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Temperature-dependent selectivity of hydrogenation/ hydro-genolysis during phenol conversion over Ni catalysts Yanchun Shi, Enhui Xing, Jimei Zhang, Yongbing Xie, He Zhao, Yuxing Sheng, and Hongbin Cao ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 22, 2019

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ACS Sustainable Chemistry & Engineering

Temperature-dependent selectivity of hydrogenation/hydrogenolysis during phenol conversion over Ni catalysts Yanchun Shi,a Enhui Xing,b Jimei Zhang,a Yongbing Xie,a He Zhao,a Yuxing Sheng,a and Hongbin Caoa*

a.

Beijing Engineering Research Center of Process Pollution Control, Division of Environmental Engineering and Technology, Institute of Process Engineering, Chinese Academy of Sciences.

b.

State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, Sinopec, China.

Corresponding author: [email protected] (Prof. H. Cao)

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ABSTRACT: It is challenging to selectively upgrade phenolic compounds to aromatics, because of much weaker adsorption of hydroxyl than that of phenyl upon Ni catalysts. With 10% Ni loading in theory, there Ni catalysts with different Ni nanoparticle sizes were prepared with the wet impregnation method on SiO2 and Silicalite-1, and in situ encapsulation method (Silicalite-1). Based upon results, we proposed a general rule concerning temperature-dependent selectivity control on phenol hydro-conversion over Ni catalysts. As well as benzene saturation in consecutive mode, hydrogenation of phenyl ring was more dramatically inhibited at elevated temperature via decreased adsorption of benzene rings than hydroxyl to selectively favor hydrogenolysis over hydrogenation in parallel mode. Among three Ni catalysts, Ni@Silicalite-1 with 3-5 nm Ni nanoparticle sizes encapsulated, imposed the restricted adsorption conformation of phenol via end-up mode within channels of Silicalite-1 zeolite to further improve benzene selectivity. Due to restriction of channels and smaller Ni nanoparticle sizes, better activity and stability were simultaneously achieved over Ni@Silicalite-1 catalyst, as well as superior benzene selectivity at higher temperature via thermodynamic hindrance on phenyl adsorption to facilitate benzene formation in kinetics rather than hydrogenation of phenyl without further saturation of benzene via hindrance on benzene adsorption.

KEYWORDS: phenol, hydrogenolysis, benzene, temperature-dependent selectivity control, encapsulated Ni catalyst


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INTRODUCTION With increasing awareness on environments and sustainability, catalytic upgrading of phenolic compounds derived from biomass to fuels and chemicals, especially to aromatics (benzene, toluene and xylene), has attracted much attention from industrial and academic scope.1-30 Hydrotreatment has been considered as the most attractive method, with two distinct mechanisms on phenolics conversion based upon selective activation of phenyl/hydroxyl. (i) Activation of phenyl was typically conducted over bi-functional catalysts to achieve complete deoxygenation by consecutive reactions with hydrogenation of phenyl over metal sites (Pt, Pd, Ni, etc) dehydration of alcohols over acid sites (H+, H-type zeolite, etc) - hydrogenation to produce cycloalkanes.4-10 In such processes, reforming of cycloalkanes could enhance aromatics selectivity, but suffered from thermodynamic limitation. (ii) Activation of hydroxyl could facilitate hydrogenolysis over oxophilic metal (Fe, Ru, Mo, etc) and/or supports (ZrO2, TiO2, Nb2O3, etc) by strong interaction with oxygen atom to reduce the energy barrier for the direct cleavage of hydroxyl, accomplishing C-O bond scission to form aromatics.11-26 From phenolics to aromatics, hydrogenolysis is regarded as the ideal route with little thermodynamic limitation31 and minimized H2 consumption; however, hydrogenolysis of hydroxyl is less favorable than hydrogenation of phenyl in kinetics, because of the difficulty in direct C-O scission because of ring stabilization. Extensive researches have been devoted to the selection of metals and supports, among which Ni exhibited promising performance in non-noble metal catalysts.7-8,10-11,16-21,27-28 Significantly, Ni particle sizes directly determined activity; smaller particles, higher conversion and better selectivity to aromatics.27 From the view of reaction chemistry, it has also been a great challenge to understand the reaction mechanism behind the reaction network consisting both



parallel and consecutive reactions for product optimization, apart from the obvious economic driving force in minimizing H2 consumption. Hydro-treatment of phenolics was proposed to follow the Langmuir-Hinshelwood (L-H) mechanism via hydrogenation/hydrogenolysis.13,19 Adsorption mode of phenolics is probably one of determining factors to final products distribution. However, the gap between weakly physical adsorption of hydroxyl and strong chemical adsorption of phenyl seemed too large to overcome in kinetics on mono metal catalysts during phenolics conversion. Here, we designed encapsulated Ni particles within confined space within Silicalite-1 to retain end-up adsorption of phenol to improve catalytic activity, selectivity and stability. With temperature-dependent selectivity proven, catalytic upgrading of phenol (model of phenolics) to benzene was conducted from 200 oC to 400 oC over three Ni catalysts (10% loading) with different Ni nanoparticle sizes to achieve selective adsorption, activation and conversion of phenyl/ hydroxyl via hydrogenation/hydrogenolysis as well as better activity/stability.

EXPERIMENTAL Synthesis of Ni catalysts All materials (Tetraethyl orthosilicate, Tetrapropylammonium hydroxide, SiO2, Ni(NO3)2∙6H2O and phenol) were used as provided. The water used in all experiments was deionized water. Synthesis of Silicalite-1 zeolite: Tetraethyl orthosilicate (TEOS) and Tetrapropylammonium hydroxide (TPAOH) were selected as silicon source and organic structure directing agent (OSDA) to synthesize S-1 zeolite (MFI). Firstly, TEOS was dissolved in the aqueous TPAOH solution, and the molar ratios of typical batch composition was TPAOH/SiO2 = 0.06 and H2O/SiO2 = 30. Secondly, this mixture was heated to 70 oC for 2 - 6 h to completely evaporate ethanol. Thirdly, the hydrothermal synthesis was conducted in Teflon-line vessels (50 mL) under


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rotating conditions at 170 oC for 12 h. The synthesized samples were filtrated, washed with deionized water until pH = 7, and dried at 100 oC overnight. As-synthesized Silicalite-1 samples were calcined at 550 oC in ambient air for 6 h in a muffle furnace to remove organics. Synthesis of Ni/SiO2 and Ni/Silicalite-1 catalysts: The pristine Ni/SiO2 and Ni/Silicalite-1 catalysts were prepared with wet impregnation method. In a representative procedure, the desired quantities of Ni(NO3)2∙6H2O (0.5 g) were dissolved in deionized water (10 mL) and then slowly dropped this solution onto supports (1.0 g) with continuous stirring for 4 h at ambient temperature. The obtained products were dried at 105 °C overnight and calcined at 550 °C for 5 h in air atmosphere. Prepared catalysts were reduced at 500 °C for 4 h (ramp: 10 °C ·min-1) in hydrogen flow (flow rate: 80 mL· min-1). Synthesis of encapsulated Ni@Silicalite-1 catalysts: Firstly, TEOS was dissolved in the aqueous TPAOH solution with molar ratios of typical batch composition as TPAOH/SiO2 = 0.1 and H2O/SiO2 = 30, and then the clathrate of Ni-ethanediamine was added into this mixture. Secondly, this mixture was heated to 70 oC for 2 - 6 h to completely evaporate the ethanol. Thirdly, the hydrothermal synthesis was conducted in Teflon-line vessels (50 mL) under rotating conditions at 170 oC for 12 h. The synthesized samples were filtrated, washed with deionized water until pH = 7, and dried at 100 oC overnight. As-synthesized Ni@Silicalite-1 samples were calcined at 550 oC in ambient air for 6 h in a muffle furnace to remove organics. Reduction of Ni@Silicalite-1 catalysts were described above. Characterization Wide-angle X-ray diffraction patterns (WXRD) of samples were collected on a D/MAX-III Xray diffractometer (Rigaku Corporation, Japan) with filtered Cu K α radiation at a tube current of 35 mA and a voltage of 35 kV. The scanning range of 2θ was 5 - 80°. The relative crystallinity of



catalysts was calculated by the sum of the peak intensities at 2θ of 23.1, 23.3, 23.7, 24.0 and 24.4°, with Silicalite-1 as the reference. The crystal morphology was measured on scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The elemental analyses of the solids were performed on an X-ray fluorescence (XRF) spectrometer MagiX (Philips). Nitrogen adsorption-desorption isotherms were recorded on a Micromeritics ASAP 2010 instrument. The samples were first out-gassed under vacuum at 90 oC for 1 h and at 350 oC for 15 h. The total surface area was obtained by application of the BET equation using the relative pressure range of 0.05 - 0.16 in the nitrogen adsorption isotherm as range of linearity (using a molecular cross-sectional area for N2 of 0.162 nm2). The micropore volume was calculated by the t-plot method. H2 temperature-programmed desorption (H2-TPD) were performed in microreactor monitored by a auto CHEM II 2920. In situ FT-IR spectra study on the interaction between phenol and Ni@Silicalite-1: IR spectra of phenol (heating 100 oC) adsorbed over Ni@Silicalite-1 catalyst was obtained on Nicolet Impact 410 FT-IR spectrometer by scans of 64 with a resolution of 4 cm-1. The samples were pressed into a self-supporting wafer (10 mg) and placed in a quartz IR cell with CaF2 windows. Then it was purged with N2 flow at 400 °C for 1 h and subsequently cooled down to 100 oC for phenol adsorption. The IR spectra of the samples before phenol adsorption were recorded at different temperatures. After adsorbing for 30 s, the samples were purged with N2 flow to higher temperatures ((200, 250, 320 and 400 oC).) at a heating rate of 10 °C/min all the time at 0.25 MPa H2. Then the IR spectra of phenol on Ni@S-1 were recorded at different temperatures. All the spectra given in this work were difference spectra to deduct H2O background. Catalytic tests


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Vapor-phase hydro-treatment of phenol was carried out in a stainless-steel fixed bed reactor. The reduced catalysts (2 mL) was placed in the center of the reactor and purged with hydrogen under reaction pressure by heating to reaction temperatures (200, 250, 300, 350 and 400 oC). Phenol was pumped into the reactor to fill the bed fully. The hydro-treatment conditions were as follows: 0.25, 0.5 and 1.0 MPa of H2 pressure; 200 to 400 mL/min of H2 flow rate; 0.1 mL/min of phenol flow rate; 200 to 400 oC of reaction temperatures. After the reaction system was given 120 min to achieve a steady state conditions before samples were collected for analysis. The reaction products were analyzed by an Shimadzu 2030 gas chromatograph (GC) equipped with a flame ionization detector and a capillary column (SH-RTX-WAX: 30 m ⅹ 0.25 mm ⅹ 0.25 μm).

RESULTS AND DISCUSSION Physico-chemical properties: Clearly, wide-angle X-ray diffraction patterns (WXRD) show characteristic peaks of Ni[111] at 44.4o and Ni[200] at 52.1o for Ni/SiO2 (11.6 wt% Ni loading in Table 1) and Ni/Silicalite-1 (11.3 wt% Ni loading in Table 1) catalysts in Figure 1a and 1b. However, the pattern for Ni@Silicalite-1 catalyst (10.6 wt% Ni loading) shows no obvious peaks of Ni crystals, disclosing that ultra small Ni particles throughout Silicalite-1 zeolite. Figure 2 presents N2 adsorption-desorption isotherms and BJH pore size distribution plots, with corresponding textual porosities in Table 1. Compared to Silicalite-1 zeolite (451.2 m2/g, 66.8 m2/g and 0.19 cm3/g for micro surface area, external surface area and micro volume), there were obvious blocking effects on Ni/Silicalite-1 (406.7 m2/g, 84.8 m2/g and 0.12 cm3/g for micro surface area, external surface area and micro volume) and Ni@Silicalite-1 (272.2 m2/g, 124.9 m2/g and 0.14 cm3/g for micro surface area, external surface area and micro volume). With similar relative crystallinity, significant decrease in micro surface from 451.2 m2/g (Silicalite-1)



to 272.2 m2/g (Ni@Silicalite-1) exactly indicates encapsulation of Ni nanoparticles within Silicalite-1 zeolite. According to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis, there were many Ni crystals on amorphous SiO2 (Figure 3a and 3d) and Silicalite-1 zeolite external surface (Figure 3b and 3e), with Ni particle sizes of SiO2 (Figure 3g) larger than those of Silicalite-1 zeolite (Figure 3h). Significantly, Ni nanoparticles at about 3-5 nm with uniform distribution, were observed over encapsulated Ni@Silicalite-1 catalyst (Figure 3c and 3f), much smaller than the impregnated Ni/SiO2 and Ni/Silicalite-1 catalysts. Active centres: H2 temperature-programmed desorption (H2-TPD) analysis of Ni/SiO2, Ni/Silicalite-1 and Ni@Silicalite-1 catalysts were given in Figure 4. Obviously, the peaks at lower temperature region (peaks around at 78 oC for Ni/SiO2, 103 oC for Ni/Silicalite-1 and 88 oC

for Ni@Silicalite-1), assigned to hydrogen species desorption from weakly adsorbed

hydrogen or subsurface NiHx decomposition, increased in the order Ni/SiO2 < Ni/Silicalite-1 < Ni@Silicalite-1 with decrease sizes of Ni nanoparticles (Figure 3).27 That is, the smaller Ni nanoparticles, the more active sites for phenol conversion. The high temperature region from 250 oC

to 400 oC, was attributed to the spillover of hydrogen species over SiO2/Silialite-1 supports at

different temperatures. In situ FT-IR analysis of phenol adsorbed on Ni@Silicalite-1catalyst: we have tried to characterize the adsorption conformation of phenol on Ni/SiO2 and Ni/Silicalite-1 catalysts by in situ FT-IR spectra analysis. However, there was no transmission spectrum on Ni/SiO2 and Ni/Silicalite-1 catalysts, because of their black color. Therefore, it is hard to detect phenol adsorption-desorption on Ni/SiO2 and Ni/Silicalite-1 catalysts.


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Figure 5 presents the in-situ FT-IR analysis for phenol adsorption on Ni@Silicalite-1 catalyst was performed to prove thermodynamic adsorption at different temperatures (100 oC, 200 oC, 250 oC, 320 oC and 400 oC). Clearly, at 100 and 200 oC, the vibration of aromatic ring were presented at 1472 cm-1, 1498 cm-1 and 1598 cm-1, which directly indicated that prevailing adsorption of phenyl over Ni@Silicalite-1 catalyst.17,25 However, at 250 oC, the intensity of these two bands decreased dramatically, which disclosed that phenyl adsorption over Ni@S-1 catalyst was inhibited. With temperature at 320 and 400 oC, there was almost no characteristic band observed at 1472 cm-1, 1498 cm-1 and 1598 cm-1, demonstrating dramatic hindrance on phenyl adsorption. Whereas there was still the very weak peaks, assigned to adsorption of hydroxyl (3628 cm-1) over Ni@Silicalite-1 at all desorption temperatures, partially for high temperatures (350 and 400 oC). That is, high temperature favors selective adsorption of hydroxyl over phenyl via dramatic thermodynamic hindrance on phenyl adsorption. Catalytic performance over Ni/SiO2, Ni/Silicalite-1 and Ni@Silicalite-1 catalysts: As well known, temperature is the most important factor in both thermodynamics and kinetics. Thus, the catalytic upgrading of phenol in a broad range from 200 to 400 oC was conducted over three Ni catalysts, as presented in Figure 6. Interestingly, phenol conversion decreased firstly with increasing reaction temperatures, but improved further with rising of reaction temperature up to 400 oC (Figure 6A). Before the minimum value, phenol conversions decreased dramatically with increasing reaction temperatures, ascribed to thermodynamic limitation.11 From the view of adsorption in L-H mechanism, higher temperature decreased the concentration of adsorbed phenol, leading to lower activity of the overall reaction. After the minimum conversion, further decrease in adsorption amount caused by temperature, became less influential than reaction temperature, therefore the conversion increased gradually (Figure 6A). As well as in situ FT-IR



results of phenol adsorbed on Ni@Silicalite-1 catalyst, the temperature-dependent selective adsorption of phenyl/hydroxyl was proposed in this paper. With increasing temperatures, the intensity of phenyl bands decreased dramatically and finally disappeared beyond 300 oC, which was consistent with dramatic decrease in phenol conversion (Figure 6A). On the contrary, the existence of the weak adsorption of hydroxyl (3628 cm-1) at all desorption temperatures, partially for high temperatures (350 and 400 oC), demonstrated that high temperature favors selective adsorption of hydroxyl over phenyl to achieve better selective activation of hydroxyl to favor hydrogenolysis for benzene via dramatic hindrance on phenyl adsorption without further saturation of benzene via hindrance on benzene adsorption. The activity of three Ni catalysts exhibited the minimum conversion at different temperatures (350 oC for Ni/SiO2 and Ni/Silicalite-1, 250 oC for Ni@Silicalite-1). Particularly, the minimum conversion over Ni@Silicalite-1 was anticipated to occur at 250 oC, lower than those of Ni/SiO2 and Ni/S-1 catalysts at 350 oC. This activity result is directly related to ultra small Ni nanoparticles (3-5 nm in Figure 3) over Ni@Silicalite-1, consistent with H2-TPD analysis (Figure 4c). Zhu et al also proved that decrease in Ni particle size could enhance the reaction activity during hydro-treatment of m-cresol over Ni/SiO2 catalysts.27 Therefore, with smaller Ni nanoparticle sizes, better activity could be achieved, which decreased the temperature corresponding to the minimum conversion with the downsizing of Ni particle sizes. That is the reason why the temperature corresponding to the minimum conversion is so different for encapsulated Ni@Silicalite-1 compared to the other impregnated Ni/SiO2 and Ni/Silicatlite-1 catalysts, 250 v. s. 350 oC. Additionally, the smallest Ni nanoparticles of Ni@Silicalite-1 catalyst also exhibited the highest conversion of phenol at same reaction conditions among three Ni


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catalysts, especially for 400 oC (71.2% for Ni@Silicalite, 31.1% for Ni/SiO2 and 37.8% for Ni/Silicalite-1 catalysts in Figure 6A). As shown in Figure 6B, the distribution of products mainly contained cyclohexnone, cyclohexanol, benzene, cyclohexene and cyclohexane, which were subdivided into parallel and consecutive modes based upon reaction network in Scheme 1: parallel reaction I was undergone by hydrogenation of phenol with phenyl adsorption to produce cyclohexnone and cyclohexanol (consecutive reaction I), and parallel reaction II was hydrogenolysis of phenol with hydorxyl adsorption to form benzene. It's well know that there was almost no acidity of SiO2 and Silicalite-1 supports, thus cyclohexene and cyclohexane could only be formed by hydrogenation of benzene (consecutive reaction II), not via dehydration of cyclohexanol "phenolics pool"

29-30

4-10

or cracking of

over requisite catalysts with acid sites. Therefore, higher temperature

facilitated hydrogenolysis over hydrogenation in the parallel mode, disclosing that reaction temperature is the crucial factor to define the selective adsorption, activation and conversion of phenyl/hydroxyl over all Ni catalysts (Figure 6B and 6C) via hydrogenation/hydrogenolysis. Based upon exothermic essence of adsorption, higher temperature favored the selective adsorption of hydroxyl because of much less adsorption heat than phenyl in parallel mode. With small Ni nanoparticles encapsulated within confined space of MFI structure (Figure 3), it was worth reporting that Ni@Silicalite-1 catalyst presented high activity, as well as superior benzene selectivity at higher temperature via thermodynamic hindrance on phenyl adsorption to facilitate benzene formation in kinetics rather than hydrogenation of phenyl without further saturation of benzene via hindrance on benzene adsorption. Different from activity fluctuations, benzene became predominant product continuously from 200 to 400 oC in Figure 6B. Especially at 400 oC, benzene selectivity were 95.4 % for



Ni/SiO2, 96.8 % for Ni/S-1, 99.2 % for Ni@S-1, regardless of slight products with the hydrogenation of aromatic rings (1.8 % cyclohexene, 2.4 % cyclohexanone and 0.4 % cyclohexanol for Ni/SiO2; 1.7 % cyclohenxanone and 1.5 % cyclohenxanol for Ni/S-1; 0.8 % cyclohexanone for Ni@S-1). High reaction temperatures not only thermodynamically favor selective hydroxyl adsorption of phenol molecules to facilitate the hydrogenolysis, but also restrain the hydrogenation of benzene (consecutive reaction II) in Scheme 1, which could also be proven by inhibited adsorption of phenyl (Figure 5) to maximize benzene selectivity. At 400 oC, no formation of cyclohexane excluded reforming reaction over Ni catalysts without acidity. Compared with impregnated Ni/SiO2 (63 % at 350 oC and 95.4 % at 400 oC) and Ni/Silicalite-1 (66.5 % at 350 oC and 96.8 % at 400 oC), encapsulated Ni@Silicalite-1 exhibited the highest benzene selectivity (81.9 % at 350 oC and 99.2 % at 400 oC). Since Ni@Silicalite-1 with 3-5 nm Ni nanoparticle sizes encapsulated, imposed the restricted adsorption conformation of phenol via end-up mode within channels of Silicalite-1 zeolite to further improve benzene selectivity. Due to restriction of channels and smaller Ni nanoparticle sizes, better activity is achieved over Ni@Silicalite-1 catalyst at the same time (Figure 6A). Influence of H2 pressure and flow rate on catalytic performance over and Ni@Silicalite-1: we also check the influences of H2 pressure and H2 flow rate on phenol upgrading over Ni@Silicalite-1 catalyst in order to verify our proposal on selective adsorption of phenyl/hydroxyl. Firstly, H2 is one of the reactants to improve phenol conversion with higher H2 pressure (Figure 7A); meanwhile, H2 has formed the competitive adsorption with phenyl over active sites catalysts 27, 37-39 surface based upon L-H mechanism. Correspondingly, the selectivity of hydrogenated products decreased, resulting in the high ratio of hydrogenolysis/hydrogenation (Figure 6B). Secondly, varying H2 flow rate at 400 oC (hydrogenolysis zone) from 200 ml/min to


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400 ml/min (Figure 8), it can be seen that too fast H2 flow rate lead to kinetic controlled region favoring phenyl adsorption with insufficient contacting time to reach thermodynamical balance between hydroxyl and phenyl adsorption, thus phenol conversion and benzene selectivity both decreased with cyclohexanone selectivity slightly enhanced. Stability of impregnated Ni/Silicalite-1 and encapsulated Ni@Silicalite-1: Migration and aggregation of Ni particles is the bottleneck hindering the industrial application of Ni catalysts especially at high temperature. In this paper, we consciously prepared Ni@Silicalite-1 catalyst via encapsulation to enhance the catalytic stability, which has been proven effective in hydrogenation processes.32-36 Impregnated Ni/Silicalite-1 showed a gradual deactivation (Figure 9A), because Ni particles increased obviously (Figure 10a and 10e), which was common sense of catalysts prepared by wet impregnation method. As expected, encapsulated Ni@Silicalite-1 catalyst presented stable catalytic performances in both phenol conversion and benzene selectivity (Figure 9). There was no obvious difference in sizes of Ni nanoparticles between fresh Ni@Silicalite-1 and used Ni@Silicate-1 (Figure 10b and 10f), which disclosed superiority of encapsulation method than conventional methods to avoid migration and aggregation of Ni particles. A general rule on how to control reaction selectivity is proposed in Figure 11. Temperaturedependent selective adsorption of phenyl/hydroxyl is the determining factor to product distribution, under which smaller sizes of Ni particles presented high activity and benzene selectivity. Based upon this rule, selective adsorption of hydroxyl was preferred in region IV to achieve high selectivity of aromatic via inhibition in adsorption of phenyl and benzene. In all, temperature-dependent thermodynamic adsorption lead to dramatic decrease in adsorption of phenyl at high temperature, which in turn influenced the overall reaction toward benzene



formation kinetically to diminish stabilization effects of phenyl on C-O bond. Finally, excellent benzene selectivity and catalytic stability and were accomplished over encapsulated Ni@Silicalite-1 catalyst by in-situ synthesis with high phenol conversion because of end-up adsorption of phenol and the temperature-dependent essence. CONCLUSIONS With increasing interests in biomass utilizations, it is challenging to selectively upgrade phenolic compounds to aromatics due to much weaker adsorption of hydroxyl than that of phenyl on Ni catalysts. As the raw material and the key intermediate during the upgrading of phenolic compounds, phenol was selected as the modal compound to investigate the reaction network consisting both parallel and consecutive reactions over mono Ni catalysts for product optimization. The catalytic upgrading of phenol was carried out over impregnated Ni/SiO2, Ni/Silicalite-1, and encapsulated Ni@Silicalite-1 catalysts in a broad range to optimize the activity and selectivity, based upon which we proposed a new methology via thermodynamic hindrance on phenyl adsorption to facilitate benzene formation in kinetics rather than hydrogenation of phenyl with minimized H2 consumption. Subsequently, further hydrogenation of benzene, the hydrogenolysis product of phenol, could also be prevented via dramatic decrease in adsorption of benzene to maximize benzene yield. With better activity and stability, further improvement in benzene selectivity was achieved over Ni@Silicalite-1 catalyst with Ni 3-5 nm nanoparticle sizes encapsulated, because of restriction form channels on the restricted adsorption conformation of phenol with Silicalite-1 zeolite to achieve end-up adsorption. With the temperature-dependent essence of phenol adsorption on Ni catalysts, this synthesis concept of encapsulation Ni catalysts may highlight a new strategy for enhancing catalytic activity,


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aromatics selectivity and stability, which should be applicable to catalytic upgrading of phenolic compounds.

AUTHOR INFORMATION Corresponding Author [email protected] (Prof. H. Cao) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT Dr. Yanchun Shi and Dr. Enhui Xing express their dedication to Prof. Xingtian Shu on the occasion of his 80th birthday. This work was supported by the National Key Research and Development Program of China (Grant No. 2016YFB0600505) and Youth Innovation Promotion Association, CAS (2014037). Special thanks for IR results afforded by Dr. Fuping Tian and Zhenhua Qiao student from Dalian University of Technology.. REFERENCES (1) Liu, H. Z.; Jiang, T.; Han, B. X.; Liang, S. G.; Zhou, Y. X. Selective phenol hydrogenation to cyclohexanone over a dual supported Pd-lewis acid catalyst. Science, 2009, 326, 12501253, DOI 10.1126/science.1179713. (2) Wang, Y.; Yao, J.; Li, H.; Su, D. S.; Antonietti, M. Highly selective hydrogenation of phenol and derivatives over a Pd@Carbon nitride catalyst in aqueous media. J. Am. Chem. Soc. 2011, 133, 2362-2365, DOI 10.1021/ja109856y.



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(3) Shao, T.; Xia, Q. N.; Dong, L.; Liu, X. H.; Han, X.; Parker, S. F.; Cheng, Y. Q.; Daemen, L. L.; Ramirez-Cuesta, A. J.; Yang, S. H.; Wang, Y. Q. Selective production of arenes via direct lignin upgrading over a niobium-based catalyst. Nat. Commun. 2017, DOI: 10.1038/ncomms16104. (4) Wang, G. H.; Cao, Z. W.; Gu, D.; Pfänder, N.; Swertz, A. C.; Spliethoff, B.; Bongard, H. J.; Weidenthaler, C.; Schmidt, W.; Rinaldi, R.; Schüth, F. Nitrogen-doped ordered mesoporous carbon supported bimetallic PtCo nanoparticles for upgrading of biophenolics. Angew. Chem. Int. Ed. 2016, 55, 8850-8855, DOI 10.1002/anie.201511558. (5) Yoon, Y.; Rousseau, R.; Weber, R. S.; Mei, D.; Lercher, J. A. First-principles study of phenol hydrogenation on Pt and Ni catalysts in aqueous phase. J. Am. Chem. Soc. 2014, 136, 10287-10298, DOI 10.1021/ja501592y. (6) Liu, X.; Xu, L.; Xu, G.; Jia, W.; Ma, Y.; Zhang, Y. Selective hydrodeoxygenation of ligninderived phenols to cyclohexanols or cyclohexanes over magnetic CoNx@NC catalysts under mild conditions. ACS Catal. 2016, 6, 7611-7620, DOI 10.1021/acscatal.6b01785. (7) Kordouli, E.; Pawelec, B.; Kordulis, C.; Lycourghiotis, A.; Fierro, J. L. G. Hydrodeoxygenation of phenol on bi-functional Ni-based catalysts: Effects of Mo promotion and

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Figure and Table captions: Figure 1. XRD patterns of Ni/SiO2 (a), Ni/Silicalite-1 (b) and Ni@Silicalite-1 (c) catalysts. Figure 2. N2 adsorption-desorption analysis of Ni/SiO2 (A and C), Ni/Silicalite-1 (B and D) and Ni@Silicalite-1 (B and D) catalysts. Figure 3. SEM and TEM images (a-f) and the statistics particle size distribution bar charts (based upon TEM analysis) of Ni/SiO2 (a, d, g), Ni/Silicalite-1 (b, e, h) and Ni@Silicalite-1 (c, f, i) catalysts. Figure 4. H2-TPD curves of Ni/SiO2 (a), Ni/Silicalite-1 (b) and Ni@Silicalite-1 (c) catalysts. Figure 5. In situ FT-IR spectra analysis of phenol adsorption on encapsulated Ni@Silicalite-1 catalyst at different desorption temperatures. Figure 6. Conversion (%, A) of phenol, distribution of products (mol%, B) and the selectivity of parallel reaction (C, hydrogenation and hydrogenolysis) over Ni/SiO2, Ni/Silicalite-1 and Ni@Silicalite-1 catalysts under different temperatures (1.0 g catalysts; 0.1 ml/min phenol flow rate; 300 ml/min H2 flow rate; 0.25 MPa H2). Figure 7. Effect of pressure on phenol conversion (A), products selectivity (A) and selectivity of hydrogenation/hydrogenolysis (B) over Ni@Silicalite-1 catalyst (1.0 g catalysts; 0.1 mL/min phenol flow rate; 300 mL/min H2 flow rate; 350 oC) Figure 8. Effect of rate of H2 flow on distribution of products selectivity over Ni@Silicalite-1 catalyst (1.0 g catalysts; 0.1 mL/min phenol flow rate; 400 oC; 0.25 MPa H2) Figure 9. Stability of phenol conversion catalytic performance over Ni@Silicalite-1 catalyst (1.0 g catalysts; 0.1 mL/min phenol flow rate; 400 oC; 300 mL/min H2 flow rate; 0.25 MPa H2) Figure 10. TEM images of used Ni/Silicalite-1 (a) and Ni@Silicalite-1 (b) with the statistics particle size distribution bar charts (e and f) over used catalysts. 20 ACS Paragon Plus Environment

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Page 23 of 37 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 11. Representation of factors (reaction temperature and Ni size) limiting phenol hydro-treatment over Ni catalysts.

Scheme 1. Phenol reaction network over Ni catalysts in a board temperature range. Table 1. Textural properties of Ni/SiO2, Ni/Silicalite-1 and Ni@Silicaite-1 catalysts

21 ACS Paragon Plus Environment


a (Ni/SiO2) b (Ni/Silicalite-1) c (Ni@Silicalite-1)

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 24 of 37

Ni [111] Ni [200]

10

20

30

40

50

60

70

o

2 Theta ( )

Figure 1. XRD patterns of Ni/SiO2 (a), Ni/Silicalite-1 (b) and Ni@Silicalite-1 (c) catalysts.


Page 25 of 37

A

a (Ni/SiO2) N2 volume adsorbed (cm /g)

500

Ni/S-1 Ni@S-1

B

200

3

3

N2 volume adsorbed (cm /g)

600

400

300

200

100

150

100

50 0 0.0

0.2

0.4

0.6

0.8

1.0

0.0

0.2

0.4

Relative pressure (p/p0)

0.6

0.8

1.0

Relative pressure (p/p0) 0.6

0.5

b (Ni/S-1) c (Ni@S-1)

D

a (Ni/SiO2)

C

0.5

3

3

Desorption dV (logd) (cm /g)

0.4

Desorption dV (logd) (cm /g)

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


0.3

0.2

0.1

0.4

0.3

0.2

0.1

0.0 0.0 0

5

10

15

20

25

30

35

0

40

5

10

15

20

25

30

35

40

Pore Diameter (nm)

Pore Diameter (nm)

Figure 2. N2 adsorption-desorption analysis of Ni/SiO2 (A and C), Ni/Silicalite-1 (B and D) and Ni@Silicalite-1 (B and D) catalysts



Figure 3. SEM and TEM images (a-f) and the statistics particle size distribution bar charts (based upon TEM analysis) of Ni/SiO2 (a, d, g), Ni/Silicalite-1 (b, e, h) and Ni@Silicalite-1 (c, f, i) catalysts.


Page 26 of 37

Page 27 of 37

a (Ni/SiO2)

88

b (Ni/S-1) c (Ni@S-1)

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


103

250 - 400

78

100

200

300

400

500

o

Temperature ( C)

Figure 4. H2-TPD curves of Ni/SiO2 (a), Ni/Silicalite-1 (b) and Ni@Silicalite-1 (c) catalysts.



Ni@S-1

o

1598

b (200 C-Desorption) o

c (250 C-Desorption)

1498

o

a (100 C-Adsorption)

0.5

o

o

e (400 C-Desorption)

3600

3400

3200

3000

1472

1605

d (320 C-Desorption)

3628

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 37

1600

1500

1400

-1

Wavenumber (cm )

Figure 5. In situ FT-IR spectra analysis of phenol adsorption on encapsulated Ni@Silicalite-1 catalyst at different desorption temperatures.


Benzene Cyclohexanone

Cyclohexane Cyclohexene

o

--

c

--

d

--

350

300

o

Catalysts and Temperature ( C) e

--

400

0

c

0

b

40

a

0

c

35

b

0

a

c

30

b

25

a

0 40 0

c

b

35

a

250

Temperature ( C) b

0

0

0

400

0

350

300

25

250

20

30

200

40

0

0

20

60

40

20

40

c (Ni@Silicalite-1)

80

0

40

60

b (Ni/Silicalite-1)

a (Ni/SiO 2)

0

60

80

Selectivity of phenol hydrogenolysis

Selectivity of phenol hydrogenation

35

Distribution of products (%)

80

100

Selectivity of parallel reaction (%)

b (Ni/Silicalite-1) c (Ni@Silicalite-1)

C

Cyclohexanol

0

B 100

30

A 100

a (Ni/SiO2)

Conversion (%)

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


25

Page 29 of 37

o

Reaction temperature ( C )

f

a-2 a-3 -- b-1 (mol%, b-2 b-3 -- c-1 c-3 -- the d-1 d-2 d-3 200 300 350 400 450 500 550(C, 600 hydrogenation 650 700 750 800 850 900and 950 Figure 6. Conversion (%, A) of phenol, distribution ofa-1 products B)c-2 and selectivity of 250 parallel reaction

hydrogenolysis) over Ni/SiO2, Ni/Silicalite-1 and Ni@Silicalite-1 catalysts under different temperatures (1.0 g catalysts; 0.1 ml/min phenol flow rate; 300 ml/min H2 flow rate; 0.25 MPa H2)



A

Cyclohexane Cyclohexene

Conversion (%) and Selectivity (mol/mol %)

100

Benzene Cyclohexanone

Con.% Cyclohexanol

80

60

40

20

0

0.25

1.0

0.5 Presure (MPa H2)

B

b Selectivity of phenol hydrogenation

c d Selectivity of phenol hydrogenolysis

100

Selectivity of parallel reaction (%)

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 30 of 37

80

60

40

20

0

0.25

0.5

1.0

Presure (MPa H2)

Figure 7. Effect of pressure on phenol conversion (A), products selectivity (A) and selectivity of hydrogenation/hydrogenolysis (B) over Ni@Silicalite-1 catalyst (1.0 g catalysts; 0.1 mL/min phenol flow rate; 300 mL/min H2 flow rate; 350 oC)


Page 31 of 37

Con.%

Cyclohexanone

Benzene 100

Conversion (%) and Selectivity (mol/mol %)

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


80

60

40

20

0

200

300

400

The rate of hydrogen flow (mL/min) a

b

c

Figure 8. Effect of rate of H2 flow on distribution of products selectivity over Ni@Silicalite-1 catalyst (1.0 g catalysts; 0.1 mL/min phenol flow rate; 400 oC; 0.25 MPa H2)



100

A

a (Ni/S-1) b (Ni@S-1)

Conversion (%) of phenol

80

60

40

20

0 150

180

210

240

270

300

330

360

390

420

450

480

390

420

450

480

t (min)

100

B

99

Selectivity of products (mol/mol %)

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 37

98

a (Benzene over Ni/S-1) b (Cyclohexanone over Ni/S-1) c (Cyclohexanol over Ni/S-1) d (Benzene over Ni@S-1) e (Cyclohexanone over Ni@S-1)

97 96 95 4 3 2 1 0 150

180

210

240

270

300

330

360

t (min)

Figure 9. Stability of phenol conversion catalytic performance over Ni@Silicalite-1 catalyst (1.0 g catalysts; 0.1 mL/min phenol flow rate; 400 oC; 300 mL/min H2 flow rate; 0.25 MPa H2)


Page 33 of 37

35

55

e (used Ni/Silicalite-1)

f (used Ni@Silicalite-1)

50

30

45 40

Frequency (%)

25

Frequency (%)

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


20 15 10

35 30 25 20 15 10

5

5 0

0 20

25

30

35

40

50

45

55

60

65

70

80

75

85

2

90

5

4

3

7

6

Particle size (nm) a

b

c

d

e

f

g

h

i

9

8

10

11

12

13

14

15

Particle size (nm) j

k

l

m

n

a

b

c

d

e

f

--

--

--

--

--

--

--

Figure 10. TEM images of used Ni/Silicalite-1 (a) and Ni@Silicalite-1 (b) with the statistics particle size distribution bar charts (e and f) over used catalysts.



Figure 11. Representation of factors (reaction temperature and Ni size) limiting phenol hydro-treatment over Ni catalysts.


Page 34 of 37

Page 35 of 37 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


Scheme 1. Phenol reaction network over Ni catalysts in a board temperature range.



Page 36 of 37

Table 1. Textural properties of Ni/SiO2, Ni/Silicalite-1 and Ni@Silicaite-1 catalysts

Samples

Ni loading (XRF, %)

SBET (m2/g)

Smicro (m2/g)

Sext (m2/g)

Vmicro (cm3/g)

Vtotal (cm3/g)

Ni/SiO2

11.6

201.8

23.7

178.1

0.01

0.99

Ni/Silicalite-1

11.3

491.5

406.7

84.8

0.12

0.33

Ni@Silicalite-1

10.6

397.1

272.2

124.9

0.14

0.31

Silicalite-1

-

518.0

451.2

66.8

0.19

0.34


Page 37 of 37

(IV) ✓ Hydrogenolysis of hydroxyl

(II)

✓ High activity

➢ Hydrogenolysis of hydroxyl

✓ High selectivity of aromatics

➢ Low activity

✓ Better stability

➢ High selectivity of aromatics

Temperature (oC)

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


50 nm

(I)

Ni@S-1

(III)

➢ Hydrogenation of phenyl

➢ Hydrogenation of phenyl

➢ Low activity

➢ High activity

➢ High selectivity of alcohols

➢ High selectivity of alcohols

Minimizing Ni sizes (nm) Bio-based phenol is selectively upgraded to benzene over Ni@Silicalite-1 via thermodynamic ACS Paragon Plus Environment hindrance on phenyl/benzene

Hydrogenolysis

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