Mesoporous Silicates

Article pubs.acs.org/EF

Hydrodeoxygenation of Guaiacol over MoO3‑NiO/Mesoporous Silicates: Effect of Incorporated Heteroatom Marimuthu Selvaraj,† Kannan Shanthi,*,† Rajamanickam Maheswari,† and Anand Ramanathan*,‡ †

Department of Chemistry, Anna University, Chennai 600025, India Center for Environmentally Beneficial Catalysis (CEBC), The University of Kansas, Lawrence, Kansas 66047, United States

‡

ABSTRACT: Ordered mesoporous silicate SBA-15 and Ti-SBA-15 (Si/Ti = 10) and mesoporous aluminosilicate MAS (Si/Al = 20) supports were prepared by a hydrothermal method. Peptization of γ-alumina with these supports was carried out with 2% acetic acid, and the resulting composite materials were loaded with 3 wt % NiO and 10 wt % MoO3 by a simple impregnation method. The physicochemical properties of these catalysts were examined with the following techniques, such as XRD, N2 sorption, diffuse reflectance UV−vis spectra, TEM, H2-TPR, NH3-TPD, and H2 chemisorption. Ti and Al incorporated supported catalysts strongly influence the hydrodeoxygenation (HDO) activity of guaiacol, investigated in a fixed-bed down-flow quartz reactor at atmospheric pressure. The MoO3-NiO/MAS supported catalyst shows maximum conversion compared to MoO3-NiO/Ti-SBA-15 and MoO3-NiO/SBA-15. γ-Alumina composite catalysts displayed an improved HDO activity attributed to an increased acidity and dispersion of active sites. under low H2 pressures.20−24 In these systems, the actual catalyst is MoO3 supported on high surface area oxides, whereas Ni promotes the catalytic action of Mo by creating a suitable oxidation state of Mo and an oxygen vacant site for hydrogenolysis reaction. On the other hand, there are some reports that deal with nonsulphided catalysts, such as Pt,25 Pd,26 MoO3-NiO/γ-Al2O3,27 and Ni-Cu/Al2O3,28 on neutral and acidic supports for the HDO process. The role of supports plays a crucial role in the HDO activity and selectivity.29 Mesoporous silicates and aluminosilicates supports were shown to be active compared to conventional catalysts for hydrotreating reactions.30,31 In addition, the uniform and larger pore diameter of these supports facilitates relatively much easier diffusion of substrates compared to conventional catalysts. The role of SBA-15 supports for hydrodesulfurization has been reviewed recently.32 The activity of NiMo catalysts is greatly enhanced by the presence of heteroatoms, such as Al and Ti.32,23 Taking these into account, we have prepared three supports, SBA-15, Ti-SBA-15, and mesoporous aluminosilicate (MAS). Composite materials of these supports with γ-alumina were also prepared. The active species NiO and MoO3 were impregnated with the support and the composite materials. These catalysts were then subjected to HDO reaction of guaiacol in a fixed-bed reactor at different temperatures under atmospheric pressure. The HDO activity was correlated with the physicochemical properties of catalysts.

1. INTRODUCTION Depletion of fossil fuels triggered the search for alternate fuels to counterbalance the energy gap creation and demand. Biomass is one of the possible resources that can provide viable solutions to fulfill the energy requirements. Biomass derived fuels have more advantages over conventional petroleum fuels as they are renewable and sustainable in nature.1−4 Direct utilization of biomass is not possible due to high oxygen content (usually 35−40 wt %) in the pyrolysis oil resulting in a low energy density.5 Hence, deoxygenation of biomass derived oil is mandatory for further processing as fuels, value added products, and chemical resources.6−9 Generally, thermo-mechanical and thermo-chemical pathways are used for the conversion of lignocellulose biomass to pyrolysis oil, though the thermo-mechanical route deconstructs the biomass into intermediates like bio-oils at high temperature and/or pressure, and this route is unsuitable because of its complex nature and high cost. In contrast, the thermo-chemical method consists of two processes as chemical and catalytic upgrading and favors for the conversion of biomass.10 Catalytic upgrading of biomass involves removal of oxygen and C−C bond formation, which are the deciding factors for the product formation. The investigation of the HDO process over noble metals like Pt-, Pd-, and Ru-supported catalysts showed excellent activities at high pressure.11−13 However, the cost and high pressure requirements limit its application in the scale-up processing and leads one to seek an alternative catalyst. In petroleum industries, sulphided CoMo or NiMo on Al2O3 is mostly used as a catalyst for the effective removal of N and S from fossil fuels.14−16 These sulphided catalysts are not promising for the HDO process due to modification of the sulphide structure by oxygen interaction, resulting in rapid deactivation of the catalyst.17−19 Nevertheless, sulphided and reduced MoO3-NiO supported catalysts were shown to be active for conversion of biomass containing esters, phenolics, cyclic ketones, and aldehydes © 2014 American Chemical Society

2. MATERIALS AND METHODS 2.1. Materials. Polyethylene glycol (PEG-4000), Pluronic P123 (EO20PO70EO20, Mw = 5800), and tetrapropyl orthotitanate (C12H28O4Ti, 97%) were purchased from Sigma Aldrich. Cetyltrimethylammonium bromide (CTAB, 98%) was purchased from Loba Chemie. Hydrochloric acid (37%), aluminum nitrate (Al(NO3)3·9H2O, 96%), Received: December 16, 2013 Revised: March 17, 2014 Published: March 17, 2014 2598

dx.doi.org/10.1021/ef402529k | Energy Fuels 2014, 28, 2598−2607

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Figure 1. Low-angle XRD (left) and high-angle XRD (right) patterns of MoO3-NiO modified (a) SBA-15, (b) SBA-15+γ-alumina, (c) Ti-SBA-15, (d) Ti-SBA-15+γ-alumina, (e) MAS, and (f) MAS+γ-alumina supported catalysts (⬡, MoO3; ★, NiO; ☆, γ-alumina; and ●, NiMoO4). and tetraethyl orthosilicate (TEOS, 98%) were obtained from Merck. Aqueous ammonia (Thomas Baker, 25%), aluminum oxide (γ-phase), and guaiacol (98%) were obtained from Alfa Aesar. All the chemicals were used as such without any further purification. 2.2. Preparation of Supports: SBA-15, Ti-SBA-15 (Si/Ti = 10), and MAS (Si/Al = 20) and Their Composite with γ-Alumina. SBA-15 and Ti-SBA-15 were synthesized by a hydrothermal method as described in ref 34 using triblock copolymer Pluronic P123 as a template, and tetraethylorthosilicate (TEOS) and tetrapropylorthotitanate (TPOT) as Si and Ti sources, respectively. SBA-15 was synthesized with a gel composition of 1 TEOS/0.54 HCl/0.016 P123/ 100 H2O. In a typical synthesis of SBA-15, 12 g of P123 was dispersed in a calculated amount of distilled water, and the resultant solution was mixed with 2 M HCl at 40 °C. Then, 25.5 g of TEOS was added to the above solution. The mixture was stirred at 40 °C for 24 h and then aged at 100 °C for 48 h without stirring. The obtained solid product was recovered by filtration, washed with deionized water, and dried at 80 °C overnight. Calcination was carried out in a flow of air at 550 °C for 6 h. For Ti-SBA-15, TPOT with the desired Si/Ti ratio of 10 was additionally added along with TEOS addition. MAS (Si/Al = 20) was synthesized in a double surfactant system using CTAB and PEG-4000 as structure-directing agent and particle size regulator, respectively, at room temperature.35 After homogenizing the surfactant solution mixture, required amounts of aluminum nitrate were added, followed by TEOS. The pH of the solution was adjusted to 9.0 with 25% aqueous ammonia solution. After the gel formation, stirring was continued for another 48 h at room temperature. The solids were filtered, dried at 100 °C, and calcined at 550 °C. SBA-15, Ti-SBA-15, and MAS supports were mixed with γ-alumina with a composite ratio of 3:1 by mechanical mixing in 2% acetic acid as a peptizing agent. Subsequently, mixed supports were dried at 100 °C and then calcined at 550 °C for 5 h. 2.3. Catalyst Preparation. MoO3-NiO was supported on the prepared SBA-15, Ti-SBA-15, and MAS and their γ-alumina composite supports by an incipient wet impregnation method. The order of impregnation of active metals plays a vital role in the activity of catalysts; hence, MoO3 was impregnated after NiO.36 NiO (3%) was impregnated using nickel nitrate, and then the catalyst was dried and calcined at 550 °C for 5 h in a static air atmosphere. On this catalyst, 10% MoO3 was impregnated using ammonium heptamolybdates, followed by drying and calcination for 5 h under a static air atmosphere. The catalysts were labeled as MoO3-NiO/SBA-15 and MoO3-NiO/SBA-15+γ-alumina. Similarly, the same naming format has been used for Ti-SBA-15 and MAS. The prepared catalysts were prereduced at 400 °C for 3 h with H2 gas at a flow rate of 50 cm3/min, prior to the catalytic HDO process. 2.4. Catalyst Characterization. XRD patterns of support as well as catalysts were recorded in a Phillips X″pert X-ray diffractometer

with Cu−Kα radiation (λ = 0.1548 nm) in the ranges of 0.5−5° and 10−80° 2θ values, to identify the phase constitutions in the samples. N2 sorption was carried out at 77 K on a Quadrasorb SI automated surface area and pore size analyzer. The sample was degassed at 300 °C for 3 h prior to analysis. Elemental compositions were identified using inductively coupled plasma−optical emission spectroscopy (ICP-OES) on a PerkinElmer Optima 5300 DV equipped with a concentric nebulizer and cyclonic spray chamber. The metal coordination environment of active species over these supports was studied on a Schimadzu UV-2450 UV−visible spectrometer and using BaSO4 as reference. The temperature-programmed reduction (TPR) and desorption (NH3-TPD) studies were carried out on a ChemBET TPD/TPR instrument using the following gaseous mixtures: 5%H2/95%Ar and 5%NH3/95%He for TPR and TPD, respectively. About 30 mg of the sample was loaded in a U-shaped quartz tube, and the TPR analysis was monitored in the temperature range of 100−1000 °C at a heating ramp of 10 °C/min with a flow rate of 60 cm3/min. Before analysis, the samples were heated in an inert atmosphere at 300 °C for 3 h. Similarly, for TPD analysis, about 30 mg of sample was heated to 200 °C for 2 h under N2 flow (80 cm3/min). Then, the sample temperature was brought down to 40 °C. Ammonia was adsorbed (10% NH3/90% He, 50 cm3/min) for 30−40 min, followed by removal of physisorbed NH3 with He gas. The ammonia desorbed was recorded from the temperature range of 40−800 °C at the rate of 15 °C/min. Metal dispersion, metal surface area, and average crystalline size were measured from H2 chemisorption using a ChemBET TPD/TPR instrument. About 30 mg of sample was reduced in the flow of H2/Ar gas mixture from room temperature to 400 °C at a ramp of 15 °C/min in a quartz U-tube. After reaching the desired temperature, the sample was cooled down to room temperature in an inert atmosphere. In the room temperature, H2 pulse (pure H2, 500 μL) was injected into the sample. The metal dispersion, metal surface area, and average crystallite size values were calculated using TPRWin software. 2.5. Catalytic Activity. Guaiacol has been selected as a model compound for the current study because it is an intermediate product found in pyrolysis oil. The HDO of guaiacol was carried out in a fixedbed down-flow reactor at atmospheric pressure. The reactor setup was made of a glass tube with a 1 cm radius and 30 cm length. About 0.5 of catalysts was packed in between the Pyrex glass wool and was activated at 500 °C in the flow of air and purged with N2 gas using a 30 mL/min flow rate for 2 h. The MoO3-NiO supported catalysts were prereduced at 400 °C, and reaction was carried out on the prereduced catalyst between 275 and 300 °C. The reactant was passed through the reactor tube made up of a pyrex glass tube with a spiral wound coaxially using an infusion syringe pump (RH-SY10, Ravel Hitecks, India), and the WHSV (weight hourly space velocity) of the feed was varied from 2.22 to 8.88 h−1.37 The formed products were condensed and collected in 2599


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Figure 2. N2 sorption isotherms (left) and pore size distributions (right) of MoO3-NiO modified (a) SBA-15, (b) SBA-15+γ-alumina, (c) Ti-SBA15, (d) Ti-SBA-15+γ-alumina, (e) MAS, and (f) MAS+γ-alumina supported catalysts. an ice-cooled trap. The conversion and selectivity of the products were analyzed in a GC-17A Shimadzu gas chromatograph instrument with a DB-5 column. The catalyst was recovered and regenerated at 500 °C for 6 h in a CO2-free air atmosphere for reproducibility tests. The conversion and selectivity of guaiacol was calculated from the initial and final amounts of guaiacol obtained from GC results after every hour of reaction

X guaiacol % =

15, and MAS support catalysts possibly due to partial collapse in mesoporous ordering after making the composite with alumina. The high-angle XRD patterns are presented in Figure 1b. MoO3 diffraction patterns corresponding to the orthorhombic phase (JCPDS card 35-609) are observed at 2θ 26− 27°, and NiMoO4 phase formation was also identified at 2θ 28.8° (JCPDS card 33-0948).33 MAS supported catalysts showed these peaks with low intensity compared to other supported catalyst. Similarly, the intensity of all these peaks decreases in the composite catalyst as well with small diffraction patterns of Ni and Mo metals.33,39 Again, the MAS composite catalyst has a less intense diffraction peak compared to SBA-15 and Ti-SBA-15 due to increased dispersion of active species in the presence of alumina. In the composite catalyst, peaks at 45.8° 2θ and 66.8° 2θ are due to γ-alumina (JCPDS 010-0425). The N2 isotherms of SBA-15, Ti-SBA-15, MAS, and their composite catalysts displayed typical type IV isotherms with inflections observed at 0.3−0.4, 0.5−0.7, and 0.7−0.8 P/Po, respectively, for MAS, Ti-SBA-15, and SBA-15 catalysts (Figure 2). Highly ordered arrangements of mesopores are confirmed by the presence of an H1-type hysteresis loop in SBA-15 and MAS catalysts. The shape of the hysteresis loop depends upon the amount of metal ions introduced in the framework as observed for Ti-SBA-15. The composite catalysts also have a similar type of isotherm; further, the textural properties (Table 1) of composite supported catalysts were lower than those of the parent catalyst. It may be due to the partial blockage of pores after the composite preparation; hence, they have reduced textural properties. The pore diameters of these

m(guaiacol)in − m(guaiacol)out × 100 m(guaiacol)in

C p Selectivity % =

mC p ∑ mC p

× 100

where Cp represents moles of individual products. The specific reaction rate (r, mol g−1 s−1) was calculated using the equation r=

F0Xguaiacol w

where F0 = molar flow rate of guaiacol (mol s−1) and w = catalyst weight (g). Turnover frequency was calculated employing the following equation

Turnover frequency [s−1] =

specific reaction rate [r , mol g −1 s−1] quantity of sites [μmol/g]

Quantity of sites (μmol/g) were derived from the amount of NiO and MoO4 present (ICP) and their actual sites exposed for reaction (dispersion %).

3. RESULTS AND DISCUSSIONS 3.1. Catalyst Characterization. The low-angle XRD of catalyst supports (Figure 1a) displayed an intense peak at 0.8− 1.2° 2θ ascribed to formation of ordered mesopores. The intense (100) reflection at 0.8−1.2° and two small (110) and (200) reflections are observed for SBA-15 and Ti-SBA-15 due to their highly arranged hexagonal pore structure. MAS also has the same diffraction planes at a different (a higher) 2θ region compared to that of SBA-15 and Ti-SBA-15. The peaks of TiSBA-15 and MAS are highly ordered as that of SBA-15 with p6mm symmetry.38 The two small humps in MAS at 2θ values of 1.7° and 2.2° indicate the long-range ordered hexagonal symmetry. The intensities of these peak are found to be lower in the composite supports as compared with SBA-15, Ti-SBA-

Table 1. Textural Properties of MoO3-NiO Catalysts MoO3-NiO catalysts

SBETa (m2/g)

SBA-15 SBA-15+γ-alumina Ti-SBA-15 Ti-SBA-15+γ-alumina MAS MAS+γ-alumina

852 653 391 336 937 725

Vp,

b

BJH

(cm3/g)

0.63 0.52 0.98 0.76 0.96 0.76

dP,

c

BJH

(nm)

6.0 5.8 6.6 6.3 3.3 3.0

a SBET = Specific surface area determined using Brunauer−Emmett− Teller (BET) equation. bVp, BJH = Total pore volume at 0.99 P/P0. c dP,BJH = BJH adsorption pore diameter.

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Temperature-programmed reduction results of MoO3-NiO supported catalysts in the temperature range of 100−1100 °C are given in Figure 4, and the quantity of H2 consumption is given in Table 3. Two reduction peaks were observed for SBA15 and Ti-SBA-15 supported catalysts and their γ-alumina composite catalysts. Interestingly, four reduction peaks were observed for the MAS supported catalyst; one at 375−465 °C and three small shoulder peaks with peak maxima at 490, 560, and 700 °C. The low-temperature peak observed between 320 and 520 °C is due to reduction of Ni2+ to Ni0 that are bound weakly to the surface of the support,42 and the hightemperature broad H2 consumption peak around 550−680 °C is due to reduction of the MoO3 phase and NiMoO4 phase. The reductions of these two phases overlapped with each other and are in good agreement with the literature.42,43 The reduction temperature decreases with the types of heteroatoms (Ti and Al) incorporated in the silica framework. The strong interactions between the metal and supports seem to reduce the cluster formation in Ti-SBA-15 and MAS compared to the SBA-15 supported catalyst.44 The amount of H2 consumed was quantified from TPR peaks, and the MAS supported catalyst revealed a higher consumption of H2 (726 μmol) compared to Ti-SBA-15 (427 μmol) and SBA-15 (220 μmol), suggesting fine dispersion and strong interaction of active metal species in the MAS catalyst. Similar observation is also noted for their composite catalyst as well, however, with a higher H2 uptake compared to the unmodified catalyst possibly due to enhanced dispersion of MoNi species by alumina addition. The results of NH3 TPD measurements of various MoO3NiO catalysts are presented in Figure 5. A weak NH3 desorption peak with a Tmax of 115 °C was noted for the SBA-15 catalyst. However, two desorption peaks (T = 140 and 345 °C) with an increase in total acidity was observed for SBA15+γ-alumina composite catalyst. Similar observation is also made for Ti-SBA-15 catalyst and its composite catalyst. For TiSBA-15+γ-alumina catalyst, the high-temperature desorption peak observed at 420 °C implies a higher acid strength compared to SBA-15+γ-alumina catalyst. However, both MAS and MAS+γ-alumina catalysts displayed a higher total acidity compared to these catalysts. The acidity values follows the order SBA-15 < Ti-SBA-15(10) < MAS. The results of H2 chemisorption of MoO3-NiO supported catalysts are given in Table 3. In the SBA-15 supported MoO3NiO catalysts, the metal dispersion and metal surface area were found to be 10% and 8.3 m2/g, respectively. The presence of framework Ti and Al in Ti-SBA-15 and MAS supports, respectively, decreased agglomeration and cluster formation (see average crystalline size, Table 3); hence, higher metal dispersion and metal surface area were obtained. However, the composite catalysts were found to have higher dispersion compare to SBA-15, Ti-SBA-15, and MAS supported catalysts. The better dispersion of composite supported catalysts was attributed to an increase in total acidity and higher acid strength (see NH3-TPD discussion). The morphology of synthesized catalysts clearly displayed an accumulation of particles in SBA-15 (Figure 6a−c), which is observed to be less in both Ti-SBA-15 (Figure 6d−f) and MAS (Figure 6g−i) catalysts attributed to framework Ti and Al, respectively. In the MAS catalyst, because of the fine dispersion of active metals, particle accumulations were not seen at this magnification. The Al and Ti incorporation in MAS and TiSBA-15, respectively, enhances the dispersion and different

catalysts were observed in the range between 3.0 and 6.6 nm and the pore volumes between 0.56 and 0.98 cm3/g. The elemental compositions of SBA-15, Ti-SBA-15, MAS, and their composite catalysts were examined, and results are presented in Table 2. The estimated values are slightly higher Table 2. Elemental (ICP-OES) Analysis of MoO3-NiO Catalysts Si/Al or Ti MoO3-NiO catalysts SBA-15 SBA-15+γ-alumina Ti-SBA-15 Ti-SBA-15+γ-alumina MAS MAS+γ-alumina

fresh

12 15 22 27

Mo wt%

Ni wt%

spent

fresh

spent

fresh

spent

19 25 29 35

9 9 9.5 9.4 9.6 9.5

8.6 8.6 9.3 9.1 9.2 9.2

3 2.8 2.6 2.5 2.9 2.8

2.8 2.7 2.5 2.2 2.6 2.5

than the targeted value possibly due to loss of heteroatoms during washing and filtration of synthesized precipitate. The active metal and promoter concentration was found be nearly closer to the theoretical value. The coordination of nickel and molybdenum with the support was identified by recording their diffuse reflectance UV−vis spectra, and the results are presented in Figure 3. A

Figure 3. Diffuse reflectance UV−vis of MoO3-NiO modified (a) SBA15, (b) SBA-15+γ-alumina, (c) Ti-SBA-15, (d) Ti-SBA-15+γ-alumina, (e) MAS, and (f) MAS+γ-alumina supported catalysts (⬡, MoO3 and ★, NiO).

mixture of tetrahedral (Td) and octahedral (Oh) Mo species (charge-transfer transition from O2− to Mo6+) as observed in the regions of 230−280 and 300−330 nm, respectively.40 The weak bands observed between 405 and 420 nm and between 600 and 800 nm (for composite catalysts) are attributed, respectively, to Ni2+ (Oh) and Ni2+ (Td) from the spinel-like phase.41 The octahedral Mo species was predominantly detected for SBA-15 catalyst, and a distribution of both Td and Oh Mo species were noticed in Ti-SBA-15 and MAS catalysts, respectively. For Ti-SBA-15, an additional band at 220 nm was observed due to tetrahedral coordination of Ti species. A similar observation was also noted for composite catalysts with respective peaks slightly blue-shifted, indicating strong interaction and fine dispersions of Mo species. 2601


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Figure 4. TPR profiles of MoO3-NiO modified (a) SBA-15, (b) SBA-15+γ-alumina, (c) Ti-SBA-15, (d) Ti-SBA-15+γ-alumina, (e) MAS, and (f) MAS+γ-alumina supported catalysts.

Table 3. H2 Consumption, Acidity, Dispersion, Surface Area, and Average Crystalline Size of MoO3-NiO Species MoO3-NiO catalysts

H2 consumed (μmol/g)

dispersion (%)

metal surface area (m2/g)

average crystalline size (nm)

acidity (mmol/g)

SBA-15 SBA-15+γ-alumina Ti-SBA-15 Ti-SBA-15+γ-alumina MAS MAS+γ-alumina

220 250 427 433 726 747

10 12 15 18 19 25

8.3 11.6 14.2 18.0 16.3 19.2

52 46 44 40 38 30

0.024 0.065 0.106 0.125 0.232 0.252

Figure 5. NH3-TPD profiles of MoO3-NiO modified (a) SBA-15, (b) SBA-15+γ-alumina, (c) Ti-SBA-15, (d) Ti-SBA-15+γ-alumina, (e) MAS, and (f) MAS+γ-alumina supported catalysts.

low guaiacol conversion (possibly catalyst deactivation). Moreover, hydrogenation of benzene to the desired cyclohexane also decreased. Having optimized the temperature as 300 °C, WHSV was varied from 2.22 to 8.88 h−1 over MoO3NiO/MAS catalyst, and the results are given in Figure 7b. A slight increase in conversion of guaiacol was noticed at a WHSV of 4.44 h−1. As expected, benzene selectivity increased with WHSV due to insufficient contact time for hydrogenation activity. Hence, a lower WHSV (up to 4.44 h−1) may be favorable for cyclohexane formation. Normally, there will be an optimum WHSV for any catalysts working in the fixed-bed flow reactor. At low values of WHSV, side reaction leading to undesirable products will be formed. At high values of WHSV, the contact time with the catalyst would be insufficient for the reactant to undergo conversion. The conversion and selectivity shown in Figures 7 and 8 refer to the values obtained after steady state is reached. A measurable quantity of products of the reaction is obtained after 2−3 h of the reaction. Hence, for the actual steady state,

phase formations, such as NiO, NiMoO4, and MoO3, also confirmed from high-angle XRD and H2 chemisorption study. The hexagonal arrangements of both SBA-15 and Ti-SBA-15 were preserved even after metal loading, and the dark patches in the pictures were due to metal particles. The TEM images of composite mixtures (not shown) also showed a similar morphology with less agglomeration of active metals. 3.2. HDO of Guaiacol. The activity of the catalysts MoO3NiO/MAS was tested in a fixed-bed down-flow reactor at various temperatures between 275 and 350 °C with a constant flow rate of 2.2 WHSV h−1 and a H2 flow of 50 cm3/min (Figure 7a). The MAS catalyst was chosen for the temperature optimization study as it has more acid sites and higher dispersion of active species compared to SBA-15 and Ti-SBA15 supported catalysts. Guaiacol conversion of about 50% was noticed at 275 °C; however, approximately 68% deoxygenation activity was obtained at 300 °C. Further increase in temperature decreased the hydrodeoxygenation activity due to cracking reactions45−48 and coke deposition on the surface, leading to 2602


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Figure 6. TEM images of MoO3-NiO modified (a−c) SBA-15, (d−f) Ti-SBA-15, and (g−i) MAS supported catalysts.

Figure 7. HDO activity of MoO3-NiO/MAS catalyst with variation in (a) temperature and (b) WHSV.

considerable time is given before measurement. The initial activity of all the catalysts was very low. This may be because

the catalyst goes through some induction period to attain its steady-state activity.49 2603


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Figure 8. Variation of conversion with time over (a) SBA-15, Ti-SBA-15, and MAS and (b) their γ-alumina composite catalyst. Typical product distribution at steady state over (c) SBA-15, Ti-SBA-15, and MAS and (c) their γ-alumina composite catalyst. Reaction conditions: T = 300 °C, WHSV = 4.44 h−1, t = 8−10 h, and molar flow rate of guaiacol = 9.933 × 10−6 mol/s.

enhances the catalytic performance but also remarkably changes the dispersion of active and promoter metal species. The formation of bulk particles in SBA-15 and Ti-SBA-15 supports reduces the activity of the catalysts, which is evident from the XRD pattern, BET, TPR profile, TEM, and H2-chemisorption study. The selectivity trend is slightly varied with composite catalysts. The increase in acidity by γ-alumina addition enhances both conversion and formation of benzene in the cases of SBA-15 and Ti-SBA-15 composites. However, the MAS composite yielded a significant increase in cyclohexane formation at the expense of benzene and other byproducts. Even though the pore size of MAS was smaller compared to that of other catalysts, higher surface area, larger H2 uptake (TPR), and relatively higher percentage of active metal species dispersion lead to a higher HDO activity in the case of MAS and its composite catalyst. In addition, the hydrogenation of aromatics occurred on the well-dispersed Ni active sites, evidenced from the several individual reduction peak maxima in TPR profiles of MAS catalysts. The higher selectivity of aromatics in SBA-15 and Ti-SBA-15 supported catalysts may be attributed to the formation of NiMoO4 species. Normalizing the guaiacol conversion with the weight of catalyst and the moles of active sites (% dispersion MoO3-NiO sites from chemisorption), we calculated the specific reaction rate and turnover frequencies, respectively, which are tabulated in Table 4. Compared to SBA-15 catalyst, the specific reaction

Figure 8a,b shows the variation of conversion with time over SBA-15, Ti-SBA-15, and MAS and their composite supported catalysts at 300 °C. Guaiacol conversion was significantly low for the initial 2−3 h49 with relatively lower mass balance; however, a stable conversion and selectivity of products were noticed from 3 to 4 h of reaction time. About 20% guaiacol conversion was noticed over SBA-15 catalyst, and the conversion was improved to approximately 55% over Ti-SBA15. However, a remarkable conversion of ∼74% was obtained over the MAS catalyst. These findings suggest that the presence of incorporated heteroatoms, such as Ti and Al, enables fine dispersion of active metal species, thereby affecting the conversion of guaiacol. A similar trend with much improved HDO activity was observed in the composite catalysts. For instance, addition of γ-alumina on SBA-15 catalyst increased its activity (guaiacol conversion) from ∼20% to 64%. An analogous profound effect of γ-alumina addition was observed over Ti-SBA-15 and MAS composite catalysts as well with a guaiacol conversion of ∼95% noticed for the MAS composite catalyst. The byproducts, such as phenol and cyclohexanol, were also greatly suppressed in the composite catalysts. The selectivity of products varies with support morphology and the presence of heteroatoms (Ti or Al). Cycohexane was observed as a major product over MAS compared to other catalysts (see Figure 8c,d) where benzene was the major product, implying that aluminum incorporation not only 2604


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considerable loss of activity was noticed for four cycles (Figure 9), and the chemical analysis was almost similar as that of fresh

Table 4. TOF and Specific Reaction Rate for HDO of Guaiacol over Different MoO3-NiO Catalysts at 300 °C MoO3-NiO catalysts

specific reaction rate (r) × 10−4 (mol g−1 s−1)

TOF s−1

SBA-15 SBA-15+γ-alumina Ti-SBA-15 Ti-SBA-15+γ-alumina MAS MAS+γ-alumina 9%MoO3-3NiO/SBA-15+γ-alumina 11%MoO3-3NiO/SBA-15+γ-alumina

1.99 6.36 5.46 7.95 7.45 9.44 5.07 5.27

3.3 8.9 6.1 7.4 6.6 6.3 8.4 8.2

rate and TOF was observed higher in Ti-SBA-15 and MAS catalysts. Interestingly, γ-alumina composite catalysts (SBA-15 and Ti-SBA-15) displayed relatively higher TOF compared to unmodified catalysts. These results again support the fact that the reaction is positively influenced by the presence of heteroatoms (Ti) and the composite (γ-alumina). However, only a marginal variation in TOF was noticed for MAS and its γ-alumina composite catalysts. Since the support MAS already has Al present, compositing with γ-alumina did not have a profound effect on TOF. The absence of external and internal mass transfer limitations were checked by the Mears criterion50 (eq 1) and the Weisz− Prater criterion (eq 2), respectively rAρb Rn < 0.15 kcCA b (1)

Figure 9. Reusability and stability of MoO3-NiO/MAS+γ-alumina catalyst. Reaction conditions: T = 300 °C, WHSV = 4.44 h−1, t = 8−10 h, and molar flow rate of guaiacol = 9.933 × 10−6 mol/s.

catalyst (see Table 3). The preservation of chemical properties of the catalysts’ surface during these runs may be attributed to the presence of framework Al and its moderate acidity, which may prevent the agglomeration of active species by different metal support interactions.

where rA is the measured reaction rate in kmol kgcat−1 s−1; ρb is the bulk density of the catalyst bed in kg m−3; R is the catalyst particle radius in m; n is the reaction order; kc is the mass transfer coefficient in m s−1; and CAb is the bulk gas concentration of guaiacol in kmol m−3. C WP =

rAρc R2 DeCA s

MoO3-NiO/Ti-SBA-15 > MoO3-NiO/SBA-15. The nature of active species formation on the surface is directly related to the nature of the heteroatom present in the mesoporous matrix. Incorporation of Al in the framework or catalyst formulation with γ-alumina support led to a high dispersion NiMoOx phase. However, bulk MoO3 formation was observed in Ti-SBA-15 and SBA-15. The composite supported systems yielded small cluster formation, which may be due to strong interaction between the support and active metal species. The conversion also follows the same trend as that of MoO3-NiO/MAS > MoO3-NiO/Ti-SBA-15 > MoO3-NiO/ SBA-15. The hydrogenation of aromatics is more on MAS supported catalysts, and aromatic formation is more on SBA-15 and Ti-SBA-15. It may be due to their variation in the acidity and dispersion.

(2) −3

where ρc is the catalyst density kg m ; De is the effective gasphase diffusivity in m2 s−1 given by De = DAB ϕp σc/τ, where ϕp, σc, and τ are pellet porosity, constriction factor, and tortuosity, respectively; and CAs is the concentration of guaiacol at the catalyst surface in kmol m−3. We observed the highest rate of 9.44 × 10−4 mol g−1 s−1 for MAS+γ-alumina catalyst and a total gas flow rate of 0.834 cm3 s−1 and a particle radius of 1.25 × 10−4 m; the following were calculated: CAs ≈ CAb = 2.505 × 10−3 kmol/m3, De = 1.08 × 10−5 m2/s, kc = 0.2231 m/s. Using these values, the Mears criterion value was calculated to be 0.0021 < 0.15, and the Weisz−Prater criterion CWP is 2.88 × 10−3 < 1. These results clearly suggest the absence of mass transfer limitations in these studies. Further, the Koros and Nowak test was also employed to check for the absence of mass transfer limitations. In this test, the amount of MoO4 was varied narrowly over the SBA-15+γalumina composite support, keeping a constant amount of NiO, and the results are presented in Table 4. A close TOF was noticed (see Table 4, entries 2, 7, and 8), indicating the absence of any mass transfer limitations in our studies. The reusability of MoO3-NiO/MAS+γ-alumina catalyst was studied by activating them in the flow of air at 500 °C for 3 h, followed by reducing them in a flow of H2 at 400 °C. No

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AUTHOR INFORMATION

Corresponding Authors

*(K.S.) E-mail: [email protected]. Tel: +91-4422358654 Fax: +1-785-864-6051+91-44-22200660. *(A.R.) E-mail: [email protected]. Tel.: +1-785-864-1631. Fax: +1-785-864-6051. Notes

The authors declare no competing financial interest. 2605


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ACKNOWLEDGMENTS The authors thank Prof. R. V. Chaudhari, Center for Environmentally Beneficial Catalysis, University of Kansas, for mass transfer calculation and helpful discussions. The authors are thankful to the Department of Chemistry for providing the lab facility and infrastructure facility to carry out this work. The authors are also grateful to DST, DRDO, UGC-DRS, and DSTFIST for providing financial facility and instruments like TPR, TPD, BET, GC, and DR-UV−vis spectrophotometer.

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