Layered MWW zeolite-supported Rh catalysts for the hydrodeoxygenation of lignin model compounds

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Layered MWW zeolite-supported Rh catalysts for the hydrodeoxygenation of lignin model compounds Ji Sun Yoon a,b,1 , Taehee Lee c,1 , Jae-Wook Choi a , Dong Jin Suh a,d,f , Kangtaek Lee b , Jeong-Myeong Ha a,e,f,∗ , Jungkyu Choi c,f,∗ a

Clean Energy Research Center, Korea Institute of Science and Technology, Seoul, 02792, Republic of Korea Department of Chemical and Biomolecular Engineering, Yonsei University, Seoul, 03722, Republic of Korea c Department of Chemical and Biological Engineering, Korea University, Seoul, 02841, Republic of Korea d Department of Green Process and System Engineering, Korea University of Science and Technology, Daejeon, 34113, Republic of Korea e Department of Clean Energy and Chemical Engineering, Korea University of Science and Technology, Daejeon, 34113, Republic of Korea f Green School (Graduate School of Energy and Environment), Korea University, Seoul, 02841, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 12 August 2016 Received in revised form 19 October 2016 Accepted 24 October 2016 Available online xxx Keywords: Hydrodeoxygenation 1,3,5-trimethoxybenzene Guaiacol Layered MWW zeolite-supported Rh catalysts MCM-22 MCM-36

a b s t r a c t The improved catalytic activity of Rh nanoparticles deposited on the swollen and pillared zeolites was observed in the hydrodeoxygenation (HDO) reactions of 1,3,5-trimethoxybenzene (1,3,5-TMB), a bulky lignin model compound, and guaiacol, one of the most frequently used lignin model compounds. As the high dispersion of metal nanoparticles increases HDO activity, the swelling/calcination and pillaring of crystalline MCM-22 zeolites increased the dispersion of Rh metal nanoparticles on the external surface area, and thus the corresponding HDO activity with respect to 1,3,5-TMB. On the contrary, although the mesopores in the amorphous MCM-41 and silica-alumina aerogel (SAA) supports accommodated higher Rh dispersion, overall, the resulting catalysts suffered from mass transfer limitation, and thus showed poor HDO reaction activity for 1,3,5-TMB. Finally, the Rh nanoparticles supported on the pillared zeolite showed the highest HDO activity for guaiacol, mainly due to the higher Rh dispersion and acid sites on the external surface among the zeolite-supported Rh catalysts. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Zeolites are crystalline solid acid catalysts that became popular in commercial chemical plants owing to their strong acidic catalytic activity and robust stability. Indeed, they have been used in many catalytic systems for applications including petrochemical processes, fine chemical production, and environmental processes [1–5]. The strong acidity generated on the stable crystalline zeolites is known to catalyze the chemical reactions commonly undertaken in petrochemical plants, such as isomerization, condensation, alkylation, and cracking [6–18]. In addition to these conventional applications of zeolites as single-component catalysts, they have also been employed as supports or components of multifunctional catalysts for the preparation of supported metal nanoparticle catalysts.

∗ Corresponding authors at Jungkyu Choi: Department of Chemical and Biological Engineering, Korea University, Seoul 02841, Republic of Korea and Jeong-Myeong Ha: Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea. E-mail addresses: [email protected] (J.-M. Ha), jungkyu [email protected] (J. Choi). 1 These authors equally contributed to this work.

Hydrodeoxygenation (HDO) is a catalytic process that converts oxygenates to deoxygenated hydrocarbons, frequently used for the upgrading of biomass-derived compounds to petroleum-like fuels and chemicals by removing oxygen atoms in the form of water, alcohols, and other oxygenates [19–24]. In general, solid acid catalysts contribute to the deoxygenation reaction with hydrogen atoms that are supplied via dissociative adsorption on a metal catalyst surface [19]. For the synthesis of these bifunctional catalysts, zeolites have been often selected as acidic components or supports to remove oxygen atoms by dehydration or dealcoholization in cooperation with hydrogen-adsorbing metal nanoparticle catalysts [25–27]. Although zeolites have been used as supports and/or active catalysts for the HDO of biomass-derived fragments [25,28,29], their micropores (less than ∼1 nm) are not able to host most biomass-derived reactants during the reaction, preventing the complete utilization of the acid sites inside the zeolites. Therefore, mesoporous analogues of zeolites have been developed in an attempt to overcome their limited accessibility [25,30–35]. However, conventional mesoporous supports, including MCM-41 [36], SBA-15 [37], and KIT-1 [38], are usually amorphous, resulting in lower acidity and catalytic activity compared to crystalline zeolites [39–42].

http://dx.doi.org/10.1016/j.cattod.2016.10.033 0920-5861/© 2016 Elsevier B.V. All rights reserved.

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MWW type zeolites and their derivatives can serve as model acid catalysts as well as supports for noble metal nanoparticles in bifunctional catalysts [30,43]. In particular, they are valuable for investigating any potential effects of the accessibility of reactants towards the acid sites on the apparent HDO reaction performance. Specifically, a layer-structured MCM-22(P; precursor) with intralayers can be converted to three-dimensional MWW type zeolites (MCM-22(C)), via condensation between two adjacent intra-layers during calcinations. Concomitantly, new inter-layers are created, while 12-membered ring (12-MR) supercages can be accessible through the 10-MR inter-layers. Interestingly, the intra-layers in MCM-22(P) can be further widened by swelling with a cationic surfactant and subsequent calcination or pillaring. Accordingly, the inter-layered structure containing 12-MR supercages is removed, while the 12-MR supercages are exposed. While the former zeolite is referred to as MCM-22(SC; swollen and calcined), the latter is known as MCM-36 [44]. Lignin is a complex polymer composed of aromatic compounds containing methoxy and hydroxyl functionalities as observed in 2-methoxyphenol (guaiacol) and 2,6-dimethoxyphenol (syringol). The effective HDO reaction of lignin is necessary for its upgrading towards highly valuable chemicals [45] and fuels [46,47]. In order simulate the high complexity of lignin with oxygenate functionalities and aromatic phenyl groups, mono-phenolic oxygenates such as guaiacol, anisole, vanillin, and eugenol, were used as model compounds of lignin fragments in the HDO. These simplified HDO reactions using such model compounds provided important insights into the HDO of lignin-derived small molecules [19–21,23,24,48]. In this study, MCM-22(C), MCM-22(SC), and MCM-36 zeolites are employed as acid catalyst and supports for the preparation of catalysts, which are used for the HDO of lignin-derived reactants. In addition, Rh is selected as an active metal catalyst because it exhibited the highest deoxygenation performance in the HDO reaction of guaiacol in our previous study [24]. The modification of nanostructures by swelling and pillaring of MCM-22(P) may adjust acid properties and/or metal dispersions in the final bifunctional catalysts and accordingly, the catalytic HDO activity because the HDO of lignin-derived molecules depends on both acidic properties of supports and hydrogen-supplying ability of metals. By increasing a non-microporous external surface area through a swelling process, the acid sites on the external surface, involved in the HDO reaction, can be easily accessed by reactants. Thus, the increased accessibility towards the external surface in MCM-22(SC) and MCM-36, as compared to that in MCM-22, is expected to account for the improved HDO reaction performance. In order to prove this, the HDO of model compounds of lignin monomers, namely 1,3,5-trimethoxybenzene (1,3,5-TMB) and guaiacol was performed. To this end, Rh nanoparticles were impregnated on these three types of supports in order to form bifunctional HDO catalysts. First, their catalytic performance was examined in the conversion of bulky 1,3,5-TMB, which has only three methoxy groups, thus allowing to evaluate its accessibility in the MCM-22 support and its derivatives and the HDO activity. In addition, mesoporous, non-crystalline MCM-41 and silica-alumina aerogel (SAA) were used as acid catalysts together with Rh nanoparticles in order to compare their HDO activity with that of Rh nanoparticles on MCM-22 and its derivatives. Specifically, MCM-41 is an amorphous material with hexagonally placed unidirectional mesopores, while crystalline MCM-22 and its crystalline derivatives contain bidirectional sinusoidal microporous channels of 10-MR in the intra-layers with different degree of the interlayer spacing. Finally, the Rh nanoparticles deposited on the three zeolite supports were further used to investigate their activity in the HDO of guaiacol, which contains both methoxy and hydroxyl groups.

2. Experimental 2.1. Materials Tetraethyl orthosilicate (TEOS, 98%), propylene oxide (PO, 99%), n-decane (99%), 1,3,5-trimethoxybenzene (1,3,5-TMB, 99%), rhodium (III) chloride hydrate (RhCl3 ·xH2 O, 38–40 wt% Rh), sodium aluminate (NaAlO2 ; 55% Al2 O3 , 45% Na2 O), hexamethyleneimine (HMI), cetyltrimethylammonium bromide (CTAB), sodium hydroxide (NaOH, 98%), and mesostructured aluminosilicate (MCM-41) were purchased from Sigma-Aldrich (Milwaukee, Wisconsin, USA). Tetrapropylammonium hydroxide (TPAOH, 40% aq.) was purchased from Alfa Aesar (Ward Hill, MA, USA). Fumed silica (SiO2 ; Cab-OSil M5) was purchased from Cabot (Boston, MA, USA). Methanol (extra pure grade, 99.5%) and 1-pentanol (98%) were purchased from Dae-jung (Seoul, Republic of Korea). Aluminum (III) chloride hexahydrate (AlCl3 ·6H2 O, 97%) and nitric acid (HNO3 , 69–70%) were purchased from Junsei (Tokyo, Japan). Deionized (DI) water (18.2 M cm) was prepared using the aqua MAX Ultra 370 Series water purification system (Young Lin Instrument, Anyang, Republic of Korea). Helium (99.999%), hydrogen (99.999%), and air cylinders were purchased from Shinyang Sanso (Seoul, Republic of Korea). Argon (99.999%) and nitrogen (99.999%) cylinders were purchased from Sungkang Gas (Seoul, Republic of Korea).

2.2. Preparation of catalysts MCM-22(C) was synthesized as reported in the literature [49]. HMI (16.2 g) and fumed silica (19.9 g) were added to the solution of NaAlO2 (0.59 g) and NaOH (2.03 g) in DI water (261.3 g). The mixture was stirred overnight at room temperature; it was then placed into a Teflon-lined, stainless steel autoclave and reacted at 135 ◦ C for 11 d. The resulting crystalline product (MCM-22(P)) was dried at 80 ◦ C overnight after being washed several times with DI water. MCM-22(C) was obtained through calcination of MCM-22(P) at 450 ◦ C for 12 h under air. MCM-22(SC) was synthesized by swelling MCM-22(P) as described in the literature [50] and subsequent calcinations. CTAB (10.15 g) and TPAOH (aq., 11 g) were dissolved in DI water (64.2 g, the amount of water was doubled in comparison with that reported in the literature). MCM-22(P) powder (1.8 g) was added to the solution, and the pH was maintained at 13.5. The mixture was then stirred for 16 h at room temperature. The swollen material (swollen MCM-22(P); MCM-22(S)) was dried at 80 ◦ C overnight after being collected by centrifugation. MCM-22(SC) was obtained by conducting calcination of MCM-22(S) at 550 ◦ C for 12 h under air. MCM-36 was synthesized by pillaring MCM-22(S) as described in the literature [50]. After swelling at room temperature, MCM22(S) was centrifuged and washed 10 times with DI water, followed by overnight drying at 80 ◦ C. The dried swollen material (1.0 g) was added to TEOS (4.0 g) and stirred for 25 h at 90 ◦ C under Ar. After that, the product was recovered by filtering and dried at room temperature. The dried powder was further hydrolyzed with an aqueous NaOH solution (10.0 g, pH ∼8) for 6 h at 40 ◦ C. The hydrolyzed product was obtained by centrifugation and dried overnight at room temperature. Finally, the recovered product was calcined at 450 ◦ C under a N2 flow for 6 h and at 550 ◦ C under air for 12 h. The SEM images and XRD patterns of MCM-22(P), MCM22(C), MCM-22(SC), and MCM-36 are shown in Figs. S1 and S2 in the Supporting information, respectively. The morphologies of these particles were almost comparable under the SEM resolution. The XRD analysis confirmed the synthesis of MCM-22(C), while MCM22(SC) exhibited a disordered stacking in the direction of the c-axis. The poor signal-to-noise ratio and broadening of the XRD pattern of MCM-36 suggests structural damage to some extent.

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50 ◦ C. The BA and LA sites were assigned at 1550 and 1450 cm−1 , respectively. 2.4. HDO reactions

Scheme 1. Schematic structures of MCM-22(C), MCM-22(SC), and MCM-36 along with those of MCM-41 and SAA.

In addition to MCM-22 and its derivatives, silica-alumina aerogel (SAA, Al/(Si + Al) = 0.01 (mol/mol)) was synthesized by sol-gel method as described in the literatures [20,23]. Schematics for the supports used in this study are illustrated in Scheme 1. Rh (∼3 wt%) was impregnated on MCM-22(C), MCM-22(SC), MCM-36, MCM-41, and SAA supports by using an incipient wetness method. After drying for 1 d at 105 ◦ C under ambient conditions, the catalysts were reduced under a H2 flow at 350 ◦ C for 4 h. The prepared catalysts were further ground and sieved to form pellets smaller than 75 ␮m.

2.3. Characterization The composition of the catalysts was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES; Polyscan-61E, Thermo Electron Corp., Winsford, Cheshire, UK). The ICP-AES results of the prepared catalysts are summarized in Table 1. CO-chemisorption was carried out using a pulse injection of CO (10% CO/He) (BETCAT-B, BELCAT, Osaka, Japan) equipped with a thermal conductivity detector (TCD) as previously described [23]. Scanning electron microscopy (SEM) images were obtained by using a Hitachi S-4300 instrument. The surfaces of samples were Pt-coated via ion sputtering (Hitachi E-1030) with 20 mA for ∼60 sec. X-ray diffraction (XRD) patterns were obtained by using a Rigaku Model D/Max-2500V/PC diffractometer (Japan) with Cu K␣ radiation (40 kV, 100 mA, ␭ = 0.154 nm). High-angle annular darkfield (HAADF) images were obtained via a scanning transmission electron microscope (STEM) using a Tecnai G2 F20 (FEI, Hilsboro, OR, USA). Prior to N2 isotherm measurements, the samples were degassed at 250 ◦ C for 5 h. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method and the micropore surface area was determined by the t-plot method. The total pore volume was determined through DFT calculations. The quantities of Lewis (LA) and Brønsted acid (BA) sites were titrated by using pyridine based on a FT/IR-4100 type A spectrometer (JASCO International Co. Ltd., Tokyo, Japan) equipped with a MCT-M detector at a frequency range of 1400–1600 cm−1 with a resolution of 4 cm−1 , as previously described [23]. A self-pelletized catalyst (0.02–0.03 g) was evacuated at 250 ◦ C for 3 h to remove any physisorbed moisture on the catalyst in an IR cell equipped with a CaF2 window. Prior to the pyridine adsorption process, the baseline was already recorded for the evacuated catalyst pellet. Subsequently, pyridine vapor was adsorbed onto the catalyst surface in the IR cell at 50 ◦ C for 10 min and the IR cell was evacuated at 150 ◦ C for 1 h. The FT-IR spectra were acquired after the cell temperature was decreased to

The HDO reaction of 1,3,5-TMB (0.2 M, 40 mL) or guaiacol (0.45 M, 40 mL) in n-decane was performed in a 160-mL Hastelloy C® -276 batch reactor in the presence of Rh/MCM-22(C), Rh/MCM22(SC), and Rh/MCM-36 catalysts to investigate the effect of zeolites in bifunctional catalysts on the HDO performance along with Rh/MCM-41 and Rh/SAA. The amounts of catalysts used were 0.01 and 0.02 g for 1,3,5-TMB and guaiacol, respectively. Prior to the reaction, the batch reactor was filled with H2 (40 bar) at room temperature and the reaction was conducted at 250 ◦ C for 80 min. The products, identified by using a GC–MS (gas chromatograph mass spectrometer; Agilent Technologies, 7890A with an HP-5 capillary column (60 m × 0.25 mm × 0.25 m)), were quantified by using a gas chromatograph (Younglin, YL6500 with an HP-5 capillary column (60 m × 0.25 mm × 0.25 ␮m) equipped with a flame ionization detector (FID). The conversion and selectivity of the reactions were defined by the following equations: Conversion(%) = Selectivity(%) =

C i - Cf Ci A

i

× 100, × 100,

Ai

i

where Ci and Cf are the moles of 1,3,5-TMB or guaiacol before and after the reaction, respectively, and Ai is the product area of the i-th species. The oxygen removal that describes the fraction of deoxygenation was defined as [(Selectivity of 0-Os) + (Selectivity of 1-Os) × 2/3 + (Selectivity of 2-Os) × 1/3] × (Conversion). The yield of the compounds can be calculated by multiplying conversion and selectivity. 3. Results and discussion 3.1. Pore structure of zeolites Prior to performing the HDO reaction, the pore structure of Rh/MCM-22(C), Rh/MCM-22(SC), Rh/MCM-36, Rh/MCM-41, and Rh/SAA was characterized by using N2 -physisorption isotherms (Figs. 1 and S3 ). The corresponding BET, micropore, and external surface areas are summarized in Table 2. All the zeolite-supported Rh (Rh/zeolite) catalysts, including Rh/MCM-22(C), Rh/MCM22(SC), and Rh/MCM-36, exhibited micropores (< 2 nm) with negligible mesopores, except Rh/MCM-36 which contained some mesopores (∼2–4 nm). The observed mesopores of Rh/MCM36, apparently created by silica pillars between the intra-layers in MCM-22, were confirmed by the presence of hysteresis at p/p0 = ∼0.4 in the N2 adsorption-desorption isotherms and DFTcalculated pore size distributions (Fig. 1), while Rh/MCM-22 and Rh/MCM-22(SC) exhibited little or none of these features. Among the Rh/zeolite catalysts, Rh/MCM-22(C) exhibited the largest micropore surface area with the smallest external surface area. In contrast, Rh/MCM-36, prepared by pillaring of MCM-22(P), exhibited the smallest BET surface area because of the much reduced micropore surface area (Table 2). In addition, MCM-22(SC) exhibited the smallest pore volume seemingly because of disordered inter-layers via calcination of MCM-22(S). Compared to the Rh/zeolite catalysts, Rh deposited on mesoporous MCM-41 (Rh/MCM-41) and SAA (Rh/SAA) exhibited considerably larger external and mesopore surface areas with a negligible microporous surface area (Table 2). Specifically, Rh/MCM-41 exhibited negligible micropores but uniform, pronounced mesopores of

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Table 1 Al content of porous supports obtained from the ICP-AES characterization and the content, dispersion, and particle size of Rh nanoparticles on the supports.

Al/(Si + Al) (10−2 mol/mol) Rh (wt%) [CO]/[Rh] (mol/mol) Calculated particle size (nm)

MCM-22(C)

MCM-22(SC)

MCM-36

MCM-41

SAA

1.8

1.6

2.3

2.8

1.0

Rh/MCM-22(C) 2.7 0.27

Rh/MCM-22(SC) 2.8 0.41

Rh/MCM-36 3.0 0.44

Rh/MCM-41 2.9 0.53

Rh/SAA 3.0 0.75

4.1

2.7

2.5

2.1

1.5

Fig. 1. Pore size distributions of Rh/solid acid catalysts (Rh/MCM-22(C), Rh/MCM-22(SC), Rh/MCM-36, Rh/MCM-41, and Rh/SAA) by using (a) the BJH method (desorption branch) and (b) DFT calculations.

Table 2 Pore structure of Rh/solid acid catalysts measured by N2 physisorption. Catalysts

SBET (m2 /g)a

Smicro (m2 /g)b

Sext (m2 /g)c

Sext /SBET

Vpore (cm3 /g)d

Dpore (nm)e

Dpore (nm)f

Rh/MCM-22(C) Rh/MCM-22(SC) Rh/MCM-36 Rh/MCM-41 Rh/SAA

626 368 366 792 763

616 329 274 7 80

10 39 92 785 683

0.016 0.11 0.25 0.99 0.90

0.28 0.19 0.22 0.67 0.75

0.8, 1.3, 1.7 0.8, 1.4 0.8, 1.4, 1.7, 2.5 1.4, 1.7, 3.9 1.6, 2.1, 4.5, 5.0, 6.7

– 3.9 3.9 2.9, 3.8 4.3

a b c d e f

BET surface area. Micropore surface area measured by using the t-plot analysis. External surface area measured by subtracting the micropore surface area from the BET surface area. Total pore volume measured by using DFT calculations. Peak pore width measured by using DFT calculations. Peak pore width measured by using the BJH method based on the desorption branch.

approximately ∼3–4 nm, while Rh/SAA exhibited a broad pore width distribution of ∼2–10 nm that reflects the mesopores of SAA (Fig. 1). It was noted that the fraction of external surface areas, Sext /SBET , increased with the order Rh/MCM-22(C) « Rh/MCM22(SC) < Rh/MCM–36 « Rh/SAA < Rh/MCM-41 (Table 2). 3.2. Particle size of Rh nanoparticles and acid titration in supported Rh catalysts The quantity of active metal atoms has been reported to improve the HDO activity [24]. From the HAADF-STEM mea-

surements, the sizes of the Rh particles in Rh/MCM-22(C), Rh/MCM-22(SC), Rh/MCM-36, Rh/MCM-41, and Rh/SAA were estimated to be 3.5 ± 0.8 nm, 3.4 ± 0.7 nm, 2.9 ± 0.6 nm, 2.2 ± 0.5 nm, and 1.8 ± 0.4 nm, respectively (Fig. S4 and Table 1). This reveals that the Rh particles in the Rh/zeolite catalysts were larger than those supported on mesoporous MCM-41 and SAA. Larger Rh particles, or smaller Rh dispersion, of the Rh/zeolite catalysts were also confirmed by CO chemisorption (Table 1). In general, it appears that the Rh dispersion increased with the increasing Sext /SBET ratio (Fig. S5 and Tables 1 and 2). The Rh dispersion of Rh/MCM-41 and Rh/SAA, which exhibited the very high mesopore surface area, was

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Fig. 2. HDO reaction results of 1,3,5-TMB on Rh/MCM-22(C), Rh/MCM-22(SC), Rh/MCM-36, Rh/MCM-41, and Rh/SAA.

larger than those of the Rh/zeolite. These observations indicate that the micropores of MCM-22(C), MCM-22(SC), and MCM-36 were too small to host the Rh nanoparticles. Instead, the majority of Rh particles were likely to exist on the external surface of these zeolite supports, while the pronounced mesopores of MCM-41 and SAA allowed the deposition of the Rh nanoparticles. Among the Rh/zeolite catalysts, MCM-36, which apparently had some degree of mesopores due to pillaring, accommodated the deposition of Rh particles on the external surface area and accordingly, the particle sizes of Rh estimated from both TEM and CO chemisorption analyses were smaller than those of Rh/MCM-22(C) and Rh/MCM-22(SC) (Fig. S4 and Table 1). In addition, the acidity of the catalysts, which also have been reported to improve the HDO activity [19], was measured by analyzing the FT-IR spectra with pyridine (Fig. S6 and Table 3). All the Rh/zeolite catalysts exhibited a large quantity of Brønsted acid sites, while Rh/MCM-41 and Rh/SAA contained a smaller degree of both BA and LA sites. Among the Rh/zeolite catalysts, Rh/MCM-36 showed a little reduced amount of both BA and LA sites compared to those in Rh/MCM-22(C) and Rh/MCM-22(SC), suggesting that the pillaring process with silica led to a decrease of the specific acid sites with the same degree of reduction for both BA and LA sites.

3.3. HDO reaction of 1,3,5-TMB The HDO reaction of 1,3,5-TMB was performed using the supported Rh catalysts (Fig. 2). The conversion of 1,3,5-TMB decreased in the order Rh/MCM-36, Rh/MCM-22(SC), Rh/MCM-41, Rh/SAA, and Rh/MCM-22(C). In addition the selectivity towards the fully deoxygenated products (0-Os) decreased in the order Rh/MCM-36, Rh/MCM-22(SC), Rh/MCM-22(C), Rh/MCM-41, and Rh/SAA. These results reveal the better deoxygenation activity of the Rh/zeolite catalysts. For the Rh/MCM-41 and Rh/SAA catalysts, the 1,3,5-TMB conversion and the selectivity towards 0-Os were slightly larger for Rh/MCM-41 (containing ordered uniform mesopores) than for Rh/SAA (containing random mesopores). This can be ascribed to the better mass transfer in the uniform and less tortuous cylindrical pores of the MCM-41 support. Compared to MCM-41, SAA contains complex networked non-uniform pores that retard the mass transfer rate of reactants and/or products and accordingly, suppress the overall HDO reaction. In addition, the limiting pore aperture in Rh/SAA mainly due to the lower bound of its wide pore size distribution will be smaller than in Rh/MCM-41. Although Rh/SAA exhibited a larger Rh dispersion and a greater quantity of total acid sites than Rh/MCM-41, the smaller conversion on Rh/SAA indicates that the slow mass transfer rate in the Rh/SAA catalyst is a key factor and in this case decreases the overall HDO reaction rate.

Fig. 3. Plausible pathway of HDO of 1,3,5-TMB (1,3,5-TMCH: 1,3,5trimethoxycyclohexane, 3,5-DMPhOL: 3,5-dimethoxyphenol, 3,5-DMCHOL: 3,5-dimethoxycyclohexanol, 3,5-DMB: 3,5-dimethoxybenzene, 3,5-DMCH: 3,5dimethoxycylcohexane, MB: methoxybenzene, MCH: methoxycyclohexane, CHON: cyclohexanone, CHOL: cyclohexanol, CHE: cyclohexene, CH: cyclohexane).

The products of the HDO of 1,3,5-TMB were classified into molecules containing three oxygen atoms (3-Os), two oxygen atoms (2-Os), one oxygen atom (1-Os), and no oxygen atom (0-Os). Cyclohexane (CH, 0-O), methoxycyclohexane (MCH, 1O), cyclohexanol (CHOL, 1-O), dimethoxybenzenes (DMBs, 2-O), dimethoxycyclohexanes (DMCHs, 2-O), and trimethoxycyclohexane (TMCH, 3-O) were frequently observed as major products (Fig. 3 and Table S1). A plausible pathway for the HDO of 1,3,5TMB is depicted in Fig. 3. Hydrogenation of the phenyl rings and demethanolization are the major reaction steps. While previous studies using supported noble metal catalysts reported that the phenyl rings were saturated and then the oxygen functionalities were hydrodeoxygenated [19–21,23,24], the HDO pathways of 1,3,5-TMB were more complex; the demethanolization or the substitution of methoxy with hydroxyl groups were observed before the hydrogenation of phenyl rings occurred. The compounds whose rings were saturated were further demethanolized or deoxygenated to produce fully deoxygenated cyclohexene or cyclohexane products. In this study, two descriptors were selected to analyze the reaction results in terms of catalyst structure; (1) the accessibility of 1,3,5-TMB to the active sites of Rh particles and (2) the exposed acid sites in the pores. These two factors were considered since the HDO reaction is likely to be mainly based on the hydrogenation of 1,3,5-TMB on the Rh particles and the dehydration/dealkoxylation

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Table 3 Estimated number of acid sites titrated by using pyridine through FT-IR analysis. Catalysts

Brønsted acid (mmol/gcatalyst )

Lewis acid (mmol/gcatalyst )

(Brønsted acid)/(Lewis acid)

Total (mmol/gcatalyst )

Rh/MCM-22(C) Rh/MCM-22(SC) Rh/MCM-36 Rh/MCM-41 Rh/SAA

0.12 0.12 0.10 0.04 0.06

0.06 0.06 0.04 0.02 0.02

2.0 2.0 2.5 1.5 3.7

0.18 0.18 0.14 0.06 0.08

Fig. 4. Catalytic activity as a function of (a) the dispersion of Rh particles ([CO]/[Rh]) and (b) the number of total acid sites. The results for the Rh/zeolite catalysts are marked by red symbols. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

on the acid sites [19,20,24]. The high dispersion of the Rh particles facilitates the dissociative adsorption of hydrogen molecules on the Rh surface and the adsorbed hydrogen atoms initiate the conversion of 1,3,5-TMB. Therefore, we compared the Rh dispersion with the catalytic activity described by the conversion of 1,3,5-TMB, the yield of 0-Os, and the oxygen removal (Fig. 4a). While a general trend was not observed for all the five catalysts, it was found that the catalytic activity increased with the increasing [CO]/[Rh] in the Rh/zeolite catalysts. Notably, Rh/MCM-41 and Rh/SAA did not exhibit a better catalytic activity in spite of their higher [CO]/[Rh]. In addition to the active sites of the Rh particles, we also attempted to associate the catalytic activity with the number of acid sites in the catalysts (Fig. 4b). No reliable general correlation between the HDO reaction activity and the number of acid sites could be found. Interestingly, Rh/MCM-22(C) and Rh/MCM-22(SC), which possess the largest number of acid sites (Table 3), did not exhibit the best catalytic activity, while the most active catalyst Rh/MCM-36 contained a moderate number of acid sites. The results in Fig. 4 indicate that the catalytic HDO activity cannot be determined by a single descriptor, but rather by a combination of several factors. Therefore, in addition to the quantities of surface Rh atoms and acid sites, the pore structure of the catalyst supports was further investigated as a possibly additional important factor. All MWW type zeolites are microporous crystals, and MCM-22(SC) and MCM-36 include imperfect structures because of swelling/calcination and pillaring, respectively [49,51]. As mentioned above, the formation of Rh nanoparticles embedded in the micropores was unlikely to occur, and thus most Rh particles would exist on the external surface and mesopores of zeolites. Both the HAADF-STEM images and CO chemisorption results of the Rh/zeolite catalysts confirmed the formation of Rh particles whose diameters were ∼2.9–3.5 nm (by HAADF-STEM) and ∼2.5–4.1 nm (by CO chemisorption), i.e., considerably larger than their micropores of Rh/zeolite catalysts (Fig. S4 and Table 1). Therefore, the Rh/zeolite catalysts only allowed the formation of Rh nanopar-

ticles on the external and/or mesoporous surface of MWW type zeolite supports. Even though a small fraction of Rh particles were present in the microporous region, the micropores (∼0.4–0.55 nm) in the intra-layers of MCM-22 zeolites were unlikely to accommodate the bulky reactant (1,3,5-TMB) and products (e.g., kinetic diameter of cyclohexane: 0.6 nm), suppressing the HDO reaction. On the contrary, the mesopores of MCM-41 and SAA possibly allowed for embedding Rh nanoparticles whose diameters were ∼1.8–2.2 nm (by HAADF-STEM) or ∼1.5–2.1 nm (by CO chemisorption), as expected from the larger mesopore size than the estimated size of the supported Rh nanoparticles. Therefore, the Rh dispersion in the Rh/zeolite catalysts was smaller than that in the Rh/MCM-41 and Rh/SAA catalysts. Based on these observations, the activity of the Rh/zeolite catalysts need to be separately discussed from that of Rh/MCM-41 and Rh/SAA. For the activity of Rh/zeolite catalysts, the conversion of 1,3,5-TMB, the yield of 0-Os, and the oxygen removal increased with the increasing quantity of exposed surface Rh atoms as shown by our previous results using carbon supports [24], while the acidity in Table 3 did not clearly account for the HDO activity of the Rh/zeolite catalysts. Although the Rh dispersion appears to determine the HDO activity of the Rh/zeolite catalysts, the HDO activities of Rh/MCM41 and Rh/SAA, whose Rh dispersions are larger than those of the Rh/zeolite catalysts, were worse compared to the Rh/zeolite catalysts. Since the HDO activity of Rh/MCM-41 and Rh/SAA was rather comparable to that of Rh/MCM-22(C), whose acidity is much higher than that of Rh/MCM-41 and Rh/SAA, the acidity could also not describe the catalytic activity of Rh/MCM-41 and Rh/SAA. Because both Rh dispersion and acidity cannot fully account for the difference in HDO activity between Rh/zeolite and other catalysts, an additional descriptor, namely the pore structure, was proposed. The micropores of the Rh/zeolite catalysts cannot accommodate Rh nanoparticles, suppressing the reaction inside the micropores and the HDO reaction seemingly occur at the Rh nanoparticles formed on the external surface of these zeolite sup-

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ports and in the mesopores between the intra-layers. Compared to that, the HDO reaction on Rh/MCM-41 and Rh/SAA would occur at the Rh nanoparticles formed at the mesopores of MCM-41 and SAA, whose predominant mesopores can accommodate the deposition of Rh nanoparticles and also allow for the diffusion of reactants and products. Considering the possibility that the diffusion rates are significantly decreased in the mesoporous regime, the final HDO activity was decreased because of the prohibited diffusion to and from the deposited Rh nanoparticle in the unidirectional (Rh/MCM41) and the disordered (Rh/SAA) mesoporous structures; however, the enhanced HDO activity of Rh/MCM-36 could be ascribed to the formation of Rh metal nanoparticles on the external surface area. Specifically, considering that the sizes of the uniform mesopore in Rh/MCM-41, Rh nanoparticles in Rh/MCM-41, and 1,3,5-TMB are estimated to be ∼3 nm (Fig. 1a), ∼2.1–2.2 nm (Fig. S4 and Table 1), and ∼1 nm (a possible maximum length), respectively, the diffusion of 1,3,5-TMB along the corresponding mesopores that contain hydrogen atom adsorbed Rh nanoparticles would be highly inhibited. Similarly, Rh nanoparticles (∼2.5–3.4 nm) present in the mesopores (∼4 nm from Fig. 1a) in both Rh/MCM-22(SC) and Rh/MCM-36 would not contribute to the HDO reaction considerably. Nevertheless, we cannot completely rule out a contribution of some Rh nanoparticles in the two-dimensional mesopores of MCM-36 to the overall HDO reaction, in contrast with those in the one-dimensional mesopores of MCM-41. Under this circumstance, it appears that for the Rh/zeolite catalysts, Rh nanoparticles on the external surface, not in the mesopores between the adjacent intralayers, primarily contributed to the catalytic activity of the HDO reaction. Although an increase in the HDO reaction time will be beneficial for increasing the yield towards the fully-deoxygenated compounds [23,52], the comparison of the catalytic activities after a given reaction time (here, 80 min) suggested the desirable use of a zeolite support with a high external surface area.

3.4. HDO reaction of guaiacol On the basis of the understanding of the model reaction based on 1,3,5-TMB that contains only methoxy groups, the HDO of guaiacol, one of the most frequently used model compounds of lignin and lignocellulose fragments, was also performed with the three Rh/zeolite catalysts that held a linear correlation between the HDO reactivity of 1,3,5-TMB and the Rh dispersion (Fig. 4a). As expected, Rh/MCM-36 exhibited the best HDO activity affording the highest yield of 0-Os, while the difference in the HDO activity between Rh/MCM-22(C) and Rh/MCM-22(SC) was rather insignificant rela-

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tive to the HDO reaction of 1,3,5-TMB shown above (Fig. 5 and Table S2). This suggests that the combined Rh dispersion/acid distribution/pore structure effect plays an important role in determining the overall HDO activity with respect to guaiacol, which includes both methoxy and hydroxyl groups. Nevertheless, among the three Rh/zeolite catalysts used in this study, the MCM-36-supported Rh catalyst showed the best performance for the HDO reaction of lignin model compounds, indicating that its structural configuration through the pillaring of MCM-22(P) was optimal for suitable bifunctional catalysts. We recognized that the degree of deoxygenation of guaiacol (Fig. 5) was much lower than that of 1,3,5-TMB (Fig. 2) for all three Rh/zeolite catalysts. The marked conversions, comparable among the three catalysts, indicate that the hydrogenation effectively occurred at the active sites of the Rh nanoparticles, though the subsequent demethanolization was largely prohibited based on a previous study [19]. Considering that the demethanolization reaction for 1,3,5-TMB occurred even before saturation of the benzene ring (Fig. 3), we hypothesize that the demethanolization reaction of guaiacol requires stronger and/or more acid sites than that of 1,3,5-TMB. Assuming that the acid sites were uniformly distributed, their numbers on the external surface area including mesoporous area in the Rh/zeolite catalysts would be in the order Rh/MCM-36 (∼0.035 mmol/gcatalyst ) > Rh/MCM-22(SC) (∼0.02 mmol/gcatalyst ) » Rh/MCM-22(C) (∼0.003 mmol/gcatalyst ). The much improved HDO activity observed for Rh/MCM-36 strongly suggests the high sensitivity of acid sites on the overall HDO reaction activity.

4. Conclusions The dispersion of Rh nanoparticles, quantity of acid sites, and mass transfer of reactants and products at the mesopores of these supported heterogeneous catalysts contributed to determining their overall catalytic activity in the HDO of 1,3,5-TMB. The HDO activity of the corresponding Rh/zeolite catalysts was primarily determined by the Rh dispersion. Thus, Rh/MCM-36, which possessed the highest Rh dispersion value, showed the highest HDO activity. On the contrary, Rh/MCM-41 and Rh/SAA, whose Rh dispersion was much larger than that of the Rh/zeolite catalysts, exhibited the low HDO activity seemingly due to the poor mass transfer in both the unidirectional and disordered mesopores where the active Rh nanoparticles were predominantly placed. As for guaiacol, the number of acid sites on the external surface area along with the Rh dispersion presumably played an important role in the HDO reaction. The Rh/MCM-36 catalyst, which had the high-

Fig. 5. HDO reaction results of guaiacol on Rh/MCM-22(C), Rh/MCM-22(SC), and Rh/MCM-36.

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est Rh dispersion value and number of acid sites on the external surface area showed the highest HDO activity among the Rh/zeolite catalysts. These observations suggest that an optimal combination of structural change, Rh dispersion, and acidity is critical for improving the HDO activity of these supported Rh catalysts.

Acknowledgments This work was supported by the National Research Council of Science & Technology (NST) grant by the Korea Government (MSIP) (No. CAP-11-04-KIST). This research was also supported by the Basic Science Research Program (No. 2015R1A1A1A05027663) and by the Super Ultra Low Energy and Emission Vehicle Engineering Research Center (SULEEV ERC)(No. 2016R1A5A1009592) through the National Research Foundation of Korea (NRF) funded by the Korea government (Ministry of Science, ICT & Future Planning).

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2016.10. 033.

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