Selective Hydrogenolysis of Dibenzofuran over Highly Efficient Pt

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Selective hydrogenolysis of dibenzofuran over highly efficient Pt/MgO catalysts to o-phenylphenol Jie Zhang, Lei Wang, Chuang Li, Shaohua Jin, and Changhai Liang Org. Process Res. Dev., Just Accepted Manuscript • DOI: 10.1021/acs.oprd.7b00339 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Organic Process Research & Development

Selective hydrogenolysis of dibenzofuran over highly efficient Pt/MgO catalysts to o-phenylphenol Jie Zhang, Lei Wang, Chuang Li, Shaohua Jin, Changhai Liang*

Laboratory of Advanced Materials and Catalytic Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian 116023, China

* To whom correspondence should be addressed: Fax: + 86-411-84986353; E-mail: [email protected]; Homepage: http://amce.dlut.edu.cn

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KEYWORDS: Selective hydrogenolysis, Pt/MgO, dibenzofuran, o-phenylphenol, basicity， dehydrogenation.

ABSTRACT Direct selective hydrogenolysis of dibenzofuran (DBF) derived from coal and shale oil to value-added chemical, ortho-phenylphenol (OPP), with high selectivity (80 %) and yield (48 %) has been achieved over Pt/MgO at 400 °C and 1.0 MPa by controlling the C-O bond cleavage as well as minimizing the extent of hydrogenation of aromatic rings. Meanwhile, Pt/SiO2, Pt/Al2O3 and Pt/MgO/Al2O3 catalysts were used for the DBF hydrogenolysis and showed lower selectivity to OPP. The influence of reaction parameters has been studied to unveil the optimal reaction conditions. And the phenomenon of OPP dehydrogenation is found over various catalysts for the hydrogenation reaction of OPP. Extensive reactions and catalyst characterizations demonstrated that the OPP selectivity and the dehydrogenation of OPP to DBF follows the order of supports basicity: MgO-900 > MgO > SiO2 > MgO/Al2O3 > Al2O3, which shows that the acid-base properties of supports impact the adsorption and desorption behaviors of DBF and OPP, and result in the disparity of OPP selectivity and the dehydrogenation of OPP. Finally, a plausible reaction pathway and mechanistic understanding are proposed.

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INTRODUCTION The realization of sustainable routes to liquid fuels and chemicals from oxygen -containing liquids derived from coal and shale oil relies on the effective transformation of highly oxygenated compounds into hydrocarbon products and value-added chemicals.1-4 Additionally, the inherent disadvantage of the oxygen-containing compounds found in coal derived chemicals, such as phenols, aryl ethers and benzofurans5, is that their uses could lead to environmental violation in case of oxygen-containing waste releases to the environment. Addressing the challenge of sustainable generation of value-added chemicals by using of oxygen-containing waste6 will require highly efficient catalysts with desirable selectivity, which is also inherent problem for biomass conversion. Therefore, selective hydrogenolysis of C-O bond to value-added products are attracting increasing attention and will inevitably benefit the sustainable development of coal-derived liquids and biomass conversion. To the best of our knowledge, due to the high dissociation energy of C-O bond, e.g. dibenzofuran (DBF, as one typical oxygen-containing compound derived from coal), the C-O bond is quite difficult to be cleaved. o-Phenylphenol (OPP), as an intermediate product of DBF hydrogenolysis, is one of the most valuable and versatile chemicals which is widely used as thermal stabilizer, surfactant and biocide and is also an excellent starting precursor for fuel conversion. The demand of it could reach 60000 tons per year in the world and will increase with time going. Grubbs demonstrated that the use of the combination of triethylsilane with organic base can efficiently obtain OPP with 95 % selectivity in homogeneous catalysis7, nevertheless the catalyst tends to be difficultly separated, expensive and thus impractical in industry. It is thus urgent to develop suitable catalysts and optimize 4


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reaction conditions aiming at selective hydrogenolysis of DBF to OPP in heterogeneous catalysis via cleaving just one C-O bond with preservation of the aromatic rings. The reaction pathway of catalytic DBF hydrogenation is presented in Scheme 1.5,8-10 Previous research had been established that all the phenolic intermediates are more reactive5, 6, 11

than dibenzofuran that it is very difficult to obtain OPP with the selective cleavage of

only one aromatic C-O bond. In our previous work,9 the hydrodeoxygenation products of DBF were obtained and hydrodeoxygenation pathway (HYD) was discussed in detail. Considerable research has been focused on the hydrodeoxygenation of DBF to fuels via HYD pathway over acid catalysts.12-18 Only a few studies of the DBF hydrogenolysis to OPP have been reported in the literature,19 in which the selectivity to OPP reached 73 % and the yield was just 16.8 % over carbon supported Pd catalyst. As a consequence, C-O bond hydrogenolysis has been extensively studied over the past decade.20 Various metals have been used as the active components, such as Ru,21 Pt,15-17, 22 Pd,23 Rh,24 Ni,13, 25-28 Cu,29, 30 Fe, Au,31 W and Co,20 and important progresses have been made toward the cleavage of C-O bond. In particular, the noble metals, Pt, Pd, Ru and Rh, have attracted considerable attention because of their high hydrogenolysis activity. Accordingly, a rationally designed catalyst should easily activate molecular hydrogen and the C-O bond without hydrogenation for the purpose of acting selectively towards the hydrogenolysis of the oxygen-aromatic bonds. Indeed, there is also a need to optimize the suitable temperature and pressure to transform dibenzofuran into high-value o-phenylphenol over a highly efficient catalyst. The acidic and basic properties of supports have important impacts on the catalytic performances of supported metals because they could affect the adsorption and desorption

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behaviors of reactants and products. It is reported that the aromatic rings in which π electrons act as a Lewis base due to the enriched electron densities favor the adsorption and activation on acidic support than basic support, even for acidic phenol. Therefore, acidic supports could easily achieve the hydrogenation of the aromatic rings.32,

33

The basic

supports could weaken the hydrogenation of aromatic rings through hydrogenation route (HYD) due to decreased interaction of aromatic rings and support. Tanabe et al.34 studied the adsorption of phenol on acidic SiO2-Al2O3 and basic MgO, in which the aromatic ring adsorbed parallelly and perpendicularly on the catalyst surface, respectively, due to basic π-electrons. The vertical adsorption favored the direct deoxygenation route (DDO) for hydrogenolysis and the coplanar adsorption promoted the HYD route for hydrogenation. Moreover, Shin et al.35 reported that the nonplanar adsorption could promote hydrogenolysis of phenol due to the difficulty of interaction between the aromatic ring and active surface. Additionally, the basic supports have been widely used in catalysis for hydrogenation reaction and biomass pyrolysis,36-39 which has unexpected high selectivity and anti-coking in many reactions.40-43 Hence, it could be hypothesized that the basic MgO could be a desired support for hydrogenolysis of DBF. In an attempt to develop an environmentally benign and economically feasible route of dibenzofuran hydrogenolysis, a combination catalyst system composed of various supports (Al2O3, SiO2, MgO/Al2O3 and MgO) loaded with Pt was explored for their catalytic activity and selectivity in DBF hydrogenolysis. Interestingly, the Pt/MgO catalyst exhibited a great catalytic activity and selectivity to OPP. The different catalytic behaviors observed over various supports were interpreted in terms of acid-base properties. The hydrogenolysis

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ability and high OPP selectivity of Pt/MgO catalyst were attributed to the high activity of Pt and the basic character of the MgO support.

Scheme 1 Reaction pathways of hydrodeoxygenation of DBF

EXPERIMENTAL SECTION Materials Commercial available Al2O3, SiO2, Mg(CH3COO)2, Mg(OH)2 and H2PtCl6·6H2O were purchased from Sinopharm. Dibenzofuran and o-phenylphenol were obtained from Aladdin with 98.0 % purity.

Preparation of supports and catalysts MgO was obtained by calcining Mg(OH)2 at 500 °C or 900 °C for 4h, and the obtained supports were denoted as MgO and MgO-900. Other metal oxides Al2O3 and SiO2 were calcined in O2/Ar (20 mL/40 mL) at 400 °C for 2h before use.9 Pt/MgO, Pt/Al2O3 and Pt/SiO2 were prepared by impregnation method using H2PtCl6·6H2O as precursor and methanol as the solvent. Additionally, Pt/MgO/Al2O3 was prepared by soaking Pt/Al2O3 into methanol solution of Mg (CH3COO)2, followed by calcination in O2/Ar (20 mL/40 mL) at 400 °C for 4 h. The weight percent of Mg in Pt/MgO/Al2O3 is 3.5 wt. %. All of the above catalysts were heated from room temperature to

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400 °C at a rate of 5 °C min-1 and reduced for 2 h under 40 mL min-1 H2 before further catalytic test. The actual Pt contents in as-prepared Pt catalysts are close to nominal 0.50 wt. % Pt loading.

Characterization of supports and catalysts X-ray diffraction (XRD) analysis of the supports was performed on a Rigaku D/MAX-RB instrument using a Cu Kα monochromatized radiation source in the 2θ range of 5o-90o with a scan speed of 10o min-1, operated at 40 kV and 100 mA. The specific surface area of supports was calculated by the Brunauer-Emmett-Teller (BET) method and pore volume was calculated by the volume of liquid nitrogen at p/p0=0.99 based on nitrogen gas physisorption measurements over Quantachrome Autosorb IQ. The acidity and basicity features of the supports were evaluated by temperature-programmed desorption (TPD) using NH3 and CO2 as the probe molecule, respectively. TPD was conducted on CHEMBET-3000 and OBP-1 chemisorption instrument with a thermal conductivity detector (TCD), respectively. A 0.10 g support was loaded into the reactor and was performed with a heating ramp rate of 10 min-1 in a stream of Ar from room temperature to 500 °C holding 1 h with a total flow rate of 40 mL min-1, then saturated at 30 °C in a 10 % NH3/He or CO2 stream for 1 h to ensure the maximum adsorption. The data were recorded in He from room temperature to 500 °C. Elemental analysis for Pt was recorded using inductively coupled plasma atomic emission spectroscopy (ICP-AES) over (Perkin-Elmer Optima 2000 DV). Pt dispersion was determined according to CO chemisorption measurements using CO pulse adsorption on CHEMBET-3000. CO adsorption was considered to be completed after three successive peaks showed the same peak areas. A stoichiometry of CO/metal =1 was taken to calculate

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the number of metal active sites. The particle size of catalysts was analysed using a FEI Tecnai G2 F30 TEM.

Catalytic tests Typically, hydrogenolysis of DBF or OPP was performed at 400 °C and 1.0 MPa total pressure in a continuous-flow fixed-bed reactor (considered as plug flow reactor) over 80 mg catalyst diluted with 5.0 mL 60-80 mesh quartz sands. Prior to catalytic tests, the as-prepared catalysts were pretreated in-situ under 40 mL min-1 H2 at 1.0 MPa and 400 °C for 1 h. Then, the temperature and the pressure were adjusted to reaction conditions. Next, dibenzofuran was delivered into the reactor and mixed with H2 gas at the inlet of the reactor. And the mixture of reactant and H2 gas moves downward and reacts in the middle of the reactor. The liquid reactants were consisted of 2.0 wt. % DBF or OPP, 1.0 wt. % n-dodecane (as internal standard) and 97.0 wt. % n-decane (as solvent). The reaction products being condensed in a cold trap were collected and analyzed off-line by an Agilent gas chromatograph 7890A equipped with flame ionization detector and a 0.5 µm×0.32 mm×30 m FFAP capillary column. Product identifications were conducted on an Agilent 7890B with 5977A MSD and a 0.25 µm×0.25 mm×30 m HP-5 capillary column. Conversion (X) and selectivity (S) are defined as follows:

= ( − )/ × 100%

(1)

= /∑ × 100%

(2)

Where n0 and nDBF are the moles of DBF in the feed and product, respectively, ni is the mole of i product molecule and ∑ni are the total moles of products.The carbon balance of products is 100 % ± 5 %. Turnover frequency values (TOF) are calculated from the formula:

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= (F/M′ ) × X/(M × W)(F/M′ )

(3)

τ = W/F

(4)

Where M and M' are the moles of active sites and molecular weight of reactant, respectively, τ represents the contact time, W denotes the catalyst weight and F denotes the total weight flow rate of the reactant.

RESULTS AND DISCUSSION Characterization of supports and catalysts The obtained Pt content, BET specific surface area, pore volume and pore diameter of the samples are summarized in Table S1. The actual Pt contents of as-prepared Pt catalysts are close to nominal 0.50 wt. % Pt loading. MgO has a specific surface area of 81 m2 g-1 and a pore volume of 0.44 cm3/g. After calcination at 900 °C, the obtained MgO sample shows some reduction in specific surface area and pore volume, which is resulted from the collapse of the partial framework. SiO2, MgO/Al2O3, and Al2O3 have higher specific surface areas of 285, 235, and 308 m2 g-1, respectively. It is clearly that the surface area and pore volume of Al2O3 decrease sharply after the deposition of MgO. The power XRD patterns of supports are shown in Figure S1a. As can be seen, calcination of Mg(OH)2 at 500 °C and 900 °C produce the different intensity of peaks (2θ = 43, 62, 78, 37, 75o).44 It is probably due to the fact that calcining Mg(OH)2 at higher temperature should give the periclase magnesium oxide with enhanced crystallinity. No obvious diffraction peaks were observed clearly for the patterns of SiO2 and Al2O3 supports, confirming that they have amorphous structures. Same situation occurred in MgO/Al2O3,

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which may be resulted from either low loading or high dispersion of MgO. When adding Pt to the supports, no diffraction peaks of Pt were discernable after reduction as shown in Fig. S1b, indicating that the reduced Pt metal was highly dispersed and the particle size was too small to detect by XRD.

Figure 1. TPD profiles of (a) CO2 and (b) NH3 adsorbed on supports The different product selectivity may be related to the nature of the support. Therefore, the acidic and basic properties of the supports were measured by temperature-programmed desorption of ammonia (NH3-TPD) and carbon dioxide (CO2-TPD) as shown in Figure 1 and the amount of acid and base were shown in Figure S1. Regarding to basic properties of supports, as shown in Figure 1a, one broad peak is displayed at 280 °C over MgO, which can be attributed to moderately basic sites. At 120 °C, the small peak suggests weakly basic sites. Between them, the weak peak is covered up at 230 °C. These results are in close agreement with previously reported literature value.41 There is slightly higher basic strength for MgO with calcined temperature increasing to 900 °C.44 Additionally, no peak was observed for Al2O3 and SiO2 supports. However, Al2O3 support, modified by a small amount of MgO, exhibited strong peak at 120 °C, which can be ascribed to weakly basic sites. Apart from the

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basicity of the support, the acidity was determined by NH3-TPD. As shown in Figure 1b, there is a broad desorption peak detected for the Al2O3 support. Apparently, the desorption peaks over MgO/Al2O3 support decreased, which is due to the fact that acidic sites was covered with MgO. Generally, as for the MgO and SiO2 supports, there are hardly any acidic sites. When adding Pt to the support, the acid-base properties of catalysts have little change compared with the pure supports. The detailed results were shown in Figure S2. With regard to the TPD analysis, the combination of the acidity and basicity strength showed that basicity of the supports decreases in the the following order: MgO > SiO2 > MgO/Al2O3 > Al2O3. Figure S3 shows the TEM images together with particle size distributions of Pt catalysts of three representative acid-base supports. As displayed from the TEM images, Pt nanoparticles are well dispersed with no apparent aggregation. The mean diameters of the Pt nanoparticles in the Pt/SiO2, Pt/MgO and Pt/Al2O3 catalysts are 3.3, 1.6 and 1.5 nm, respectively. Pt/Al2O3 having smallest particle size suggests a high metal dispersion, which results in its better activity. Correspondingly, the Pt dispersions in the Pt/SiO2, Pt/MgO and Pt/Al2O3 catalysts are 34 %, 70 % and 75 %, respectively. Considering that all the catalysts were prepared by the same method, the variation in Pt particle size could be ascribed to the varied degree of metal-support interactions.45 The nature of the surface hydroxyl groups of most oxides affect the adsorption of anion during synthesis.46

Hydrogenolysis of DBF over Pt/MgO catalyst The hydrogenolysis products, such as OPP, biphenyl (BP), cyclohexylbenzene (CHB), cyclopentylmethylbenzene (CPMB), cyclohexane (CH) and benzene (B) were determined by the GC-MS, as shown in Figure 2. According to the results，the conversion of DBF follows

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the direct deoxygenation route (DDO) under the current conditions, with no product from the hydrogenation route (HYD), as depicted in the previous proposed reaction network.9 Furimsky et al.10 suggested that the hydrogenation of DBF could follow several pathways depending on the reaction temperature. Among them, at high temperatures the reaction is more inclined to follow the direct deoxygenation route under the same pressure condition. Figure 2a presents the relative concentration versus the contact time. With contact time increasing, the relative concentration of OPP increased followed by the decrease at contact time of 0.84 min. However, the relative concentration of BP steadily increases. It can be inferred that BP is the product of intermediate species OPP by the cleavage of C-O bond. Figure 2b shows a plot of products selectivity versus DBF conversion. Surprisingly, OPP is the predominant product with a selectivity of more than 80 % at low conversions (< 12 %) over Pt/MgO. The decrease in OPP selectivity, accompanied by a simultaneous increase in BP selectivity with increasing conversion, seems that BP is formed at the expense of OPP decreases. Apparently, OPP, which is known to be an intermediate product of hydrogenolysis of DBF, was quite easy to be converted to BP through hydrogenolysis by the cleavage of C-O bond. On the other hand, the hydrogenation products, such as CHB and CPMB, gradually increased with increasing DBF conversion. Traces of CH were detected at high conversions, which is probably due to the fact that hydrogenation occurred at higher conversions. By adjusting the Pt loadings, good results were obtained with 48 % OPP yield over 1.0 wt. % Pt/MgO (Figure S4).

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Figure 2. Relative concentration (a) and selectivities (b) of the hydrogenolysis of DBF over 0.5 wt. % Pt/MgO as a function of contact time and conversion, respectively. Reaction condition: 400 oC, 1.0 MPa, H2/oil = 400. Effect of reaction temperature To evaluate the influence of the temperature on hydrogenolysis of DBF, the model reaction was carried out at different reaction temperatures over Pt/MgO, Pt/SiO2 and Pt/Al2O3 at 1.0 MPa. Variations in reaction temperature had a considerable effect on selectivity and conversion, as illustrated in Figure 3a and Figure S5. Over Pt/MgO, with temperature increasing, the selectivity to OPP gradually increases and becomes stabilized at 400℃. The conversion of DBF gradually decreases with increasing temperature. The data obtained is in conflict with the results obtained over Pt/Al2O3 (Figure S5b) and previous report9 wherein the conversion and temperature increase at the same time. It may be attributed to a temperature-induced decrease of the fraction of the catalyst surface which was

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covered by reactants at high temperature35, 45 and the dehydrogenation of formed OPP back to the reactant DBF (discussed in below). And the temperature induced acceleration of catalyst coking is unlikely for MgO catalysts due to its anti-coking nature, which is evidenced from the TEM characterization (Figure S8).

Figure 3. Effect of the reaction temperature at 1.0 MPa (a) and the reaction pressure at 400 o C (b) over 0.5 wt. % Pt/MgO. Reaction condition: τ= 0.55 min, H2/oil = 400. The

hydrogenation

product,

tetrahydrodibenzofuran

(THDBF),

dodecahydrodibenzofuran (PHDBF) and bicyclohexane (BCH), were observed (Scheme 1) and the amount gradually increased over both catalysts when the temperature was decreased to 380 °C. The major hydrogenation reaction (HYD) occurred at a lower temperature in our previous work.9 The observation is consistent with the previous result

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10

that the


hydrogenation or hydrogenolysis reaction of DBF depends on the reaction temperature, in which the hydrogenolysis of C-O bonds occurs at the high temperature and low temperature is more inclined to hydrogenation of aromatic rings. The hydrogenolysis of DBF need higher temperatures than hydrogenation because the cleavage of oxygen-sp2 carbon bonds is more difficult than oxygen-sp3 carbon bonds.8 Hence, catalytic hydrogenolysis of DBF requires high temperature due to the strong C-O bond.10, 35, 45 Moreover, the catalytic performance exhibited markedly higher selectivities to hydrogenation of aromatic ring over Pt/Al2O3 than Pt/MgO (Figure 3a, Figure S5b and Table 1), which is well agreed with the results we mentioned in the introduction. At the same temperature of 380 °C, the selectivities to hydrogenation of aromatic ring decreased in the order: Pt/Al2O3 > Pt/SiO2 > Pt/MgO. Without dehydrogenation, the conversion increases with elevated temperature over Pt/Al2O3, which is maybe that the relationship between reaction conversion and temperature is closely related to the dehydrogenation of OPP over Pt/MgO.

Effect of reaction pressure Except for the reaction temperature, the pressure is another key parameter for the hydrogenolysis of DBF. Figure 3b shows the conversion and selectivity of the hydrogenolysis reaction under different pressures at 400 °C. At the lower pressure (0.5 MPa), the conversion of DBF was just 12 %, but all the products were from hydrogenolysis but not from the hydrogenation of aromatic ring and the OPP achieved the highest selectivity of 74 %. Further raising the pressures to 2.0 MPa resulted in an increase in reaction conversion while the hydrogenation of aromatic ring starts to occur, demonstrating that hydrogenation competed with hydrogenolysis in designated condition. However, the selectivity to OPP

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gradually decreased, accompanied by the increased selectivity to BP. A higher pressure promotes product removal by hydrogen addition and hydrogen-aided desorption.47 With the pressure going up, the increased surface hydrogen coverage leads to the formation of hydrogenation products and an increased C-O bond hydrogenolysis rate.47 Hence, the generation of BP was much easier by hydrogenolysis or dehydration of OPP at higher pressure. It is confirmed that decreasing pressure favored the hydrogenolysis of DBF, and suppressed or avoided excessive hydrogenation of aromatic ring.48

Effect of supports The selectivities of the different catalysts for the hydrogenolysis of DBF are shown in Table 1. Because there were essential variations in activity from one catalyst to another, the catalyst tests were undergone to allow comparisons of selectivities at fixed conversions of about 30 % of DBF. The selectivity to OPP varied considerably with different catalysts. The presence of MgO and SiO2 supported Pt, which is known to have less prominent acidic character than Al2O3, helps on the generation of OPP at higher selectivity. The increase in the selectivity to OPP can be due to the fact that the dehydration reaction of OPP is more difficult in basic conditions. While for the acidic Al2O3, OPP was not observed from the reaction products. The reaction for DBF conversion is rather dominated by the excessive hydrogenolysis of DBF to BP and B, which is followed by a small extent of partial hydrogenation of BP to CHB and CH (Table 1). Apparently, the intermediate OPP was rapidly hydrogenolysed to BP. It is suggested that Pt/Al2O3 catalyst favored the formation of the deoxygenation products (BP and B). As we all known that the acidic support is function for hydrogenolysis and dehydration,8 and is more favored to deoxygenation.49 The basic

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MgO, which had the highest OPP selectivity in all using catalysts, significantly enhanced the formation of OPP. Table 1. TOF and selectivity for DBF hydrogenolysis for various catalysts with at 30 % conversion (All reaction were run at 400 °C and 1 MPa) Catalysts

TOF (h-1)

Pt/MgO Pt/SiO2 Pt/Al2O3 Pt/MgO/Al2O3 Pt/MgO-900

1363 722 1022 699 1123

Selectivity (%) OPP

BP

B

CHB

CPMB

CH

67 56 0 17 73

17 31 57 37 16

12 5 35 25 11

2 6 6 8 0

2 2 1 3 0

0 0 1 10 0

By comparison, the TOF analysis of catalysts indicated that the activity over Pt/MgO was higher than that over Pt/SiO2 due to the high Pt dispersion based on TEM results. Moreover, Pt/Al2O3 conversion was efficient, indicating a high reaction activity via hydrogenolysis of C-O bonds, which can be possibly explained by the strong acid that accelerates the C-O bonds cleavage and high Pt dispersion. Table 2. TOF and selectivity for OPP hydrogenolysis over various catalysts at 30 % conversion. (All reaction were run at 400 °C and 1.0 MPa) Catalysts

Selectivity (%)

TOF (h-1) DBF

BP

B

CHB

CPMB

CHEB

CH

Pt/MgO

2406

18

47

18

6

4

6

1

Pt/MgO-900

1951

20

49

7

10

5

8

1

Pt/SiO2

1123

11

49

17

11

5

6

1

Pt/Al2O3

2117

0

55

27

16

1

0

1

Pt/MgO/Al2O3

1747

3

59

11

17

4

4

2

In order to clearly analyze the respective hydrogenolysis and hydrogenation of DBF, the selectivities to C-O bond hydrogenolysis products (the total of OPP, BP and B) and phenyl

18


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ring hydrogenation products (the total of CHB, CPMB and CH) were as depicted in Table 1. There was no hydrogenation of aromatic rings over Pt/MgO-900. However, other catalysts had approximately 4 ~ 21 % of the hydrogenation of aromatic rings. From the perspective of hydrogen consumption, Pt/MgO catalysts also were superior to other catalysts. To gain preliminary insight into the effective basic sites for the production of OPP, the hydrogenolysis of DBF was carried out using Pt/MgO/Al2O3 and Pt/MgO-900. The Pt/Al2O3 was modified by MgO to generate the basic sites, as can be observed in Figure 1a. As was expected, the selectivity to OPP was raised to 17 % (Table 1). Apparently, the production of OPP can be explained by the presence of basic sites. The basic strength of MgO was changed to verify the effect of basic sites on the production of OPP by changing the calcination temperature of MgO. The higher calcination temperature would generate further stronger basic sites (Figure 1a), due to exposure of the more surface defects.50 Similarly, the selectivity to OPP on Pt/MgO-900 catalyst was 6 % higher than that for Pt/MgO (Table 1). As a consequence, the amount and strength of basic sites were crucial for the formation of OPP. To explore the differences in reactivity and selectivity of various catalysts on the process of hydrogenolysis of DBF, the performance of the Pt catalysts on different supports in hydrogenolysis of the target product OPP as substrate was carried out in the same conditions (1 MPa, 400 °C). The results were shown in Table 2. To our surprise, varying degrees of the dehydrogenation for OPP to DBF took place as literature51 reported, in which it was claimed that the hydrogenolysis of DBF to OPP is reversible and it is impossible for the conversion to completed as restricted by chemical equilibrium and thermodynamics.

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Phenylcyclohenene (CHEB) was observed in the hydrogenolysis of OPP over all catalysts except Pt/Al2O3 catalyst, in which it was suggested that the catalysts with low acidic supports are at least required. Compared with increased selectivities to BP over Pt/MgO, Pt/SiO2 and Pt/Al2O3, it may be suggested that basic catalysts have lower cleavage ability of C-OH bonds.39 Over MgO and SiO2 supported Pt catalysts, the dehydration of OPP was companied by a marked dehydrogenation generating DBF. And the dehydrogenation ability over Pt/MgO was stronger than Pt/SiO2. Compared with Pt/Al2O3, traces of DBF were found over Pt/MgO/Al2O3. With the dehydrogenation phenomenon in hydrogenolysis reaction, the TOF of the hydrogenolysis of OPP over Pt/MgO was higher than over Pt/Al2O3. The low activities of the hydrogenolysis of DBF, as revealed in Table 2, is associated with an inhibition of the dehydrogenation of OPP and the level of dehydrogenation of OPP.51 Specifically, the ability of dehydrogenation depends on the catalyst acid-base properties. This is in agreement with the findings by Diez et al.,41 who proposed that 2-propanol was converted to propylene or acetone via dehydrogenation or dehydration depending on the acid-base surface properties of catalysts. The Mg2+-O2- pairs promoted dehydrogenation and Al3+-O2- active sites facilitated dehydration. It is generally accepted that over acidic catalysts alcohols

undergo

dehydration,

and

over

basic

catalysts

alcohols

could

occur

dehydrogenation and dehydration.52-54 The more active the dehydrogenation, the stronger the base was of the catalysts (Table 2). Hence, we obtained clear evidence that dehydrogenation of OPP to DBF occur in basic sites.

20


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Figure 4. Effect of the reaction temperature over Pt/MgO on conversion of OPP and products selectivities. Reaction condition: 1.0 MPa, τ = 0.55 min, H2/oil = 400.

To gain further insight of the relationship between temperature and conversion of DBF during dehydrogenation of OPP, we conducted the experiment of the hydrogenolysis of OPP under different temperatures as illustrated in Figure 4. It was observed that an increase in the dehydrogenation product DBF with increasing temperature was reflected by the temperature-induced accelerated dehydrogenation of OPP in basic sites.35 That is to say, the dehydrogenation constrained the conversion of DBF. And in these conditions, at 400℃ or higher temperature, the selectivity of OPP or DBF and the conversion of DBF or OPP were stable, indicating that the reaction between DBF and OPP approached equilibrium.

Reaction pathway and kinetics of DBF hydrogenolysis According to our results, the overall reaction pathway of the hydrogenolysis of DBF over Pt/MgO was suggested and represented in Scheme 2. The hydrogenolysis reaction occurs through σ adsorption of the DBF molecular via the oxygen atom.55 The orientation of the aromatic rings of DBF and OPP, which is perpendicular to the support surface of MgO, decreased the hydrogenation of aromatic ring. Regarding the hydrogen activation during the 21



hydrotreating reaction, the hydrogen atom is usually formed by heterolytic dissociation of molecular hydrogen on the metal.56 where it can then be transferred to take part in the reactions by providing spillover hydrogen to MgO.57 First, the cleavage of the only one C-O bond takes place to give rise to OPP, followed by another C-O bond breakage to BP. Subsequently, the first route is via hydrogenolysis to B, followed by sequential minor hydrogenation to CH. The second one involves partial hydrogenation to active CHEB, then further hydrogenation to CHB. Isomerized product CPMB occurred at the same time, followed by small amounts of CHB hydrogenolysis to B and CH. The hydrogenolysis is the dominant reaction for C-O cleavage, affording OPP and BP as the primary products. No matter how the pressure was changed over Pt/MgO at 400 °C (Figure 3b), BCH was not found. However, when the temperature was decreased until the appearance of THDBF and PHDBF, BCH was observed. This suggested that BCH did not come from the hydrogenation of CHB but was derived from the C-O bond breakage of PHDBF. The hydrogenation of CHB to BCH is impossible since the hydrogenation of the second phenyl ring of BP is supposed to be much more difficult than that of the first ring.55

Scheme 2 Proposed reaction pathways of DBF hydrogenolysis over Pt/MgO in reaction conditions

To understand the effect of catalysts on the hydrogenolysis of DBF, the kinetic analysis was studied based on the reaction results. First of all, simplified reaction process was shown 22


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in Scheme S1. Then a pseudo-first-order model for the conversion of DBF and OPP was developed by the following equation.

−ln (/ ) = k × τ

(5)

Where C is the concentration of DBF at contact time τ and C0 is the initial concentration. The kinetic analysis was conducted with varied conversions of DBF and OPP versus the contact time. The conversion of DBF or OPP was well agreed with pseudo-first-order kinetics. The plots of ln(C/C0) versus contact time were linear with high r2 values (0.97-1.00) except for 0.95 over Pt/SiO2 in DBF conversion as shown in Figure 5. To prove the pseudo-first-order conversion of DBF and OPP, we performed experiments on the conversion of DBF and OPP at two concentration of the reactant (1 % and 2 %) over Pt/MgO catalyst. It was found that the conversion of DBF and OPP did not depend on their initial concentration, indicating that the hydrogenolysis of DBF and OPP followed first-order behavior over Pt/MgO. On account of the low reaction yield of the hydrogenolysis of OPP and DBF, the reversible reaction was neglected. Therefore, Figure 5a gives the pseudo first-order rate constants of the total conversion of OPP (k-1+k2) over the Pt catalysts. Figure 5b presents the pseudo first-order rate constants of the conversion of DBF (k1) over the Pt catalysts. From the rate constants of OPP hydrogenolysis and their initial product selectivities, we calculated the rate constants of dehydrogenation and hydrogenolysis of OPP (k-1 and k2, respectively).58 The specific values are listed in Table 3. The k1 gradually decreases following the order: Pt/Al2O3 > Pt/MgO > Pt/MgO-900 > Pt/SiO2 > Pt/MgO/Al2O3. This may be due to the more dispersed Pt nanoparticles with better activity in the conversion of DBF. The k-1 decreases

23



following the order: Pt/MgO ≈ Pt/MgO-900 > Pt/SiO2 ≈ Pt/MgO/Al2O3 > Pt/Al2O3, which reveals the dehydrogenation ability of OPP. This phenomenon could be attributed to the strength of the basic sites in the supports. The ratio of the rate constants of the DBF reaction and the OPP reaction k1/(k-1+k2) reflected the OPP formation rate, which showed that stronger basic catalyst was associated with higher OPP formation rate. Considering that no formation of OPP was found when the DBF hydrogenolysis reaction was carried out over Pt/Al2O3, we neglect the value of k1/(k-1+k2) over Pt/Al2O3. Apparently, the rate constant of OPP transformation to BP is more than twice as much as the rate constant of DBF conversion over Pt/Al2O3, probably suggesting that the Pt/Al2O3 catalyst is unsuitable for the formation of OPP despite high activity. Over acidic Al2O3, the acid sites are beneficial to the adsorption and activation of the intermediate phenols.59 From the perspective of reaction activity and selectivity, Pt/MgO catalyst is a potential catalyst for the selective hydrogenolysis of DBF to OPP.

Figure 5. The plot of ln(C/C0) versus contact time for the hydrogenolysis of OPP (a) and DBF (b) at 400 °C and 1.0 MPa over the Pt catalysts. Table 3 Rate constants in the pseudo-first-order kinetic analysis of the conversion of DBF and OPP over the Pt catalysts

24


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Catalysts

k-1+k2

k1

Pt/MgO Pt/MgO-900 Pt/SiO2 Pt/Al2O3 Pt/MgO/Al2O3

1.51 1.24 0.69 2.99 1.36

1.16 1.09 0.53 1.34 0.48

Rate constant (min-1) k-1 0.33 0.27 0.08 0 0.05

k2

k1/(k-1+k2)

1.18 0.97 0.61 2.99 1.31

0.77 0.88 0.76 0.45 0.35

The relationship between acidic-basic supports and the formation of OPP or the dehydrogenation of OPP was shown in Figure 6. Along with the increasing basicity of supports, the OPP yield of DBF hydrogenolysis and the DBF yield of OPP dehydrogenation was enhanced over various catalysts. This result gives additional evidence for the assumption that the acid-base surface properties of catalysts are an important factor for selective hydrogenolysis of DBF to OPP, reflecting the fact that basic catalysts accelerate the formation of OPP. It had already been discussed that the reaction of the alcohols, such as 2-propanol, is often used to characterize the acidic-basic properties of the supported catalysts.60 The DBF yields of OPP hydrogenolysis reflected the catalyst acid-base properties, which may actually be suitable for the acid and base characterization of catalysts. The differences of acid-base properties of metal oxide are due to the electronegativity of the neutral metal and the metal-oxygen bond energy.61 Generally, the acidic sites on Al2O3 come from the Al3+ sites (as Lewis acid sites) and hydroxyl connected with Al3+ (as Brønsted acid sites), and the basic sites on MgO supports are assigned to the O2- sites (as Lewis basic sites)62, 63 and surface OH- (as Brønsted basic sites).64,65 On the basic MgO, the Mg-O bond of reactant adsorbed on the surface of support breaks to generate the phenoxide radical and then dehydrogenation to DBF. However, on the acidic Al2O3, the group turned into an unstable surface species and did not desorb to yield BP.66 It is reported that the Lewis acid 25



sites are beneficial to the activation of the intermediate phenols via formation of phenoxide.59 Shinohara et al.66 suggested that alcohol dehydrates via scission of the C-O bond of a surface alkoxy group under interaction of its oxygen with some electrophile existing on the surface, and 2-propanol is dehydrogenated via the extraction of α-hydrogen of the isopropoxy group by metal on the oxide surface based on calculation of the activation energies of the dehydration and the dehydrogenation of 2-propanol.

Figure 6. The OPP yield in DBF hydrogenolysis and the DBF yield in OPP hydrogenolysis over supported Pt catalysts at 400 °C and 1.0 MPa, τ = 0.84 min, H2/oil = 400.

Conclusions Highly efficient Pt/MgO catalysts have been developed for selective hydrogenolysis of DBF to OPP by cleavage the only one C-O bond with preservation of their aromatic rings. The high selectivity of the catalysts for hydrogenolysis of DBF to OPP results from both strong basic sites and suitable reaction conditions (400 °C and 1.0 MPa). The dehydrogenation of OPP to DBF was found on basic catalysts and increasing reaction temperature accelerates dehydrogenation of OPP. The basic sites promote the formation of OPP due to the low cleavage ability of C-OH bonds or the dehydrogenation of OPP due to

26


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the nature of basic sites. High temperature and low pressure are in favor of the formation of OPP. Hydrogenolysis kinetics of DBF are best represented by a standard pseudo-first-order approximation. An improved reaction pathway and mechanistic understanding are established, which enables the rationalization of the hydrogenolysis performance of DBF. The dehydrogenation in basic sites makes the hydrogenolysis of DBF to OPP a reversible reaction where complete conversion is impossible due to restriction by chemical equilibrium.

ASSOCIATED CONTENT Supporting information The Supporting Information (SI) is available free of charge on the ACS Publications website at DOI: xxx. See SI for supplementary Tables and Figures.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone: + 86-411-84986353.

Funding This work was supported by the National Key Research & Development Program of China (2016YFB0600305).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge Lab of Advanced Materials＆Catalytic Engineering for providing the research facilities. The authors thank Instrumental Analysis＆Research Center for their 27



analysis assistance.

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Selective Hydrogenolysis of Dibenzofuran over Highly Efficient Pt

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