Hydroxyalkylation of Phenol with Formaldehyde to Bisphenol F

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Hydroxyalkylation of Phenol with Formaldehyde to Bisphenol F Catalyzed by Keggin Phosphotungstic Acid Encapsulated in Metalorganic Frameworks MIL-100(Fe or Cr) and MIL-101(Fe or Cr) Meng Chen, Jiaqi Yan, Ying Tan, Yongfei Li, Zhimin Wu, Langsheng Pan, and Yuejin Liu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b02746 • Publication Date (Web): 05 Nov 2015 Downloaded from http://pubs.acs.org on November 10, 2015

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Industrial & Engineering Chemistry Research

Hydroxyalkylation of Phenol with Formaldehyde to Bisphenol F Catalyzed by Keggin Phosphotungstic Acid Encapsulated in Metal-organic Frameworks MIL-100(Fe or Cr) and MIL-101(Fe or Cr) Meng Chen, Jiaqi Yan, Ying Tan, Yongfei Li*, Zhimin Wu, Langsheng Pan and Yuejin Liu* (School of Chemical Engineering, Xiangtan University, Xiangtan 411105, P.R. China;)

*Corresponding authors, Yongfei Li, Tel. : +86-18207320169; E-mail address: [email protected], Yuejin Liu, Tel. :+86-13574069061; E-mail address: [email protected].

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Abstract The hydroxyalkylation of phenol with formaldehyde to bisphenol F was investigated over the metal-organic frameworks of MIL-100(Fe or Cr) and MIL-101(Fe or Cr) encapsulated with Keggin phosphotungstic acid (PTA). The PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) with different β (the molar ratio of W/M, M = Fe or Cr) were prepared and characterized by XRD, BET, FT-IR, TG, SEM, TEM and ICP. The research results showed PTA@MIL-100(Fe or Cr)(β) had higher catalytic activity than PTA@MIL-101(Fe or Cr)(β). The PTA@MIL-100(Fe)(0.4) was the most effective among the studied catalysts and had excellent reusability. A 92.12% product yield and 93.01% selectivity for bisphenol F could be obtained over the PTA@MIL-100(Fe)(0.40) catalyst. The effects of reaction time, reaction temperature, catalyst concentration and phenol/formaldehyde molar ratio on the reaction performance were also investigated. A plausible mechanism for the hydroxyalkylation of phenol with formaldehyde to bisphenol F was proposed.

Keywords: Hydroxyalkylation; bisphenol F; phosphotungstic acid; encapsulate; metal-organic frameworks

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1. Introduction Bisphenol F (BPF) has a wide range of industrial applications, such as lacquers, varnishes, liners, adhesives plastics, epoxy resins, phenolic resins and polycarbonates etc.1-3 Traditionally, the hydroxyalkylation of phenol with formaldehyde to bisphenol F could be catalyzed by some liquid protonic acids such as oxalic acid, phosphoric acid and hydrochloric acid.4-6 However, these homogeneous catalysis systems have several drawbacks such as being toxic, disposal, corrosive and difficult in separation and recovery.7-8 Due to the increasing concern for developing environment-friendly chemical processes and requirements for product quality improvement, several attempts have been reported to replace the conventional liquid protonic acid catalytic processes by solid catalysts, such as acid functionalized molecular sieves,9-10 H-beta zeolites,11 and phosphotungstic acid (PTA) immobilized on activated carbon and activated-bentonite.12-13 Among these catalysts, PTA exhibited excellent catalytic activity, however, the low surface area and high solubility of PTA in polar solvents severely restricted its application in catalysis.14-15 Garade reported highly dispersed PTA on fumed silica (SiO2) as a catalyst for hydroxyalkylation of phenol to form bisphenol F.16 The PTA on fumed silica gave a 90.1% selectivity of bisphenol F, however, the yield of bisphenol F was only 34.2%. The bulk PTA catalyst could lead to the higher yield of bisphenol F (54.6%) due to the high acidity and partial solubility of bulk PTA in the reaction medium functioned as a homogeneous catalyst,17-18 however the substantial formation of trimer (~40%) occurred so that the selectivity of bisphenol F was low. Moreover, the partial

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solubility of bulk PTA in the reaction medium could bring difficulties in the separation and recovery from products. Therefore, it is desirable to develop more suitable carriers to immobilize PTA not only on surface but also inside supports of catalysts to prevent the solubility of PTA in reaction medium as well as extend the specific surface area of PTA for the hydroxyalkylation of phenol with formaldehyde to bisphenol F. In the past years, PTA immobilized on variety of solid materials with high surface area have been reported, such as hydrous zirconia, functionalized silica, molecular sieve MCM-41 and montmorillonite.19-22 The recently developed metal-organic frameworks (MOFs) have been widely used in catalysis due to their high surface areas, tailorable pore, easy functionalization and structural versatility.23-27 Moreover, the hydrothermal stability of MOFs are specially suitable for mild reaction28 such as the hydroxyalkylation of phenol with formaldehyde to bisphenol F. MIL-ns, a subclass of MOFs, has received growing attention due to its resistance to water, common solvents and temperature (up to 593 K).29 MIL-100(Fe or Cr) and MIL-101(Fe or Cr) materials have the same zeotype cubic structure comprised of two kinds of cages and windows (Figure 1). MIL-100(Fe or Cr) materials include two types of mesoporous cages with the pore sizes of 24 Å and 27 Å (the corresponding windows sizes are 6 Å and 9 Å), and MIL-101(Fe or Cr) materials have the characteristics in terms of mesoporous cage where the pentagonal windows with a free opening of 12 Å and the hexagonal windows with a 16 Å pore sizes (29 Å and 34 Å).30-34

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Therefore, MIL-100 and MIL-101 are the ideal carriers to encapsulate Keggin PTA within the range of 9-16 Å pore sizes. In this paper, MIL-100(Fe or Cr) and MIL-101(Fe or Cr) materials were synthesized by hydrothermal method with some modifications, and well-dispersed PTA on MIL-100(Fe or Cr) and MIL-101(Fe or Cr) were obtained. The PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) showed high catalytic activity and selectivity for the hydroxyalkylation of phenol with formaldehyde to bisphenol F.

2 Experimental 2.1 Materials and instrumentation Phosphotungstic acid (PTA, 98%), hydrofluoric acid and chromium trioxide (98%) were purchased from Sinopharm Chemical Reagent Co Ltd, Shanghai, China. Trimesic acid (98%, BTC) and p-phthalic acid (98%, BDC) were obtained from Aladdin Chemicals Co Ltd, China. All other chemicals and solvents used were obtained from commercial sources and were of the highest purity available. X-ray diffraction (XRD) data were collected on a Rigaku D/Max 2550 VB+18 kW X-ray diffractometer. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet 380 spectrometer in the wavenumber range of 400-4000 cm−1. Scanning electron microscopy (SEM) was measured on a JSM-6610LV scanning electron microscope, and Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100F transmission electron microscope with an accelerating voltage of 200 kV. ICP data were recorded on an IRIS (Ш) inductively coupled plasma Emission 5



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spectrometer (Thermo Electron Corporation). Thermogravimetric analyses (TG) were performed with a Mettler-Toledo Thermoanalyzer 1600HT in the heated rate of 10 o

C/min under nitrogen atmosphere. Nitrogen adsorption in a NOVA-2200e automated

gas sorption system was used to determine the textural properties. The reaction products were analyzed using a Shimadzu QP2010 Plus GC-MS and an Agilent 1260 HPLC with 50:50 of CH3OH:H2O as eluent at wavelength of 270 nm.

2.2 Catalysts syntheses The PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) were synthesized according to the relevant literatures33,35,42-43 with some modifications. The typical synthetic procedures are as follows: the transition metal compounds of Fe(NO3)3·9H2O, FeCl3·6H2O, CrO3 and Cr(NO3)3·9H2O were dissolved in a solvent of H2O or N,N-dimethylformamide (DMF) to form a mixture, and then the organic ligand of trimesic acid (BTC) or terephthalic acid (BDC) and PTA were added into the mixture with stirring at room temperature. The mixture was held at some specified reaction temperature (110 oC, 160 oC and 220 oC, respectively) in the Teflon-lined autoclave for some time (8 h, 12 h, 20 h and 96 h, respectively), and then were cooled to room temperature with the forming of precipitates. The precipitates were filtrated and washed with deionized water, and were further refluxed with 700 mL ethanol （95% ethanol and 5% H2O）at 80 oC for 3 h, 1000 mL aqueous solution of ammonium fluoride （30 mmol/L NH4F）at 70 oC for 3 h, and 500 mL deionized water at 90 oC for 3 h, respectively. Finally, the purified precipitates were dried in a vacuum drying

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oven overnight at 160 oC, thus, the series catalysts of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) were obtained. The molar ratios of synthesized catalysts’ ingredients, the corresponding reaction temperatures and reaction times are respectively as follows (n is the pended molar of PTA): PTA@MIL-100(Fe)(β): 1.0 Fe(NO3)3·9H2O: 0.67 BTC: n PTA: 278 H2O, 160 oC and 12 h. PTA@MIL-100(Cr)(β): 1.0 CrO3: 1.24 BTC: n PTA: 310 H2O: 0.45 HF, 220 oC and 96 h. PTA@MIL-101(Fe)(β): 1.0 FeCl3·6H2O: 1.0 BDC: n PTA: 278 H2O, 110 oC and 20 h. PTA@MIL-101(Cr)(β): 1.0 Cr(NO3)3·9H2O: 1.0 BDC: n PTA: 278 H2O, 220 oC and 8 h.

2.3 Catalytic tests The hydroxyalkylation of phenol and formaldehyde to bisphenol F was carried out in a three-necked round-bottomed flask equipped with a condenser and a mercurial thermometer at the following reaction conditions: 9.4 g (0.1 mol)-37.6 g (0.4 mol) phenol and 0.82 g (0.01 mol) aqueous formaldehyde solution, 0.0055-0.0274 g·cm-3 of catalyst concentration, reaction temperature 333-373 K. Briefly, phenol and catalyst were firstly mixed with stirring at the suitable reaction temperature, and then aqueous formaldehyde solution was slowly added into them. In the period of reaction, the concentration of formed product bisphenol F was monitored by withdrawing aliquots from the reaction mixture at fixed time intervals. The sample was dissolved

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with 10 mL aqueous methanol solution (ultrapure water/methanol=1:1, volume ratio). The property of the sample was confirmed by HPLC and GC–MS. The catalyst was recovered from reaction solution by filtration, washed with aqueous ethanol solution of 95% ethanol and 5% H2O, and then was activated for 6 h at 150 oC in a vacuum drying oven for reuse.

3 Results and discussion 3.1 Catalysts characterization As shown in Figure 2, the powder XRD patterns of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) were similar to those simulated XRD patterns of MIL-100 and MIL-101 based on the crystallography information files from the Cambridge Structure Database.33-35 Both the diffraction peak positions and the diffraction intensities of the PTA encapsulated samples match with those of the simulated patterns of MIL-100 or MIL-101 materials. These results indicated that the PTA encapsulated samples did not lose the crystalline structures of MIL-100 and MIL-101 materials. It also could be seen that the XRD patterns of PTA@MIL-100 (Fe or Cr)(β=0.18 or 0.40) were very similar to those of the simulated patterns of MIL-100, which suggested the PTA was dispersed as small crystallites in MIL-100 material. As the loading of PTA increased (β﹥0.40), the crystallites of PTA could be larger in MIL-100 (Fe or Cr) materials, and the characteristic diffraction peaks of PTA appeared between 2θ = 6o and 2θ = 8o (refer to the XRD pattern of bulk PTA) in PTA@MIL-100(Fe or Cr)(0.68) and PTA@MIL-100(Fe or Cr)(1.6) in comparison to 8


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the parent materials.36 In addition, the XRD patterns shows no degradation of PTA@MIL-100 (Fe )(0.40) structure after six consecutive cycles (Figure 2c). The IR spectra of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) were shown in Figure 3. It has been reported that PTA with Keggin structures gives several characteristic IR bands at 1079 cm-1 (the stretching frequency of P-O in the central PO4 tetrahedron), 983 cm-1 (the terminal bands for W-O in the exterior WO6 octahedron), 889 cm-1 and 805 cm-1 (the bands for the W-Ob-W and W-Oc-W bridge, respectively).37-39 As shown in Figure 3a and c, compared with the IR spectra of MIL-100(Fe or Cr), the IR spectra of PTA@MIL-100(Fe or Cr)(β) presented additional bands corresponding to PTA at 1065 (1065), 963 (965), 886 (891) and 817 (809) cm-1, respectively. In addition, The vibration bands of νas(P-Oa) and νas(W=Od) were obviously shifted in comparison to PTA(1079 and 983 cm-1, respectively). This can be explained as an effect of chemical interactions between the PTA anion and the MIL-100(Fe or Cr) framework.40-41 From Figure 3b and d, it also can be found that no indication of PTA characteristic peak were obtained in PTA@MIL-101(Fe or Cr)(β), which was in agreement with the results of ICP showed in Table 1 (the loading of PTA are only 3.61 and 2.12 wt %, respectively). It is because the pentagonal window (16 Å) of MIL-101(Fe or Cr) is larger than the size of PTA (12 Å) (Figure 1).46,47 The SEM images of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) were shown in Figure 4. The dominant crystal shape of MIL-100 and MIL-101 are octahedral ranging in size between 0.5 and 1 µm. Smaller and less defined crystals with aggregates appear in Figure 4a1, a2, b1 and b2, for the PTA@MIL-100(Fe or

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Cr)(0.4) sample, while the same morphology properties of PTA@MIL-101(Fe or Cr) are shown in Figure 4c1, c2, d1 and d2. The TEM images of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) were shown in Figure 5. It could be seen that the aggregation of PTA appearing as small dark dots in Figure 5a3 and b3 would become obvious when β value was 1.6, which was in agreement with the ICP analysis showed in Table 1 (the loading of PTA were 33.59% and 34.16%, respectively). As shown in Figure 5a2 and b2 (β value was 0.4), the aggregation of PTA was not so clear, which maybe suggest the good dispersion of PTA on MIL-100(Fe or Cr) support. It could be seen from Figure 5c1-c2 and d1-d2 that almost no aggregation of PTA occurred on MIL-101(Fe or Cr), which was in agreement with the ICP analysis showed in Table 1 (the loading of PTA were only 3.61 and 2.12 wt %, respectively). It was because the pentagonal window (16 Å) of MIL-101(Fe or Cr) was larger than the size of PTA (12 Å).46,47 Thus, the loading of PTA on MIL-101(Fe or Cr) was very small. The thermal degradation of PTA@MIL-100(Fe or Cr)(0.40) and PTA@MIL-101(Fe or Cr)(0.40) were shown in Figure 6. It could be seen that the thermal stable temperatures of both PTA@MIL-100(Fe or Cr)(0.40) and PTA@MIL-101(Fe or Cr)(0.40) could reach 300 oC, and there were two stages in the typical thermograms for both PTA@MIL-100(Fe or Cr)(0.40) and PTA@MIL-101(Cr)(0.40) samples. The first weight loss up to 100 oC is attributed to the removal of water molecules, while the weight loss up to 300 oC maybe correspond to the decomposition of the organic groups in MIL-100 or MIL-101 materials together with the collapse of the

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three-dimensional frameworks.35,44-45 The pore volumes and BET surface areas of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) with the nitrogen sorption-desorption method were showed in Table 1. The BET surface area of bulk PTA was only 8.3 m2·g-1, whereas those of MIL-100(Fe or Cr) and MIL-101(Fe or Cr) were as high as 1212.64, 1620.69, 2200.88 and 2264.48 m2·g-1, respectively. However, their surface areas would decrease markedly as the loading of PTA increased. As shown in Table 1, the BET surface area of PTA@MIL-100(Fe)(β) decreased from 1212.64 to 883.12, 761.82, 734.75 and 372.77 m2·g-1,when the loading of PTA increased from 9.53 to 18.65, 25.22 and 33.59%, respectively. These results also suggested that the dispersion of PTA on MIL-100(Fe or Cr) would be destroyed gradually as the amount of loading PTA increased.

3.2 Catalytic test The hydroxyalkylation of phenol and formaldehyde to bisphenol F was catalyzed by various catalysts as shown in Scheme 1, and the results were listed in Table 1. It could be seen that both pure MIL-100(Fe or Cr) and MIL-101(Fe or Cr) showed very low catalytic activities for the synthesis of bisphenol F (less than 10% of bisphenol F yield), although they had high surface area (1212-2264 m2·g-1). The bulk PTA showed higher catalytic activity (78.11% of BPF yield) but lower selectivity because the low surface area of bulk PTA (8.3 m2·g-1) might limit the availability of acidic sites on the surface. With PTA being encapsulated into the framework of MIL-100(Fe or Cr) or

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MIL-101(Fe or Cr), the catalytic activities of both PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) increased markedly, and the former was obviously higher than the latter. However, the catalytic activities for the synthesis of bisphenol F firstly increased and then decreased with the increasing of loading PTA. As shown in Table 1, the PTA@MIL-100(Fe or Cr)(0.40) showed the highest catalytic activity when the β value is 0.40, and the catalytic activity of these catalysts decreased in such an

order:

PTA@MIL-100(Fe)(0.40)

﹥

PTA@MIL-100(Cr)(0.40)

﹥

PTA@MIL-101(Fe)(0.40)﹥PTA@MIL-101(Cr)(0.40). With the β value ranging from 0.18 to 1.60, the loading of PTA in PTA@MIL-100(Fe)(β) varied from 9.53 to 33.59 wt %, whereas the loading PTA in PTA@MIL-100(Cr)(β) changed from 7.96 to 34.16 wt %. As for the PTA@MIL-101(Fe or Cr)(0.40), however, the loading of PTA were only 3.61% and 2.12%, respectively. This result illustrated that the encapsulating of PTA with MIL-100(Fe or Cr) was more effective than that of MIL-101(Fe or Cr). It was because the pentagonal window (16 Å) of MIL-101(Fe or Cr) was larger than the size of PTA (12 Å).46,47 OH O

+

H

acid catalyst

HO

H2 C

OH

HO

OH

OH

acid catalyst

H

+

H2O

BPF

Scheme 1. Hydroxyalkylation of phenol to bisphenol F.

It should be noted that the nature of transition metal Fe or Cr in MIL-100 or MIL-101 material determined the distribution of bisphenol F isomers. As shown in Table 1, the PTA@MIL-100(Fe)(β) and PTA@MIL-101(Fe)(β) was in favor of the formation of 2,4’-isomer, while PTA@MIL-100(Cr)(β) and PTA@MIL-101(Cr)(β) was helpful to 12


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the formation of 4,4’-isomer. As for the PTA@MIL-100(Cr)(β), the selectivity of 4,4’-isomer decreased gradually from 87.57% to 57.68% and the yield of bisphenol F increased drastically from 9.89% to 47.6% when β value increased from 0 to 0.4. However, as β value further increased up to 1.60, the selectivity of 4,4’-isomer would increase to 89.52% and the yield of bisphenol F decreased to 15.62%. The similar trend could be observed for the PTA@MIL-100(Fe)(β) catalysts. During the hydroxyalkylation of phenol catalyzed by PTA@MIL-100(Fe or Cr), the selectivity of bisphenol F just changed slightly ranging from 92% to 99%. In summary, these results indicated that the most appropriate β is 0.4 in these studied catalysts. Thus, the PTA@MIL-100(Fe) (0.4) was chosen for further investigations. The effect of reaction time on the product yield and the isomer distribution of bisphenol F have been investigated, and the results were shown in Figure 7. It could be seen from Figure 7a that the product yield increased rapidly to 71.50% over PTA@MIL-100(Fe)(0.40) catalyst within the first hour, and then the increasing rate of the product yield dropped. The yield of bisphenol F was up to 92.12% after 8 h of reaction time. Meanwhile, the selectivity of bisphenol F decreased slightly from 95.75% to 93.01% when the reaction time increased from 0.5 h to 8 h, and the distribution of all the isomers also changed only a little during the whole reaction process. As for the PTA@MIL-100(Cr)(0.40) catalyst, however, the selectivity of 4,4’-isomer decreased markedly from 71.20% to 37.49% when the reaction time increased from 0.5 h to 8 h. Although PTA@MIL-100(Cr)(0.40) was in favor of the formation of 4,4’-isomer, the PTA@MIL-100(Fe)(0.40) is the most effective among the studied catalysts

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considering

both

the

yield

and

the

selectivity

for

bisphenol

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F.

So,

PTA@MIL-100(Fe)(0.40) was chosen as the catalyst in the following experiments. The influences of reaction temperature on the yield, selectivity and isomers’ distribution of bisphenol F were shown in Figure 8. When the reaction temperature ranged from 333 K to 353 K, the yield of bisphenol F increased from 42.49% to 71.50%. However, the yield of bisphenol F would decrease to 66.61% as the reaction temperature further increased to 373 K. Meanwhile, the selectivity of bisphenol F decreased slightly and the isomers’ distribution of bisphenol F varied obviously when the reaction temperature changed from 333 K to 373 K. The amount of the kinetically controlled product 4,4’-isomers10 decreased from 32.20% to 17.04%. In summary, the optimum reaction temperature was approximately 353 K. The effects of phenol/formaldehyde molar ratio (10, 15, 20, 30 and 40 mol·mol-1, respectively) on the yield, selectivity and the isomers distribution of bisphenol F over PTA@MIL-100(Fe)(0.40) have been investigated as shown in Figure 9. When the phenol/formaldehyde molar ratio increased from 10:1 to 30:1, the yield of bisphenol F increased markedly from 31.98% to 71.50% and the selectivity of bisphenol F increased from 78.34% to 95.52%. However, both the yield and the selectivity of bisphenol F hardly increased as the phenol/formaldehyde molar ratio further increased to 40:1. The results showed that the changing trend of the yield and the selectivity of bisphenol F were similar. It was because the formaldehyde molecules could be surrounded by phenol molecules with the phenol concentration increasing, which made the initially-formed bisphenol F molecules almost could not contact with the

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formaldehyde molecule to form higher homologues. When the phenol/formaldehyde molar ratio increased, the selectivity of 2,4’-isomer increased obviously from 38.16% to 47.06%, whereas the selectivity of 2,2’-and 4,4’-isomers decreased from 36.84% to 31.02% and from 25% to 21.92% respectively. The effects of concentration of PTA@MIL-100(Fe)(0.40) on the yield, selectivity and isomers distribution of bisphenol F were shown in Figure 10. The result indicated that the yield of bisphenol F would increase from 40.68% to 71.50% as the concentration of PTA@MIL-100(Fe)(0.40) increased from 0.0055 to 0.0218 g·cm-3. The selectivity of 2,4’-isomer increased obviously from 42.97% to 50.55%, whereas the selectivity of 4,4’-isomers decreased from 29.75% to 19.34%. It was maybe because an increase in catalyst amount would increase the number of active sites and thus could accelerate intermediate 2-Hydroxybenzyl alcohol converted to 2,4’- and 2,2’-isomers. However, the yield of bisphenol F almost kept unchanged at 71.50% when the concentration of PTA@MIL-100(Fe)(0.40) further increased. The selectivity of bisphenol F only decreased

slightly

from

96.89%

to

94.82%

as

the

concentration

of

PTA@MIL-100(Fe)(0.40) increased from 0.0055 to 0.0274 g·cm-3. In summary, the optimum concentration of PTA@MIL-100(Fe)(0.40) was 0.0218 g·cm-3. The reusability of PTA@MIL-100(Fe)(0.40) catalyst were investigated, and the results were shown in Figure 11. After each run, the catalyst was filtered and washed with aqueous ethanol solution of 95% ethanol and 5% H2O to remove residual reagents. The recovered catalyst was activated at 423 K for 6 h in oven and reused in next run reaction. In the first run, the yield of bisphenol F was 71.50% and the

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selectivity of bisphenol F was 95.52%. In the third run, the yield of bisphenol F was decreased slightly to 68.28%. In the fourth run, the yield was still over 64.41% (Figure 11). In the last run, however, the yield of bisphenol F decreased obviously to 52.63%, which was maybe due to the handling losses of the catalyst. The selectivity and the isomers distribution of bisphenol F just varied slightly in all the recycle experiments because the PTA@MIL-100(Fe)(0.40) catalyst had good stability in our reaction system, and the structure of PTA@MIL-100(Fe)(0.40) was not destroyed obviously after the sixth run according to the XRD patterns (Figure 2c).

3.3 Catalytic mechanism A plausible mechanistic pathway for hydroxyalkylation of phenol with formaldehyde to bisphenol F (4,4’-isomer) was presented in Scheme 2. The protonation reaction of formaldehyde with PTA catalyst occurs to form the hydroxymethyl carbocation (a) with release of a water molecule. Then, the hydroxymethyl carbocation (a) attacks the carbon atom of phenol with a larger electron cloud density to form the resonance-stabilized carbocation (b). This is followed by a proton transfer from the carbocation (b) to the catalyst and the p-hydroxy benzyl alcohol (c) is formed with regeneration of the catalyst. The hydroxymethyl group of the carbinol of the p-hydroxy benzyl alcohol (c) abstracts a proton from the catalyst to form the hydroxy benzyl carbocation (d). The final step is the hydroxy benzyl carbocation continues to attack another carbon atom of phenol with a larger electron cloud density, generating the unstable intermediate (e) which at once takes off the proton, thus the stable

16


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bisphenol F product is finally formed with regeneration of the catalyst. H HO

O

OH

H2O

H2O H

H

H

H

OH

δδ H

(a)

Catalyst： PTA OH OH HO OH H

Provide proton

(b)

（ c） OH

OH

（ d） OH

HO H2C

（ e）

BPF (4,4'-isomer)

（ f）

H

OH

Scheme 2. Proposed plausible mechanism for hydroxyalkylation of phenol and formaldehyde to BPF (based on 4,4’-isomer) The framework of PTA catalyst is ball-and-stick (red: oxygen; blue: hydrogen; black: Tungsten; purple red: phosphorus).

4 Conclusions The PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) were prepared, and were characterized by XRD, BET, FT-IR, TG, SEM, TEM and ICP. The highly dispersed PTA in MIL-100(Fe or Cr) showed excellent catalytic activity for the hydroxyalkylation of phenol and formaldehyde to bisphenol F. PTA@MIL-100(Fe or Cr)(β)

had

higher

catalytic

activity

than

PTA@MIL-101(Fe

or

Cr)(β).

PTA@MIL-100(Fe)(0.40) gave a 92.12% product yield and 93.01% selectivity for 17



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

bisphenol F, and PTA@MIL-100(Cr)(0.40) gave a 71.20% selectivity of 4,4’-isomer bisphenol F. The nature of transition metal Fe or Cr in MIL-100 or MIL-101 material determined the isomers’ distribution of bisphenol F. MIL-Fe and MIL-Cr were in favor of the formation of 2,4’-isomer and 4,4’-isomer of bisphenol F, respectively. The PTA@MIL-100(Fe)(0.40) catalyst could be recovered and reused for six times expediently.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21276217), United Fund of Natural Science Foundation of Hunan Province and Xiangtan City (No. 09JJ9008) and Innovation Platform Open Funds of Hunan Provincial Education Department (No. 15K125).

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Figure 1. Schematic structures of MIL-100(Fe or Cr) and MIL-101(Fe or Cr) A(BTC),

C(BDC)

and

B,D(M3O)—primary

building

unit;

K(PTA),

E

and

F(supertetrahedra)—secondary buildingunits; G and H—PTA@MIL-100(Fe or Cr) and PTA@MIL-101(Fe or Cr); I and J—large cages of MIL-100(Fe or Cr) and MIL-101(Fe or Cr) Figure 2. XRD patterns of PTA@MIL-100 (Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) Figure 3. IR patterns of PTA@MIL-100 (Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) Figure 4. SEM images of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) Figure 5. TEM images of PTA@MIL-100(Fe or Cr)(β) and PTA@MIL-101(Fe or Cr)(β) Figure 6. TG patterns of PTA@MIL-100(Fe or Cr)(0.40) and PTA@MIL-101(Fe or Cr)(0.40) Figure 7. Yield, selectivity and isomers’ distribution of bisphenol F with reaction time Reaction conditions: phenol/formaldehyde molar ratio, 30; catalyst concentration, 0.0218 g·cm-3; reaction

temperature,

353

K;

catalyst,

(a):PTA@MIL-100(Fe)(0.40),

(b):PTA@MIL-100(Cr)(0.40). Figure 8. Effect of reaction temperature on the yield, selectivity and isomers’ distribution of bisphenol F Reaction conditions: phenol/formaldehyde ratio, 30; catalyst, PTA@MIL-100(Fe)(0.40); reaction time, 1 h; catalyst concentration, 0.0218 g·cm-3; The inset shows the isomers distribution of bisphenol F at different temperature. Figure 9. Effect of phenol/formaldehyde molar ratio on yield, selectivity and isomers’ distribution of bisphenol F Reaction conditions: phenol/formaldehyde ratio, 10, 15, 20, 30 and 40 mol·mol-1; catalyst, PTA@MIL-100(Fe)(0.40); reaction time, 1 h; reaction temperature 353 K; catalyst concentration,

25



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Page 26 of 39

0.0218 g·cm-3; The inset shows the isomers’ distribution of bisphenol F under different phenol/formaldehyde molar ratios. Figure 10. Effect of catalyst concentration on the yield, selectivity and isomers’ distribution of bisphenol F. Reaction conditions: phenol/formaldehyde ratio, 30 mol·mol-1; catalyst, PTA@MIL-100(Fe)(0.40); catalyst concentration, 0.0274, 0.0218, 0.0164, 0.0109 and 0.0055 g·cm-3; reaction time, 1 h; reaction temperature, 353 K; The inset shows the isomers distribution of bisphenol F under different catalysts concentration. Figure 11. Reusability of catalyst. Reaction conditions: phenol/formaldehyde ratio, 30 mol.mol-1; catalyst

concentration,

0.0218

g·cm-3;

reaction

temperature,

353

K;

catalyst,

PTA@MIL-100(Fe)(0.40); reaction time, 1 h; The inset shows the yield of bisphenol F in all reaction.

26


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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


Figure 1

27



Figure 2 a)

b） Intensity (a.u.)

PTA@MIL-100(Cr)(1.60)

Intensity (a.u.)

PTA@MIL-100(Cr)(0.68)

PTA@MIL-100(Cr)(0.40)

PTA@MIL-101(Cr)(0.40)

PTA@MIL-100(Cr)(0.18)

MIL-100(Cr) MIL-101(Cr)

Bulk PTA

10

20

30

40

50

10

20

2 Theta (θ)

30

2 Τheta (θ)

40

50

d）

c) PTA@MIL-100(Fe)(1.60)

PTA@MIL-100(Fe)(0.68) PTA@MIL-100(Fe)(0.40)

Intensity (a.u.)

PTA@MIL-100(Fe)(0.40) 6th run

Intensity (a.u.)

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

Page 28 of 39

PTA@MIL-101(Fe)(0.40)

MIL-101(Fe)

PTA@MIL-100(Fe)(0.18)

MIL-100(Fe) Simulated pattern of MIL-101

Simulated pattern of MIL-100

10

20

30

40

50

10

20

30

2 Theta (θ)

2 Theta(θ)

28


40

50

Page 29 of 39

Figure 3

a）

b）

PTA

MIL-101(Fe)

886 1065 817 963

MIL-100(Fe) PTA@MIL-100(Fe)(0.18) PTA@MIL-100(Fe)(0.40) PTA@MIL-100(Fe)(0.68)

Transmittance (%)

Transmittance (%)

1706

PTA@MIL-101(Fe)(0.40)

PTA@MIL-100(Fe)(1.60)

500

1000

1500

2000 -1

2500 500

1000

c） 8098919651065

1500

2000

2500

-1

Wavenumbers (cm )

wavenumbers/cm

d)

1708

MIL-100(Cr)

PTA@MIL-100(Cr)(0.18)

PTA@MIL-100(Cr)(0.40)

PTA@MIL-100(Cr)(0.68)

Transmittance (%)

MIL-101(Cr)

Transmittance (%)

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


PTA@MIL-101(Cr)(0.40)

PTA@MIL-100(Cr)(1.60)

500

1000

1500

-1

2000

2500 500

Wavenumber (cm )

1000

1500

-1

Wavenumbers (cm )

29


2000

2500


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

Figure 4

30


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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


Figure 5

31



Figure 6 100

80

Weight (%)

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

Page 32 of 39

60 PTA@ M IL-101(Fe)(0.40) PTA@ M IL-100(Cr)(0.40) PTA@ M IL-100(Fe)(0.40)

40

PTA@ M IL-101(Cr)(0.40)

20

0

100

200

300

o

400

Temperature ( C)

32


500

600

Page 33 of 39

Figure 7 100

80

Yield 2,4'-isomer 2,2'-isomer 4,4'-isomer Selectivity

60

60

40

40

20

20

0 0

50

100

150

200

250

300

350

400

450

100

b) 80

80


60

60

40

40

20

20

0

0 500

0

50

100

150

Reaction time (min)

200

250

300

Reaction time (min)

33


350

400

450

0 500

Bisphenol F isomers distribution (%)

a) 80

Product yield and Bisphenol F selectivity (%)

100


100


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



Figure 8 100

Yield Selectivity 80

60



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

Page 34 of 39

40

20

60 50

4,4'-isomer 2,2'-isomer 2,4'-isomer

40 30 20 10 0

333

343

353

363

373

Temperature (K) 0

333

343

353

363

Temperature (K)

34


373

Page 35 of 39

Figure 9 100

Yield Selectivity

80

60 Bisphenol F isomers distribution (%)


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


40

20

0

10

15

60


50 40 30 20 10 0

20

10

25

20 30 40 15 Mole ratio of phenol to formaldehyde

30

Mole ratio of phenol to formaldehyde

35


35

40


Figure 10 100

80

Yield Selectivity

60



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

Page 36 of 39

40

20

0

0.0055

0.0109

60


50 40 30 20 10 0

0.0055

0.0164

0.0109 0.0164 0.0218 Catalyst concentration

0.0218 -3

Catalyst concentration (g⋅cm )

36


0.0274

0.0274

Page 37 of 39

Figure 11 100

Selectivity 4,4'-isomer 2,2'-isomer 2,4'-isomer

80

60 100 Yield 80

40

yield (%)

Bisphenol F selectivity and isomers distribution (%)

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


60 40

20

20 0

1

2

3

4

5

6

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Table 1 Catalytic activities of PTA@MIL-100(Fe or Cr) and PTA@MIL-101(Fe or Cr) with different loading PTA for BPF

Catalyst （ the molar ratio of W/M ）（M=Fe, Cr）

Yield of BPF （%）

BPF isomers’ distribution（%） 4,4’-Isomer

2,4’-Isomer

2,2’-Isomer

Selectivity of BPF(%)

SBET (m2.g-1)

Pore Volume (cm3.g-1)

PTA* (wt %)

MIL-100(Fe)

4.86

37.26

42.45

20.29

98.71

1212.64

0.503

0

PTA@MIL-100(Fe)(0.18)

38.23

20.63

48.68

30.69

95.89

883.12

0.309

9.53

PTA@MIL-100(Fe)(0.40)

71.50

17.75

50.91

31.34

95.52

761.82

0.244

18.65

PTA@MIL-100(Fe)(0.68)

65.24

23.53

46.88

29.59

96.71

734.75

0.206

25.22

PTA@MIL-100(Fe)(1.60)

60.70

24.76

48.82

26.42

95.97

372.77

0.117

33.59

MIL-100(Cr)

9.89

87.57

7.95

4.48

96.81

1620.69

0.997

0

PTA@MIL-100(Cr)(0.18)

21.09

77.23

12.68

10.09

96.48

1560.94

0.235

7.96

PTA@MIL-100(Cr)(0.40)

47.60

57.68

26.50

15.82

95.46

1249.42

0.185

16.47

PTA@MIL-100(Cr)(0.68)

42.37

67.30

22.95

9.75

95.34

940.47

0.103

24.26

PTA@MIL-100(Cr)(1.60)

15.62

89.52

4.52

5.96

97.21

665.30

0.063

34.16

MIL-101(Fe) PTA@MIL-101(Fe)(0.40)

2.31 13.54

31.59 28.41

27.94 39.11

40.47 32.48

95.84 92.15

2200.88 1706.34

1.139 0.802

0 3.61

MIL-101(Cr)

1.88

91.45

6.62

1.93

92.41

2264.48

1.475

0

PTA@MIL-101(Cr)(0.40) Bulk PTA

9.57 78.11

81.09 42.04

11.84 42.96

7.07 15.00

93.15 84.45

2027.50 8.3

0.980 --

2.12 100

Reaction conditions: phenol/formaldehyde molar ratio, 30; catalyst concentration, 0.0218 g.cm-3; reaction temperature, 353 K; reaction time, 1 h; * The composition of loading PTA was determined by ICP

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Hydroxyalkylation of Phenol with Formaldehyde to Bisphenol F

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