Highly Efficient Catalytic Esterification in an âSO3H-Functionalized Cr

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Applied Chemistry

Highly efficient catalytic esterification in a –SO3H functionalized Cr(III)-MOF Yibo Dou, Heng Zhang, Awu Zhou, Fan Yang, Lun Shu, Yuanbin She, and Jian-Rong Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01239 • Publication Date (Web): 04 Jun 2018 Downloaded from http://pubs.acs.org on June 4, 2018

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Industrial & Engineering Chemistry Research

Highly efficient catalytic esterification in a –SO3H functionalized Cr(III)-MOF Yibo Dou†, Heng Zhang†, Awu Zhou†, Fan Yang†, Lun Shu†, Yuanbin She*,†,‡, JianRong Li*,† †

Beijing Key Laboratory for Green Catalysis and Separation and Department of

Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China ‡

College of Chemistry and Chemical Engineering, Zhejiang University of Technology,

Hangzhou, 310014, China ABSTRACT: A sulfonic acid functionalized metal-organic framework (MOF) Cr3(µ3-O)(H2O)3(NDC(SO3H5/6)2)3

(BUT-8(Cr)-SO3H,

NDC(SO3H)22–

=

4,8-

disulfonaphthalene-2,6-dicarboxylatlate) with high chemical and thermal stability was used for the catalytic esterification, showing excellent performance in various esterification reactions of monoacids, diacids, and acid anhydrides. In the structure of this MOF, uniformly distributed catalytic active sites of Brønsted acidic –SO3H with high density are decorated on its one-dimensional channels, which endows improved catalytic activity. The reaction conversion and corresponding yield of esters can achieve > 99% and > 90%, respectively. BUT-8(Cr)-SO3H also represents well sizeselectivity in the catalytic esterification owning to the steric or impeded diffusion effects of reactants in its pore structure. In addition, its robust structure also guarantees good reusability. Based on the above advantages and excellent catalytic performance, this acid groups-functionalized MOF could be a promising candidate for the sustainable chemical catalysis. 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

KEYWORDS: metal-organic frameworks (MOFs); brønsted acidic catalysis; sulfonic acid group; esterification

1. INTRODUCTION Esterification of carboxylic acids with alcohols as a kind of classical chemical reactions has been widely explored,1-5 because the products from the esterification, such as fatty esters and aromatic esters are valuable intermediates in the synthesis of plasticizers, drugs, perfumes, and pharmaceuticals etc.2,6 Owning to the rate of esterification depending on the autoprotolysis of the carboxylic acid, the acids based catalysts acting as a proton donor to the carboxylic acid are highly desirable in this reaction process. One of the main synthesis ways of eaters is that carboxylic acids react with alcohols in the presence of acid catalysts, such as sulfuric, phosphoric, and para-toluenesulfonic acids etc.7 However, these homogeneous catalysts are toxic, corrosive, and difficult to recover from the reaction medium, leading to the environmental pollution and separation problems. In comparison, heterogeneous catalysts with the advantages of improving catalytic activity, environmentally friendly nature, as well as good reusability, are more preferable than the conventional mineral acid catalysts. As a result, many endeavors have been devoted to exploring heterogeneous acid catalysts for this reaction, and among them great progress has been achieved by using porous materials such as zeolites,8-9 silica,10 resins,11 and clays,12 etc. However, these explored solid catalysts usually suffer from poor catalytic activity due to their low surface area to expose active sites and/or severe selfaggregation, resulting in sluggish kinetics of the catalytic reaction and the decrease of catalytic efficiency. Therefore, it is highly essential to search for new types of highefficient heterogeneous catalysts that can overcome above mentioned drawbacks and 2


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simultaneously achieve largely improved catalytic performance towards the esterification. As newly-developed porous materials, metal-organic frameworks (MOFs) constructed by metal-contained nodes and organic linkers through coordination bonds are attracting intense interest from not only chemistry and materials, but also industrial and chemical engineering communities.13-16 Owning to their large internal surface areas, uniform but tunable cavities, tailorable chemical and physical properties, MOFs have been widely explored for various applications in energy storage,17-19 gas adsorption,20-22 separation,23-24 and catalysis fields.25 Particularly, a lot of works have demonstrated that MOFs severing as platform for fixing functional species at their linkers or connectors endows the functionalized MOFs with available active sites for various catalytic applications.26-28 Currently, functionalized MOFs used as heterogeneous catalysts are mainly classified into three categories based on “engineering” active sites:29 1) the encapsulation of catalyst species within their inner pores, such as NENU-3a,30 Brønsted acid POM-encapsutated MIL-101;31 2) the fix of catalytic functional groups onto open metal sites, such as HPW@Cu3(BTC)2,32 H3PW12O40 supported MIL-101;33 and 3) the covalently modification of functional groups (as active sites) on organic linkers, such as MIL-101-SO3H,34-36 UiO-66SO3H,37 MIL-53-SO3H.38 In spite of many efforts, MOFs as catalysts for the given application are still limited by one or more of the following problems: lack of functional and selective active sites, high cost and complicated synthetic process for obtaining the MOFs, low density active sites, or serious steric hindrance effect between reactants and catalyst. Moreover, most of MOFs have poor thermal and chemical stability, which limits their application as catalysts for reactions under harsh

3



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environment.39 In order to solve these problems, it is highly desirable to explore stable MOFs with suitable-size pores functionalized by high-density catalytic active sites. In

our

recent

work,

O)(H2O)3(NDC(SO3H5/6)2)3

a

–SO3H

functionalized

(BUT-8(Cr)-SO3H,

Cr(III)-MOF

NDC(SO3H)22–

=

Cr3(µ34,8-

disulfonaphthalene-2,6-dicarboxylatlate) with flexible framework structure was synthesized, which exhibited quite high proton conductivity up to 1.27 × 10−1 S cm−1 at 100% RH and 80 °C because the high-density –SO3H group on its channel surfaces. In this work, we used this –SO3H functionalized Cr(III)-MOF for catalytic esterification reactions (Scheme 1), which demonstrated a high catalytic activity and well size selectivity in the esterification of monoacids, diacids, or acid anhydrides. The conversion of most substrates and corresponding yield of esters can achieve > 99% and > 90%, respectively. For the comparison, the catalytic esterification performance of several related MOFs, including its Na+ exchanged partner BUT-8(Cr)-SO3Na, MIL-101(Cr)-SO3H, UiO-66(Zr)-SO3H, and Cr(NO3)3·9H2O were also studied. It was found that BUT-8(Cr)-SO3H is better than other MOFs for the checked catalytic esterification reactions.

Scheme 1. (A) The porous structure of BUT-8(Cr)-SO3H and (B) the schematic representation of catalitic esterfication in high-density –SO3H functinalized pores of it (H atoms are ommited for clearity in the MOF structure, and corlour code: Cr, blackgreen; S, yellow; O, red; C, grey; and H, white). 4


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2. EXPERIMENT SECTION 2.1 Reagents Naphthalene-2,6-dicarboxylic acid (H2NDC), monosodium 2-sulfoterephthalic acid (H2BDC-SO3Na), Cr(NO3)3·9H2O, ZrOCl2·8H2O, N,N-dimethylformamide (DMF), hydrochloric acid (HCl, 37%), hydrofluoric acid (HF, 40%), fuming sulfuric acid, sodium chloride (NaCl), formic acid were purchased from Sigma-Aldrich and used without further purification. Phthalic anhydride (PA), hydrochloric acid, phthalic acid, benzoic acid, propionic acid, acetic acid, methanol (MeOH), alcohol, n-butyl alcohol, 2-ethyl hexanol, benzyl alcohol, cyclohexane, nitrobenzene, dimethyl phthalate, diethyl phthalate, dibutyl phthalate, dioctyl phthalate, dibenzyl phthalate, methyl benzoate, ethyl benzoate, n-butyl benzoate, methyl propionate, ethyl propionate, butyl propionate, methyl acetate, ethyl acetate, n-butyl acetate were purchased from Alfa Aesar and used without further purification. The Zeolite (Type 4A) was purchased from Tianjin Fuchen Chemicals Reagent Factory, which was firstly activated before use by calcination at 500 °C for 3 h. 2.2 Synthesis 4,8-Disulfonyl-2,6-naphthalenedicarboxylic

acid

(H2NDC(SO3H)2):

H2NDC(SO3H)2 was synthesized following our recently reported method.40 100 mL fuming sulfuric acid was added to a three-neck ﬂask containing 0.14 mol H2NDC and then the mixture was stirred at 140 oC for 24 h. After cooling to room temperature (RT), the mixture was dissolved in deionized water and the product was precipitated using concentrated HCl. Finally, the precipitate was collected via centrifugation and dried at 80 oC for 24 h.

5



Cr3(µ3-O)(H2O)3(NDC(SO3H5/6)2)3 (BUT-8(Cr)-SO3H): BUT-8(Cr)-SO3H was synthesized following our reported method that solvothermal synthesis of BUT-8(Cr) and subsequent modification of –SO3H group on its framework.40 BUT-8(Cr) was firstly synthesized: a mixture of Cr(NO3)3·9H2O (1 mmol), H2NDC(SO3H)2 (1 mmol), and hydrofluoric acid (2.6 × 10–3 mmol) were dissolved in 6 mL DMF, then transferred to a Teflon-lined stainless steel autoclave. The autoclave was heated at 190 oC for 24 h and then cooled to RT. After cooling down to RT, the obtained sample was firstly sucked out with a straw. Then the obtained product was harvested by centrifugation and washed with DMF solvent. The obtained product was further soaked in hot DMF solvent and water for 1 day alternatively. Finally, the prepared BUT-8(Cr) was washed with methanol and dried under vacuum. To obtain BUT8(Cr)-SO3H, the as-synthesized BUT-8(Cr) (100 mg) was treated in 0.5 M sulfuric acid aqueous solution (40 ml) for 24 h and then washed with water. This process was repeated for three times. Then, the mixture product was harvested by centrifugation and washed with water for three times. The obtained product was further soaked in water and methanol for three and two times alternatively. Finally, the prepared BUT8(Cr)-SO3H was washed with water and dried under vacuum. For the powder X-ray diffraction (PXRD) pattern, see Figure 1A. In comparison, the control samples MIL101(Cr)-SO3H and UiO-66(Zr)-SO3H were also prepared following a previously reported method.34,37 For the PXRD patterns, see Figure S1 and S2, respectively. BUT-8(Cr)-SO3Na: The BUT-8(Cr)-SO3H (~500 mg) was suspended in 20 mL saturated NaCl aqueous solution and then stirred at RT for 24 h. The suspension was collected through filtration and washed with water for three times. The BUT-8-SO3Na was finally harvested by centrifugation and dried at 60 oC for 24 h. For the PXRD pattern, see Figure 1A. 6


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2.3 The measurement of acidity strength for BUT-8(Cr)-SO3H The acidity strength of BUT-8(Cr)-SO3H was measured according to a previously reported method.41 In a typical procedure, BUT-8(Cr)-SO3H (500 mg) was suspended in 20 mL of saturated NaCl aqueous solution. The resulting suspension was stirred at RT for 12 h until equilibrium was achieved. The pH value of the filtrate was finally measured for determining the acidity strength. 2.4 Catalysis Esterification reactions were carried out according to a previously reported procedure.42 The optimal reaction conditions were obtained by systematically investigating the effect of catalyst quantity, molar ratios of the reactants, reaction temperature, reaction time, and solvents used (Figure S3). Typically, a mixture of PA (1 mmol), MeOH (12 mmol), BUT-8(Cr)-SO3H (15 mg) were added to 5 mL cyclohexane (dried), then transferred to a high pressure autoclave. The autoclave was heated at 170 oC for 8 h and cooled to RT. Then, the catalyst was removed from the heterogeneous mixture by centrifugation and the products were identified by GC-MS analysis (with the nitrobenzene as internal standard). In order to evaluate the reusability of the catalyst, the BUT-8(Cr)-SO3H after catalytic reactions was separated from reaction system by centrifugation and washed three times with acetone, and then soaked in acetone for 12 h, finally collected by centrifugation followed by drying in air at 60 oC. Then, the recycled BUT-8(Cr)-SO3H was re-activated and reused for a consecutive reaction with fresh phthalic anhydride and methanol. The corresponding amounts of –SO3H groups as Brønsted acid sites and metal ions as Lewis acid sites in the obtained MOFs catalysts are calculated according to the following equation:

7



m

Amount of acid =

Mw ×x m

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(1)

where m is the mass of MOF-SO3H, Mw is the molecular weight of MOF-SO3H, and x is the moles of Brønsted acid sites or Lewis acid sites in per mole of MOF-SO3H. The TOF value is calculated according to the following equations:

TOF =

nDMP n− SO H × h

(2)

3

where TOF is the turnover frequency, nDMP is the moles of DMP converted, n-SO3H is the moles of –SO3H groups, and h is the reaction time. 2.5 Instrumental details The powder X-ray diffraction (PXRD) patterns were recorded on a Bruker D8Focus Bragg-Brentano X-ray Powder Diffractometer equipped with a Cu sealed tube (λ = 1.54178 Å) at RT. The thermal gravimetric analysis (TGA) was performed on a TG/DTA6300 (SII) thermal analyzer and heated from 25 to 825 oC (5 oC min−1) under N2 atmosphere. The morphology of the sample was investigated using the scanning electron microscope (SEM, Zeiss SUPRA 55) with an accelerating voltage of 20 kV, combined with energy dispersive X-ray spectroscopy (EDX). The Fourier-transform infrared (FT-IR) spectra were recorded on an SHIMADZU IR with KBr pellets in the range 400~4000 cm−1. The conversions of the reactants and the yields of products were measured by Bruker Scion TQ GC-MS equipped with BR-5MS capillary column and Agilent 1260 HPLC equipped with Agilent TC C18 column.

3. RESULTS AND DISCUSSION 3.1 Characterization of structure and morphology 8


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The BUT-8(Cr) was synthesized by our recently reported method40, and the corresponding PXRD pattern is showed in Figure 1A. The reflection peaks were coincident well with the simulated pattern from isostructural Al-partner, BUT-8(Al). No additional reflection peaks were observed illustrating good crystallinity of the sample with high purity. After postsynthetic treatment, BUT-8(Cr) was functionalized with Brønsted acidic –SO3H groups to get BUT-8(Cr)-SO3H. Compared to the PXRD pattern of intrinsic BUT-8(Cr), no significant variation of crystallinity could be detected from the PXRD pattern of BUT-8(Cr)-SO3H sample. The FT-IR spectrum of the BUT-8(Cr)-SO3H was shown in Figure 1B, the peaks appeared at 1190 and 1240 cm−1 were attributed to the O=S=O symmetric and asymmetric stretching modes, respectively. Meanwhile, the peaks at 1068 cm−1 and 620 cm−1 were assigned to the S–O stretching vibration and the C–S stretching vibration. These FT-IR spectrum result illustrates that BUT-8(Cr)-SO3H was functionalized by –SO3H groups. In addition, the measurement of acidity strength shows that BUT-8(Cr)-SO3H in saturated NaCl aqueous solution has a low pH value of ~ 1. The morphology of BUT8(Cr)-SO3H was characterized by the SEM (Figure 1C), a urchin-like sphere consisted of plentiful nano-fibers with diameter of ~35 nm was observed. And, the C, O, S, and Cr elements measured by EDX spectrum were found uniform distribution on the urchin-like sphere (Figure S4). In comparison, the detected Na element in the BUT-8SO3Na (Figure S5) illustrates the –SO3H in BUT-8(Cr)-SO3H can be replaced by the –SO3Na by the treatment of aqueous NaCl saturated solution. The TGA result shows that BUT-8(Cr)-SO3H was of slight weight loss before 200 oC due to the removal of guest molecules in their pores (Figure 1D). Up to ~300 oC, the structure started to decompose. The chemical stability of this MOF has been checked in our recent reported work,40 which shows outstanding stability in water and acid aqueous solution. 9



In addition, the similar PXRD patterns (Figure 1A) between BUT-8(Cr)-SO3Na and BUT-8(Cr) imply their similar structures, and simultaneously demonstrate the structure stability of the former in water and acid aqueous solution. The good stability thus would guarantee the BUT-8(Cr)-SO3H being able to be used in subsequent esterification reactions.

Figure 1. (A) PXRD patterns of simulated BUT-8(Al), experimental BUT-8(Cr), experimental BUT-8(Cr)-SO3H and experimental BUT-8(Cr)-SO3Na; (B) FT-IR spectrum; (C) SEM image and (D) TGA curve of BUT-8(Cr)-SO3H, respectively. 3.2 Catalytic esterification The liquid-phase esterification of PA with MeOH was conducted firstly to evaluate the catalytic performance of BUT-8(Cr), BUT-8(Cr)-SO3H, BUT-8(Cr)SO3Na, MIL-101(Cr)-SO3H, UiO-66(Zr)-SO3H, and Cr(NO3)3·9H2O. As shown in Table 1, in the absence of catalyst (entry 1), the conversion of PA and the yield of DMP were only 65.4 and 23.2%, respectively. Meanwhile, the catalytic performance of type 4A zeolite as control sample in this esterification reaction was also checked (entry 2). Using this zeolite catalyst, it was found that the conversion and yield increased to 88.1 and 82.3%, respectively. In the contrast, the conversion and yield 10


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using BUT-8(Cr) (entry 3) was only 85.9 and 58.4%, respectively. The relative low yield and conversion for BUT-8(Cr) could be attributed to the fewer amounts of Lewis acidic CrIII centers. This can be confirmed by using the control sample of homogeneous Cr(NO3)3·9H2O (with much more open CrIII sites) as the catalyst (entry 4), which gave the high conversion and yield up to 92.5 and 80.1%, respectively. Although Cr(NO3)3·9H2O has a better conversion, this homogeneous catalyst is difficult to be recovered, probably leading to the environmental pollution. In comparison, heterogeneous catalyst BUT-8(Cr)-SO3H with the advantages of environmentally friendly nature and good reusability was confirmed to be more active, and the conversion and the yield could achieve up to 99 and 91%, respectively under the same condition (entry 6). While, BUT-8-SO3Na shows a moderate activity with the conversion of 92.5% and the yield of 59.6% (entry 5). These results illustrate that the introduction of –SO3H as catalytic sites display an important role for enhancing the catalytic performance. As comparison, the reaction over previously reported –SO3H functionalized MOF catalysts MIL-101(Cr)-SO3H, and UiO-66(Zr)-SO3H were investigated as well under the same reaction (entries 7 and 8). The yields of the esterification of PA with MeOH catalyzed by MIL-101(Cr)-SO3H and UiO-66(Zr)-SO3H were only 74.5 and 80%, respectively, obvious lower than that of BUT-8(Cr)-SO3H. The different catalytic performances of these –SO3H functionalized MOFs might be related with the density of –SO3H in their pores, as well as their overall pore structures. The calculated amounts of –SO3H groups and metal ions fixed on the pores of BUT-8(Cr)-SO3H (Cr3(µ3-O)(H2O)3(NDC(SO3H5/6)2)3)40, SO3H)2(BDC-(SO3))34,

MIL-101(Cr)-SO3H and

(Cr3(H2O)3O(BDCUiO-66(Zr)-SO3H

(Zr6O4(OH)4(HSO3BDC)1.08(BDC)4.9227 are about 3.7 and 2.2, 2.2 and 3.1, 0.6 and 3.4 11



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mmol g−1, respectively. Clearly, BUT-8(Cr)-SO3H functionalized with high density – SO3H groups endows more amounts of the active sites and facilitates the contact between acid groups and reactants, accounting for the best catalytic performance among these MOF catalysts.

Table 1. Esterification of PA with MeOH catalyzed by BUT-8(Cr)-SO3H and other MOF catalysts[a].

[a]

[b]

Catalyst

1

-

-

65.4

23.2

2

Zeolite

15

88.1

82.3

3

BUT-8

15

85.9

58.4

4

Cr(NO ) ·9H O

3.4

92.5

80.1

5

BUT-8-SO3Na

15

92.5

59.6

6

BUT-8-SO3H

15

>99

91

7

MIL-101-SO3H

15

87.4

74.5

8

UiO-66-SO3H

15

92.7

80

3 3

Mass (mg) Conversion(%)

[b]

Entry

2

Yield of B (%)

Reaction conditions: PA (1 mmol), MeOH (12 mmol), cyclohexane (5 mL), 170 oC,

8 h. [b] Determined by GC-MS analysis. Then, the hot filtration experiment was performed to verify the BUT-8(Cr)-SO3H is an efficient heterogeneous catalyst for the esterification of PA and MeOH. Figure 2A shows the yield of DMP as the function of reaction time with the BUT-8(Cr)SO3H catalyst in the whole reacting process (curve a) and without it after 3 hours (curve b). As expected, the yield of DMP increased with the extension of reaction time, and after 8 h the reaction reached equilibrium (DMP yield of 91%). In contrast, 12


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almost no variation for the yield of DMP was observed when the catalyst was removed from the mixture by filtration after 3 h of reaction (DMP yield is 40% now). It should be noted that this reaction can run without catalyst (Table 1, entry 1). The reaction reached equilibrium after 3 hours with the maximum DMP yield of 23.2% in the absence of catalyst. This is why no more DMP product was formed after 3 h of reaction in the absence of the catalyst under given reaction conditions. The similar situation had also been observed in previously reported works28,34. Above results thus illustrate that the esterification was truly catalyzed heterogeneously over the BUT8(Cr)-SO3H.

Figure 2. (A) The hot filtration test; (B) evaluation for the reusability; and (C) TOF values for the cycled esterification of PA and MeOH over BUT-8(Cr)-SO3H. Furthermore, the reusability of BUT-8(Cr)-SO3H catalyst was evaluated by five cycled esterification of PA and MeOH. The conversions of PA and the yields of DMP 13



maintained > 90% and > 80% respectively, after the consequently five runs (Figure 2B). No obvious decrease of the turnover frequency (TOF) values was observed for the BUT-8(Cr)-SO3H during the cycled esterification (Figure 2C), showing the high reusability of this MOF catalyst. Even so, a slight decreased peak intensity of PXRD patterns for recovered catalyst after fifth run was observed (Figure 3). This is probably due to the flexible structure of the BUT-8(Cr)-SO3H. In addition, the recycled process would also influence the crystallinity of the MOF to some extent. Fortunately, SEM image (Figure S6) result show that the morphology of used BUT8(Cr)-SO3H catalyst were almost identical to those of the fresh BUT-8(Cr)-SO3H. These results demonstrate the excellent stability and good reusability of BUT-8(Cr)SO3H in the esterification reaction, being promising for the practical application.

Figure 3. PXRD patterns of recovered BUT-8(Cr)-SO3H from consecutive cycled esterification reaction. In order to check the size selectivity in the catalysis of this BUT-8(Cr)-SO3H catalyst, the esterification of PA and various alcohols with different sizes, including MeOH, ethanol, n-butanol, 2-ethyl hexanol, and benzyl alcohol were carried out and the results are summarized in Table 2. It was found that with increasing the carbon 14


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chain length of alcohols, both the conversions of PA and the yields of products decreased correspondingly (Table 2, entries 1-5). As for the esterification of PA with MeOH, BUT-8(Cr)-SO3H represents high activity with the conversion and yield of 99% and 91% after 8 h, because MeOH molecule (3.8 Å) is small enough to diffuse through the inner channels (~10 Å in diameter) of BUT-8(Cr)-SO3H. In the cases of other reactants ethanol (4.7-5.1 Å), n-butyl alcohol (6.6 Å), and 2-ethyl hexanol (8.5 Å), BUT-8(Cr)-SO3H shows different conversions and yields of 95.4% and 87.4%, 89.3% and 62.5%, as well as 10.3% and 2.6%, respectively. The gradually lowered catalytic performances could be mainly attributed to the retarded diffusion of the reactants in the channels of BUT-8(Cr)-SO3H (entry 5). Especially, the conversion was only 9.5% and the yielded ester was hardly detected in the esterification of PA and benzyl alcohol, illustrating the benzyl alcohol is too large to enter into the channels of BUT-8(Cr)-SO3H and then interact with catalytic active sites. In addition, the conversion of PA and the yield of products in the esterification of 2-ethylhexanol or benzyl alcohol with alcohols were found to be higher in the presence of H2SO4 than those in the BUT-8(Cr)-SO3H (Table S1), further confirming the reactions took place inside the BUT-8(Cr)-SO3H channels.22 Thus, BUT-8(Cr)-SO3H has well size selectivity for the esterification of PA with alcohols.

15



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Table 2. Esterfications of PA with various alcohols catalyzed by BUT-8(Cr)-SO3H[a].

Entry

[a]

Alcohol

Conversion (%)

Yield of B (%)

[b]

1

>99

2

95.4

3

89.3

4

10.3

5

9.5

[b]

91 [b]

[b]

87.4 [b]

[b]

62.5 [c]

[c]

2.6

[c]

[c]

trace

Reaction conditions: A (1 mmol), alcohols (12 mmol), cyclohexane (5 mL),

catalyst (15 mg) 170 oC, 8 h. [b] Determined by GC analysis. [c] Determined by HPLC. Above results illustrate that BUT-8(Cr)-SO3H has high catalytic acticity in esterification reactions. To further vertify its validity in wide esterification reactions, we extend the application scope of the esterifications. As shown in Table 3, acetic acid, propionic acid, benzoic acid, and phthalic acid were checked to react with MeOH, ethanol, and n-butanol in the presence of BUT-8(Cr)-SO3H, respectively. It was found that except that of phthalic acid and n-BuOH with lower yield (entry 12), all of the other systems achieved high conversions of > 99% and the yields > 90% (entries 1-11). These results thus demonstrate that BUT-8(Cr)-SO3H can be widely used in various esterification reactions.

16


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Table 3. Esterifications of carboxylic acids with monohydric alcohols over BUT8(Cr)-SO3H. Alcohol

Conversion(%)[d]

Yield(%)[d]

Methanol

>99

>99

Ethanol

>99

>99

n-butyl alcohol

>99

95

Methanol

>99

97

Ethanol

>99

>99

6

n-butyl alcohol

>99

90

7

Methanol

>99

98.3

Ethanol

>99

96.5

n-butyl alcohol

>99

93.3

Methanol

>99

96

Ethanol

>99

85.3

n-butyl alcohol

93

70.5

Entry

Acid

1 2

Acetic acid[a]

3 4 5

Propionic acid

[a]

Benzoic acid[b]

8 9 10 11

[c]

Phthalic acid

12 [a]

Reaction conditions: acid (2 mmol), alcohols (6 mmol), cyclohexane (5 mL),

catalyst (15mg), 120 oC, 8 h.

[b]

Reaction conditions: acid (1 mmol), alcohols (6

mmol), cyclohexane (5 mL), catalyst (15 mg) 140 oC, 8 h.

[c]

Reaction conditions:

acid (1 mmol), alcohols (12 mmol), cyclohexane (5 mL), catalyst (15 mg), 170 oC, 8 h. [d]

The conversions of acids and the yields of corresponding esters are determined by

GC-MS analysis. 3.3 Mechanism As confirmed above that BUT-8(Cr)-SO3H can be effectively applied in the esterification reactions with good catalytic performances, we speculate the reaction process that probably follows a dual-site mechanism.4,43-45 As shown in scheme 2, in the first step, the contact and consequent adsorption between reactants and BUT8(Cr)-SO3H leads to the formation protonated acid molecules and non-protonated alcohol molecules, respectively. Then the non-protonated alcohol molecules attack the protonated carbonyl groups to give the intermediate (I). In the second step, a proton at 17



one oxygen atom of intermediate (II) is lost and gained at another to form intermediate (II), and then intermediate (II) connects with BUT-8(Cr)-SO3H via hydrogen bonding interactions to form intermediate (III). In the following step, a proton is transferred from oxygen atom to carbon atom to gain protonated ester molecules hydrogen bonded with BUT-8(Cr)-SO3H. In the final step, the broken of hydrogen bonds between ester and BUT-8(Cr)-SO3H renders the release of BUT8(Cr)-SO3H and leads to targeted esters.

Scheme 2. Proposed mechanism for the esterification of acids with alcohols catalyzed by BUT-8(Cr)-SO3H.

4. CONCLUSION In summary, a –SO3H functionalized MOF, BUT-8(Cr)-SO3H with onedimensional channels was demonstrated to be efficient heterogeneous catalyst in the esterification of monoacids, diacids, and acid anhydride. Owning to their high density of –SO3H sites in its pores, accessible channels, and good thermal and chemical stability, this MOF represents excellent catalytic performances, with the reaction conversion and corresponding yield of esters achieving > 99% and > 90%, 18


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respectively, as well as good reusability. Moreover, BUT-8(Cr)-SO3H also shows a good size selectivity in the esterification reactions between acids and various alcohols. It is also confirmed it can be used for various esterification reactions. Therefore, this –SO3H functionalized MOF as heterogeneous porous catalyst is highly efficient in the esterification reaction, which severed as a promising catalyst might be also accessible for other sustainable chemical synthesis.

ASSOCIATED CONTENT Supporting information PXRD patterns, SEM images and EDX spectrum for various catalysts; the effect of reaction temperature, reaction time, catalyst quantity, molar ratios of the raw materials, solvent, amount of solvent on the conversion and yield of esterification; table for esterification of PA with various alcohols catalyzed by H2SO4 are included. AUTHOR INFORMATION Corresponding Authors *[email protected] * [email protected]

ACKNOWLEDGMENTS We thank financial support from the Natural Science Foundation of China (21576006, 21606006, and 21771012) and the Beijing Natural Science Foundation (2174064).

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Table of Contents

Highly efficient catalytic esterification in a –SO3H functionalized Cr(III)-MOF Yibo Dou, Heng Zhang, Awu Zhou, Fan Yang, Lun Shu, Yuanbin She*, Jian-Rong Li*

A highly stable and –SO3H functionalized Cr(III)-MOF with one-dimensional open channels was demonstrated to be a highly efficient heterogeneous catalyst in the esterification of monoacids, diacids, and acid anhydride, giving the conversion and the yield of > 99% and > 90%, respectively.

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Highly Efficient Catalytic Esterification in an âSO3H-Functionalized Cr

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