2-MP = 2-Methoxy-1-propanol 1-MP = 1-Methoxy-2-propanol PO = Propylene oxide MeOH = Methanol IL = Ionic liquid LAS = Lewis acid site BAS = Brønsted acid site

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CONCLUSIONS In this work, ZIFs based on 2-methylimodazole (ZIF-8) and 2ethylimidazole (MAF-5 or ZIF-14) linkers were synthesized and characterized by XRD, N2 adsorption/desorption, and IR spectroscopy. The effect of the linker nature on the basicity of 10756

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LDHs = Layered double hydroxides PILCs = Pillared clays PA = Proton affinity DFT = Density functional theory 1 H MAS NMR = Solid-state 1H nuclear magnetic resonance DRIFT spectroscopy = Diffuse reflectance infrared Fourier transform spectroscopy XRD = X-ray diffraction TGA = Thermogravimetric analysis HR-TEM = High-resolution transmission electron microscopy Ea = Activation energy MeO− = Methoxide ion

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