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Hydrodeoxygenation of anisole as bio-oil model compound over supported Ni and Co catalysts: Effect of metal and support properties
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Hydrodeoxygenation of anisole as bio-oil model compound over supported Ni and Co catalysts: Effect of metal and support properties Thangaraju M. Sankaranarayanan a , Antonio Berenguer a , Cristina Ochoa-Hernández a , Inés Moreno a,b , Prabhas Jana a , Juan M. Coronado a , David P. Serrano a,b , Patricia Pizarro a,b,∗ a b
IMDEA Energy Institute, Avda. Ramón de la Sagra 3, Móstoles, Madrid, Spain Chemical and Environmental Engineering Group, ESCET, Universidad Rey Juan Carlos, Móstoles, Madrid, Spain
a r t i c l e
i n f o
Article history: Received 22 June 2014 Received in revised form 28 August 2014 Accepted 2 September 2014 Available online xxx Keywords: Hydrodeoxygenation Anisole Hierarchical zeolite ZSM-5 SBA-15
a b s t r a c t Hydrodeoxygenation (HDO) is considered one of the most promising routes to convert the pyrolysis bio-oils produced from lignocellulose into biofuels very similar to those derived from petroleum. This work reports results obtained in the catalytic HDO of anisole over non-conventional hydrotreating catalysts based on metallic Ni and Co supported on micro-mesoporous carriers of different acidity. Anisole has been chosen as representative of those compounds, containing a methoxy-phenyl group, which are abundant in lignocellulose pyrolysis bio-oils. The effect of both metal phase and support properties on their performance as HDO catalysts has been studied. With this aim, three supports with different textural and acidic properties have been employed: hierarchical ZSM-5, mesostructured pure silica SBA-15 and mesostructured Al-SBA-15. The reactions have been carried out in a stainless steel high pressure batch reactor at 220 ◦ C and with 50 bar of pure hydrogen. The interaction of the metallic species (Ni or Co) with the porous supports, as well as their dispersion, is strongly affected by the support nature and the presence of Al. Thus, it has been found a synergetic effect between the acid sites of the supports and the metallic active phases, which favors and enhances the HDO of anisole. In the case of the acidic supports, Ni-based catalysts (Ni/Al-SBA-15 and Ni/h-ZSM-5) show larger anisole conversions compared to Co-based materials. The strong interaction with the acidic supports of the latter hinders the total Co reduction prior to the reaction, being probably the major reason of their lower activity. Hydrodeoxygenation, hydrodearomatization and isomerization reactions take place extensively over Ni/h-ZSM-5, revealing it is a promising catalyst for bio-oil processing in order to attain high quality fuel production. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Biofuels can make an important contribution to overcome the energy challenges of the XXI century and significantly reduce the huge CO2 emissions associated with the transport sector. For instance, biofuels are expected to play a crucial role as substitutes for petroleum-based fuels used in heavy transport (aviation, maritime transport, etc.) where the shift from conventional engines to electric ones is still far away from being viable. According
∗ Corresponding author at: Thermochemical Processes Unit, IMDEA Energy Institute, Avda. Ramón de la Sagra 3, Móstoles, Madrid, Spain. Tel.: +34 91 737 11 20; fax: +34 91 737 11 40. E-mail addresses: [email protected], [email protected] (P. Pizarro).
to the International Energy Agency (IEA), conventional biofuels, commonly referred to as first generation biofuels, are those whose production process has already reached technological maturity and are implanted at the commercial level [1]. It is the case of bioethanol, biodiesel and biogas obtained through processes of fermentation, transesterification and anaerobic digestion, respectively. However, there are some controversies about their real environmental and socio-economic impacts, including the competition between the provision of raw materials for the production of biofuels and food supply, the risk of deforestation in tropical areas and a positive net balance of CO2 emissions by some biofuels revealed by several recent Life Cycle Assessment (LCA) studies [2]. Resulting from these drawbacks, one of the goals set by the European Union for 2020 is limiting the contribution of first generation biofuels while promoting the development of other and sustainable biofuels [1]. These advanced biofuels will be obtained from
http://dx.doi.org/10.1016/j.cattod.2014.09.004 0920-5861/© 2014 Elsevier B.V. All rights reserved.
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raw materials that do not compete with the food industry, mainly from plants cultivated in non-agricultural or marginal lands, as well as from lignocellulosic wastes. Lignocellulose, which is composed by cellulose, hemicellulose and lignin, can be transformed by either fast or flash pyrolysis, yielding up to 70% of a liquid product known as bio-oil. This bio-oil has a high potential as a liquid fuel since it retains up to 70% of the initial energy of the starting biomass and contains less nitrogen and sulphur than fossil fuels [3]. However, it possesses an excessively high content in water (15–25 wt%) and oxygen (>40 wt%) for being directly employed as a high quality fuel for transportation [4,5]. Likewise, the untreated bio-oil has other multiple deficiencies to be used as fuel: (a) relatively low calorific value, (b) high corrosiveness due to its low pH and (c) high viscosity and limited chemical stability in comparison with conventional fuels. Therefore, in order to use the pyrolysis bio-oil as liquid fuel in conventional engines, it is necessary to carry out additional processing [6,7]. Among the catalytic paths for upgrading the pyrolysis bio-oils, hydrodeoxygenation (HDO) seems to be one of the most promising [7,8]. This process consists in the treatment of bio-oil at temperatures between 250 and 450 ◦ C with hydrogen at high pressure (20–300 bar) in the presence of a catalyst, so that the oxygen contained in organic molecules is extracted in the form of water. Sulfided CoMo and NiMo catalysts have been widely studied for HDO processes, as they are conventional catalysts employed in petroleum hydroprocessing [9–15]. Other catalysts based on oxides [16–18], transition metals, supported noble metals (Pt, Pd and Ru) [19,20], metallic phosphides [21,22] or zeolites (ZSM-5) [23] are being investigated. In the present study non-conventional catalysts based on supported metallic Ni or Co are explored as potential catalysts for HDO. The effect of both metal phase and support properties, as well as their interactions, on their performance as HDO catalysts has been investigated. With this aim, three supports with different textural and acidic properties have been selected: hierarchical ZSM-5 (h-ZSM-5) with Si/Al = 47, mesostructured pure silica (SBA-15) and mesostructured Al-SBA-15 with Si/Al = 70. For the catalytic assays, anisole has been chosen as model compound because it contains a methoxy-phenyl group, which is characteristic of lignin depolymerisation fragments present in pyrolysis bio-oils obtained from lignocellulosic biomass [24].
2. Experimental 2.1. Catalysts preparation 2.1.1. Synthesis of supports Pure silica SBA-15 was synthesized by a hydrothermal method described by Zhao et al. [25]. Typically, 8 g of Pluronic 123 (Aldrich) was dissolved in 250 ml of 1.9 M HCl at room temperature. Then, it was heated to 40 ◦ C and the silica source, 16.2 g of tetraethylorthosilicate (TEOS, Aldrich), was added. After stirring for 20 h at 40 ◦ C, the mixture was aged at 110 ◦ C for 24 h under autogenous pressure. The solid product was recovered by filtration, dried overnight and calcined at 550 ◦ C for 5 h using a heating rate of 1.8 ◦ C/min under static air. Al-containing SBA-15 (Al-SBA-15) was synthesized according to a procedure described in literature [26], using aluminum isopropoxide (AIP, Aldrich) as Al source. In a typical synthesis, 8.5 g of TEOS and 0.125 g of AIP (SiO2 /Al2 O3 mol ratio = 135) were added to 10 ml of an aqueous HCl solution (pH = 1.5). After stirring for 3 h, the homogenous mixture was added to other solution containing 4 g of Pluronic 123 dissolved in 150 ml of HCl at pH = 1.5. The final mixture was stirred for 20 h at 40 ◦ C and aged under static
conditions and autogenous pressure at 110 ◦ C for 24 h. The solid product obtained was filtered, dried overnight and calcined in static air at 550 ◦ C for 5 h with a heating rate of 1.8 ◦ C/min. Hierarchical ZSM-5 zeolite was obtained by a procedure based on the silanization of protozeolitic units previously to the hydrothermal crystallization treatment [27]. Firstly, a clear solution containing tetraethylorthosilicate, tetrapropylammonium hydroxide, aluminum isopropoxide and distilled water, with the following molar composition: 1 Al2 O3 :80 SiO2 :14.4 TPAOH:2000 H2 O, was precrystallized under reflux with stirring at 90 ◦ C for 20 h. Subsequently, 8 mol% (referred to the total silica content) of the seed silanization agent (phenylaminopropyltrimethoxysilane, PHAPTMS) was added and left reacting at 90 ◦ C for 6 h. The final solution was crystallized in an autoclave at 170 ◦ C for 7 days. The solid products were separated by centrifugation, washed several times, dried overnight at 110 ◦ C and calcined in air at 550 ◦ C for 5 h. 2.1.2. Metal incorporation In all cases the catalysts were prepared by incipient wetness impregnation of aqueous solutions containing Ni(NO3 )2 ·6H2 O or Co(NO3 )2 ·6H2 O (Aldrich) in the amount required to get a final metal loading of 5 wt%, followed by calcination at 550 ◦ C for 5 h. 2.2. Catalysts characterization Porous properties were measured by gas physisorption under isothermal conditions (77 or 87 K, respectively). SBA-15 and Al-SBA-15 materials were analyzed in a Quadrasorb system (N2 adsorption–desorption) while h-ZSM-5 was measured in an Autosorb equipment (Ar adsorption–desorption). The surface area was calculated by the BET method. The pore size distribution was determined by applying the BJH method to the adsorption branch of the isotherm in the case of mesostructured materials, while the nonlocal density functional theory (NLDFT) was used for the zeolitic catalysts. Metal content was determined by ICP-OES on a Perkin Elmer Optima 7300AD instrument. X-ray diffraction (XRD) patterns of the catalysts were obtained using a Philips X’Pert PRO ˚ The phases were diffractometer with Cu-K␣ radiation ( = 1.542 A). identified by matching the peaks appearing in the XRD pattern of the test sample with JCPDS (Joint Committee on Powder Diffraction Standards) data files. TEM images of the catalysts were taken using a PHILIPS TECNAI 20T instrument, working at 200 kV and equipped with an EDX spectrometer for measuring X-ray energy dispersive spectra. The acidity of the supports was measured by temperature programmed desorption (TPD) of NH3 using an AUTOCHEM 2910 system (Micromeritics). The standard procedure for the TPD measurements involved the activation of the sample by flowing He at 600 ◦ C (1 h), cooling to 100 ◦ C, adsorbing NH3 from a He–NH3 (15%) mixture, removing the weakly adsorbed NH3 by flowing He at 100 ◦ C for 30 min, and finally carrying out the TPD experiment by raising the temperature of the catalyst sample with a ramp of 10 ◦ C/min. H2 chemisorption (H2 -TPD) was carried out using the same setup as in the NH3 -TPD measurements. In a first step, the sample (ca. 100 mg) was degassed and reduced with 50 ml of 10% H2 in Ar at 500 ◦ C (10 ◦ C/min) for 30 min. Then it was cooled down to 60 ◦ C and kept for 60 min. Subsequently, the H2 /Ar mixture was changed to 50 ml of Ar and left flowing for 30 min. Finally the dynamic chemisorption was measured by heating from 60 to 550 ◦ C at a rate of 10 ◦ C/min. The dispersion (D%), metallic surface area (S, m2 /g-M) and metal average particle size (dP , nm) were calculated from the volume of chemisorbed H2 (TPD curve up to 450 ◦ C) using an earlier established method [28]. Temperature programmed reduction (TPR) characteristics of the oxide catalysts were obtained using the same instrument as in the
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Fig. 1. Ar and N2 adsorption–desorption isotherms at 87 K (A) and 77 K (B and C), pore size distribution applying NLDFT (D) and BJH methods (E and F).
above described TPD tests (AUTOCHEM 2910, Micromeritics). Prior to the measurements, the catalyst (ca. 100 mg) was dried in the TPR cell for 1 h at 500 ◦ C in the presence of an Ar stream to remove water. The TPR profiles were obtained by passing a 10% H2 /Ar flow (50 ml/min) through the sample at temperatures between 60 and 1000 ◦ C. The temperature was increased at a rate of 10 ◦ C/min and the amount of H2 consumed was determined by calibration of the output of the thermal conductivity detector (TCD). A cooling trap (liq. N2 + isopropanol) placed between the sample and the TCD was used to retain the water produced during the reduction process. 2.3. Catalytic activity tests The anisole HDO catalytic tests were carried out in a 100 ml stainless steel (SS) high-pressure stirred batch reactor. Initially, 50 ml of a solution containing 3 wt% anisole (Sigma–Aldrich) in decalin (Sigma–Aldrich) was loaded into the reactor. Before the reaction tests, the calcined samples were pelletized, crushed, sieved with 60–40 mesh (250–420 m) and reduced in a tubular furnace using the following treatment: flowing H2 (∼60 ml/min) at 500 ◦ C for 3 h under a heating rate of 1.8 ◦ C/min, cooling down to room temperature under argon flow, and finally passivizing by progressively feeding synthetic air at room temperature. The passivation treatment forms an oxide layer that avoids the sudden re-oxidation during the transfer to the reactor. Then, the H2 reduced catalyst was introduced in the reaction system. The sealed autoclave was purged by flowing both pure N2 and H2 at room temperature (three times each). The reactor was then pressurized with pure H2 up to 50 bar and rapidly heated up
(around 24 min) to the reaction temperature (220 ◦ C) under stirring (800 rpm). The heating causes that the pressure increases up to 60 bar, approximately. After 2 h, the reactor was cooled down rapidly with cold water and ice. Reactant and products were analyzed off-line by gas chromatography (Agilent, 7890A) using a FID detector and a HP-INNOWAX column, as well as by GC–MS (Bruker, BP-5 ms; 5% diphenyl/95% dimethyl polysiloxane column). The gas products were collected and analyzed separately in other GC Agilent chromatograph provided with a TCD detector and molecular sieve 5 A˚ and Poropack QS columns. In preliminary tests, the effect of stirring was investigated in order to select a rate value (800 rpm) appropriated to avoid external mass transfer limitations. Regarding internal diffusion phenomena, their possible influence is strongly determined by the particle diameter and the pore size. Pellets of the different catalysts have been prepared by pressing the samples in powder form, being subsequently sieved to get particles with diameters in the range 250–420 m. This small particle size has been selected to decrease as much as possible the length of the intraparticle diffusional pathway, avoiding that the diffusion through the macropores generated during pelletization may influence the overall reaction rate. This fact has been also checked in previous experiments carried out varying the pellet size. On the other hand, in the case of the transport of molecules through the meso- and micropores of the catalysts, it can be assumed that its effect on the overall reaction rate is not relevant due to the large size of the mesopores in the SBA-15 type supports (about 10 nm) and the high contribution of the secondary porosity in the hierarchical ZSM-5 sample, which is formed by aggregates of really small nanounits (5–10 nm).
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3. Results and discussion 3.1. Catalysts characterization Both supports and metal-supported catalysts (calcined form) were characterized by different techniques in order to assess the main properties affecting their catalytic activity: textural properties, metal dispersion and acidity. Fig. 1 displays the N2 (mesostructured-based catalysts) or Ar (zeolite-based catalysts) isotherms and the corresponding pore size distributions. As mentioned in Section 2, the pore size distribution was determined by using the BJH method in the case of mesostructured materials and a NLDFT-based model for the zeolitic catalysts. In addition, Table 1 summarizes the corresponding BET surface area and pore volume calculated from those measurements, as well as the real metal contents measured by ICP-OES. Both SBA-15- and Al-SBA-15-based samples are mesoporous materials with ordered cylindrical pores interconnected by micropores, having average mesopore sizes of 8.3 and 10.1 nm, respectively. Hierarchical ZSM-5 zeolite is characterized by having an enhanced surface area compared to conventional ZSM-5 samples due to the generation of a secondary porosity by silanization of the protozeolitic units prior to their crystallization stage [27]. Then, additionally to the micropores associated to the MFI structure, it presents mesopores in the range 2–10 nm. More details concerning the textural properties of these three types of supports can be found in the literature [27,29,30]. For all samples, loading Ni or Co into the supports caused an evident drop on both the surface area and total pore volume, indicating that, at least partially, deposition of metal nanoparticles has occurred within the channels. Concerning the pore size distribution a shift toward lower diameters is observed when Al-SBA-15 was used as support, the mean pore diameter decreasing from 10 nm down to about 7.5 nm for the Ni- and Co-containing samples. The acid properties of the parent supports were investigated by TPD analyses using NH3 as probe molecule. As expected, NH3 chemisorption was negligible in SBA-15 (0.05 mmol NH3 /g) denoting very low acidity. Al-SBA-15 presented higher acidity (0.123 mmol NH3 /g) due to the presence of Al3+ ions with Si/Al = 70 [30–32] and exhibited a broad desorption peak centered at around 270 ◦ C. Finally, h-ZSM-5 support revealed the highest acidic features, with an ammonia release of 0.353 mmol/g, which is in agreement with its higher Al content (Si/Al = 47). As earlier reported, this acidity in hierarchical ZSM-5 zeolites comprises not only Brönsted acid sites (H+ ) but also Lewis centers, which are generated during the calcination of the as-synthesized zeolite and are probably associated to the large share of external/mesopore surface area in these materials [33,34].
X-ray diffraction analyses of both supports and metal-supported catalysts are illustrated in Fig. 2. The low angle diffraction patterns (Fig. 2A) reveal the presence of an ordered mesoporous structure before and after metal impregnation for both the SBA-15 and AlSBA-15 based materials. The well-defined peaks corresponding to (1 0 0), (1 1 0) and (2 0 0) reflections, which are characteristics of a 2D hexagonal mesostructure p6mm, are clearly observed for SBA15. Nevertheless, incorporation of Al into the silica framework leads to a slight distortion of the structure that causes a decrease on the intensity of the two last signals, and even disappearing after both metals are deposited. Since characterization was applied to calcined samples, the existence of Ni and Co oxides is expected to be detected through wide-angle XRD patterns. For comparison purposes, the resultant patterns corresponding to supports and supported materials are presented in Fig. 2B and C, respectively. The oxide phases have been examined with the help of JCPDS data. However, oxide phases are not visible in the case of h-ZSM-5, which may be attributed to the formation of highly dispersed oxide particles not detectable by this technique. On the contrary, these oxide phases are quite visible in the case of SBA-15 and Al-SBA-15 supports. TEM images of NiO and Co3 O4 supported catalysts are presented in Fig. 3, revealing different distribution of the oxides depending on the supports. In the case of NiO and Co3 O4 over h-ZSM-5, particles are smaller and more difficult to distinguish, which is in accordance with the absence of XRD reflections related to these phases, as commented above. On the other hand, large particles of up to 100 nm in size can be appreciated over SBA-15, whereas smaller metal oxide agglomerates (around 50 nm) were attained over Al-SBA-15. Indeed, primary nanoparticles of around 10 nm can be distinguished forming these aggregates. In summary, the overall dispersion of the oxides over the supports decreases in the following order: h-ZSM-5 > Al-SBA-15 > SBA-15. Fig. 4 shows the H2 -TPR profiles of the impregnated materials. Clear differences in the reduction profiles of Ni and Co over h-ZSM5, SBA-15 and Al-SBA-15 are observed, denoting different spreading behavior and interaction of the active phases with each support. Ni/h-ZSM-5 shows a quite broad temperature range of reduction indicating the presence of different NiO species. The shoulder at 370 ◦ C is attributed to NiO particles that are more easily reduced to Ni. Correspondingly, it is also distinguished the presence of a broad peak at 530 ◦ C associated with strong metal–support interactions. In addition, there is a small H2 uptake at ∼708 ◦ C, which probably corresponds with trace amounts of Ni introduced in the zeolite by ion exchange. In the case of Ni/SBA-15, the presence of a broad peak can be considered as two overlapping contributions at around 300 ◦ C and 402 ◦ C, respectively. Comparing to the zeolitic support,
Table 1 Textural properties, surface acidity and H2 chemisorption of supports and metal supported catalysts. Sample name
Metal contenta [%]
SBET b [m2 /g]
VPORE c [cc/g]
Dd [%]
Metallic surface area, S [m2 /g-M]
Metal crystallite size, dP e [nm]
h-ZSM-5 Ni/h-ZSM-5 Co/h-ZSM5 SBA-15 Ni/SBA-15 Co/SBA-15 Al-SBA-15 Ni/Al-SBA-15 Co/Al-SBA-15
47* 4.41 4.67 – 4.37 4.57 70* 4.55 4.55
563 469 507 772 544 534 922 697 760
0.601 0.545 0.579 1.38 1.10 1.10 1.25 0.82 0.91
– 11.9 16.5 – 2 1.8 – 7.6 8.2
– 79.7 111.9 – 14 12.7 – 50.4 55.9
– 8.5 6 – 48 53 – 13.3 12
* a b c d e
Si/Al ratio from ICP analyses. Actual metal content from ICP analyses. Surface area determined by BET method. Total pore volume measured at P/Po ≈ 0.98. Metal dispersion from H2 -chemisorption method [stoichiometric factor, Ni(Co):H2 ,1]. Metal crystallite size from H2 -chemisorption.
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Fig. 2. Low- (A) and wide-angle (B, C) XRD analyses of supports and supported catalysts prepared.
these lower temperatures are in accordance with the lower dispersion of the metal nanoparticles, possessing larger sizes and leading to weaker interactions with the support. For the Ni/Al-SBA-15 sample, the H2 consumption peak could be decomposed into three overlapping contributions at around 320 ◦ C, 400 ◦ C and 530 ◦ C. In
contrast to SBA-15, the highest temperature peak denotes the generation of stronger metal–support interactions caused by the acidic sites created by the incorporation of Al3+ cations. The reduction profile for Co/h-ZSM-5 shows three peaks at around 293 ◦ C, 420 ◦ C and 742 ◦ C. The first one is assigned to the
Fig. 3. TEM images of supports and supported catalysts.
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Fig. 4. TPR profiles measured for all the supported catalysts.
transition of Co3 O4 to CoO, while the second, at 420 ◦ C, corresponds with the reduction of CoO phase to Co [29]. Furthermore, the large peak observed at high temperature suggests the existence of Co species with nanosizes and really strong metal–support interactions. This peak represents the largest contribution to the H2 -TPR profile in the Co/h-ZSM-5 and could indicate that a significant part of the Co species are not reduced when this catalyst is loaded into the reactor, being instead in the form of Co oxides. Regarding to the Co/SBA-15 sample, two strong hydrogen consumption peaks are observed at 232 ◦ C and 293 ◦ C, respectively. In addition, a very small peak at ∼820 ◦ C reveals the presence probably of Co silicate-type species. Finally, comparing to the pure silica support, the reduction profile of the Co/Al-SBA-15 sample shows a shift to higher temperatures with peaks at about 360 ◦ C and 660 ◦ C, respectively. This variation can be clearly assigned to the existence of specific and relatively strong interactions between the Co species and the aluminum incorporated into the mesopore walls. On the other hand, it must be taken into account that the H2 TPR measurements are carried out in dynamic conditions, so the equilibrium is not reached in any of the points obtained along the temperature ramp period. However, prior to the reaction tests the catalysts are reduced at 500 ◦ C by passing a H2 stream and holding these conditions for a period of 3 h, which increases significantly the amount of metal reduced if compared with the corresponding value in the H2 -TPR test. By performing the H2 -TPR experiments on the reduced samples, it has been possible to check that for most catalysts the reduction degree was very high (over 95%) after this treatment. Just in the case of the Co/Al-SBA-15 and Co/h-ZSM5 samples, reduction degrees significantly lower were obtained (about 86% and 71%, respectively), which may affect negatively their catalytic activity as further commented. In summary, the results of the H2 -TPR tests indicate how the interaction of the metallic species (Ni or Co) with the porous support is strongly affected by the nature of the latter and by the
Fig. 5. Reaction network in anisole transformation: (1) dealkylation and demethylation, (2) direct deoxygenation, (3) hydrogenation, (4) Isomerization, (5) dehydration, (6) alkylation, (7) ring opening reaction.
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Fig. 6. Conversion of anisole over Ni and Co supported catalysts.
presence of Al, being enhanced in the case of the hierarchical zeolite. Accordingly, the strength of the metal–support interactions varies as follows: h-ZSM-5 > Al-SBA-15 > SBA-15. 3.2. Catalytic activity In general HDO processes of oxygenated organic compounds proceed through several parallel/consecutive transformations. The sequence of reactions taking place during the hydrogenation and hydrodeoxygenation of anisole has been reported in previous works [4,5,21,30]. Fig. 5 illustrates the possible reaction pathways and involved products proposed by those studies. The main identified products are saturated compounds such as cyclohexane and methylcyclopentane, along with intermediates like phenol, benzene, cyclohexanol, cyclohexanone, cyclohexene, ethers and transalkyl products. In the case of gaseous products, methane and trace amounts of CO, ethylene and propyl/butyl isomers have been detected. The results of the anisole HDO tests over Ni and Co supported on h-ZSM-5, SBA-15 and Al-SBA-15 are presented in this section. Identification of the main reaction pathway of anisole as a function of both the nature of support and metal is pursued. The supported metal catalysts were evaluated at 220 ◦ C and 50 bar of pure hydrogen for 2 h. As indicated in Section 2, all the prepared catalysts were pre-reduced at 500 ◦ C for 3 h before reaction and protected by controlled passivation. The anisole conversion is presented in Fig. 6, showing that high values are obtained with all the catalysts. In fact, except for Co/Al-SBA-15 and Co/h-ZSM-5, which exhibit conversions around 70%, the rest of catalysts lead to almost total conversion of anisole. For both Co/Al-SBA-15 and Co/h-ZSM-5 catalysts, a strong metal–support interaction is present, as concluded from the H2 -TPR experiments, which suggests that the high strength of this interaction affects negatively to the catalytic activity for anisole conversion. In addition, the lower activity of these catalysts can be related to the fact that Co is not completely reduced when the catalyst is loaded into the reactor (71% and 86% reduction degree for Co/h-ZSM-5 and Co/Al-SBA-15 samples, respectively). The yields of the main reaction products are shown in Fig. 7 for all the assayed catalysts. The significant differences observed in product distribution reveal the strong influence of the support nature and, specially, of their acidity. The major product of both Ni and Co supported on h-ZSM-5 was found to be cyclohexane followed by methylcyclopentane. The conversion over Ni/h-ZSM-5 was 99.8%, while it was 69% over Co/h-ZSM-5. The product yield of Ni/h-ZSM-5 was 90.3% cyclohexane, 2.1% methylcyclopentane and trace amounts of benzene,
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phenol and cyclohexene. In addition, around 5.6% of dimethyl ether was observed. As a whole, these results suggest that the transformation of anisole on this catalyst occurs preferentially through the following route: anisole → benzene → cyclohexane (Fig. 5). The major products obtained with the Co/h-ZSM-5 were cyclohexane (64.6% yield) and minor amounts of methylcyclopentane and benzene. In both cases, the ratio of isomerization versus hydrogenation products is fairly similar (0.023 and 0.026). In the case of Ni and Co supported on SBA-15, the anisole conversions were 99.8% and 98.5%, respectively. Interestingly, the product distribution is entirely different compared to that obtained when using h-ZSM-5 as support. The product yields over Ni/SBA-15 were 53.5% phenol, 35.2% cyclohexanol, 8% cyclohexane, 2% cyclohexanone and trace amounts of benzene and 2 cyclohexyl-phenol. These results denote that the demethylation/ring hydrogenation route (Fig. 5) is the main pathway with this catalyst. In contrast, the major product obtained over Co/SBA-15 was methylcyclohexyl ether (around 72%), accompanied by lower amounts of cyclohexane (12%), cyclohexanone (1%) and cyclohexene (0.5%). This product distribution corresponds with the ring hydrogenation/demethylation route: anisole → methylcyclohexyl ether → cyclohexanol → cyclohexane. Therefore, HDO is significantly lower in the case of the metal supported on the non-acidic SBA-15 material, similar conclusions being earlier reported in the literature when comparing acidic and non-acidic supports [35]. Ni and Co supported on Al-SBA-15 led to anisole conversions rather similar to those of the h-ZSM-5-based materials. Ni/Al-SBA15 produced mainly cyclohexane (95%) and marginal amounts of methylcyclopentane (0.9%) and dimethyl ether (1.6%). In the case of Co/Al-SBA-15 the product yields were 69.8% of cyclohexane, 1.3% of benzene, 0.7% methylcyclopentane and 0.5% of dimethyl ether. Hydrodeoxygenation (HDO) and hydrodearomatization (HDA) activity were calculated based on the yields of both oxygenated and non-aromatics (hydrogenated products) compounds. The comparative data are presented in Fig. 8A and B. It can be seen that Co/SBA-15, Ni/Al-SBA-15 and Ni/h-ZSM-5 catalysts exhibit the highest HDA activity. However, only the last two catalysts present very high HDO performances, evidencing that Ni, combined with acidic supports, is a highly appropriated catalytic system for upgrading compounds containing methoxy-phenyl groups. Nevertheless, it must be taken into account that this comparison is made with lower conversion levels for the Co/h-ZSM-5 and Co/Al-SBA15 samples, due to their incomplete reduction degrees (71% and 86%, respectively) compared to the rest of the catalysts. Therefore, it could be expected that the anisole conversion over both materials, and likely their HDO activity, would increase if higher reduction degrees could be achieved. Dispersion values of metallic phases deposited over the different supports were determined by H2 -chemisorption (see Table 1). The degree of dispersion was in the order: Co/h-ZSM-5 > Ni/h-ZSM5 > Co/Al-SBA-15 > Ni/Al-SBA-15 > Ni/SBA-15 > Co/SBA-15. Due to the existence of abundant surface hydroxyl groups and strong acid centers, the metal interaction is stronger with the h-ZSM-5 support compared to Al-SBA-15 and SBA-15. These factors influence the dispersion and the spreading behavior of the metal during the reduction. The lowest dispersion was observed in the case of Ni/SBA-15 (D, 2.0%) and Co/SBA-15 (D, 1.8%), although significant anisole conversion via HDA was obtained with these catalysts. This result indicates that the main HDA activity is associated to the metallic phases, acting as pure hydrogenation catalysts, while the combination of metal–acid sites is required for favoring HDO reactions. Finally, the reason for the slightly higher HDO activity of Ni/Al-SBA-15 (D, 7.6%), in spite of its lower dispersion compared to Ni/h-ZSM-5 (D, 11.9%), could be derived from a larger electron
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Fig. 7. Product distribution attained over Ni and Co supported catalysts.
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Fig. 8. (A) Anisole conversion, HDO and HDA and (B) HDO versus HDA for all the catalysts tested.
density in the metal nanoparticles of the former catalyst, due to size and geometrical factors, facilitating the Ni-mediated hydrogenolysis reaction. Therefore, from the results presented and discussed above, it can be concluded that the nature of the support strongly influences the product distribution, as well as, that metallic sites associated with acid centers of the supports exhibit a synergistic role for promoting the HDO activity. In this way, it can be envisaged that the intermediates formed on the metal sites are then transferred to neighboring acid sites of the support, where they undergo further transformations. Nevertheless, a direct interaction of anisole with the acid sites may also occur in a significant extension. Moreover, it is interesting to point out that the order of catalytic activity does not agree with the degree of dispersion of the metal phases. This result can be related to the high hydrogenation ability of relatively large metal particles, as well as to changes in the intrinsic catalytic activity of the metal sites when the metal particles show a high dispersion and strong interaction with the support. In general, the cracking of anisole takes place through hydrogenolysis of the Ar OC bond or demethylation of the ArO C bond. Cleavage of the ArO C bond requires ∼385 kJ/mol whereas breaking the Ar OC bond is more difficult since it needs to supply ∼422 kJ/mol [36]. In this work, the formation of DME over the Ni/h-ZSM-5 catalyst suggests that cleavage of the Ar OCH3 bond is likely occurring through hydrogenolysis. Although the HDO of phenolic compounds has been extensively studied during the last five years, limited information exists on the ether formation. This compound has suitable Cetane number (∼55) and it can be blended up to 30% with diesel. This fact confirms the suitability of the Ni/h-ZSM-5 catalyst for the production of synthetic second generation biofuels from the phenolic fractions of lignocellulosic-derived bio-oils.
The acidity of the supports associated with the metals strongly favors the HDO conversion of anisole, which has an important effect on the product distribution, suggesting the existence of a synergetic effect between both metallic and acid sites. In the case of the acidic supports, Ni-based catalysts show the highest anisole conversion compared to Co-based materials. The strong interaction with the supports of the latter, which hinders the total Co reduction prior to the reaction, is probably the reason of their lower activity. Interestingly, the order of catalytic activity does not agree with the degree of dispersion of the metal phases. This result can be related to the high hydrogenation ability of relatively large metal particles, as well as to changes in the intrinsic catalytic activity of the metal sites when the metal particles show a high dispersion and strong interaction with the support. Hydrodeoxygenation, hydrodearomatization and isomerization reactions take place in high extension over the Ni/h-ZSM-5 sample, revealing it is a suitable catalyst for bio-oil processing in order to attain high quality fuels. This can be assigned to an improved dispersion of the Ni particles over the mesoporosity present in hierarchical ZSM-5. In addition, it is remarkable that over Ni/h-ZSM-5 significant amounts of isomerization products, as well as of dimethyl ether coming from anisole hydrogenolysis, were obtained. Acknowledgments The authors thank to the Spanish “Ministry of Economy and Competiveness” for their financial support through the LIGCATUP (ENE2011-29643-C02-01) and INNPACTO (IPT-20120219-120000) projects, as well as to the “Regional Government of Madrid” for the RESTOENE (P2009/ENE-1743) project. References
4. Conclusions Ni and Co supported on h-ZSM-5, SBA-15 and Al-SBA15 were synthesized and evaluated as HDO catalysts using anisole as biooil model component. The results of H2 -TPR and H2 chemisorption tests indicate how the interaction of the metallic species (Ni or Co) with the porous support, as well as their dispersion, is strongly affected by the support nature and the presence of Al. In particular, Co species evidence strong interactions with the acidic Al-SBA-15 and h-ZSM-5 supports.
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Hydrodeoxygenation of anisole as bio-oil model compound over supported Ni and Co catalysts: Effect of metal and support properties Thangaraju M. Sankaranarayanan a , Antonio Berenguer a , Cristina Ochoa-Hernández a , Inés Moreno a,b , Prabhas Jana a , Juan M. Coronado a , David P. Serrano a,b , Patricia Pizarro a,b,∗ a b
IMDEA Energy Institute, Avda. Ramón de la Sagra 3, Móstoles, Madrid, Spain Chemical and Environmental Engineering Group, ESCET, Universidad Rey Juan Carlos, Móstoles, Madrid, Spain
a r t i c l e
i n f o
Article history: Received 22 June 2014 Received in revised form 28 August 2014 Accepted 2 September 2014 Available online xxx Keywords: Hydrodeoxygenation Anisole Hierarchical zeolite ZSM-5 SBA-15
a b s t r a c t Hydrodeoxygenation (HDO) is considered one of the most promising routes to convert the pyrolysis bio-oils produced from lignocellulose into biofuels very similar to those derived from petroleum. This work reports results obtained in the catalytic HDO of anisole over non-conventional hydrotreating catalysts based on metallic Ni and Co supported on micro-mesoporous carriers of different acidity. Anisole has been chosen as representative of those compounds, containing a methoxy-phenyl group, which are abundant in lignocellulose pyrolysis bio-oils. The effect of both metal phase and support properties on their performance as HDO catalysts has been studied. With this aim, three supports with different textural and acidic properties have been employed: hierarchical ZSM-5, mesostructured pure silica SBA-15 and mesostructured Al-SBA-15. The reactions have been carried out in a stainless steel high pressure batch reactor at 220 ◦ C and with 50 bar of pure hydrogen. The interaction of the metallic species (Ni or Co) with the porous supports, as well as their dispersion, is strongly affected by the support nature and the presence of Al. Thus, it has been found a synergetic effect between the acid sites of the supports and the metallic active phases, which favors and enhances the HDO of anisole. In the case of the acidic supports, Ni-based catalysts (Ni/Al-SBA-15 and Ni/h-ZSM-5) show larger anisole conversions compared to Co-based materials. The strong interaction with the acidic supports of the latter hinders the total Co reduction prior to the reaction, being probably the major reason of their lower activity. Hydrodeoxygenation, hydrodearomatization and isomerization reactions take place extensively over Ni/h-ZSM-5, revealing it is a promising catalyst for bio-oil processing in order to attain high quality fuel production. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Biofuels can make an important contribution to overcome the energy challenges of the XXI century and significantly reduce the huge CO2 emissions associated with the transport sector. For instance, biofuels are expected to play a crucial role as substitutes for petroleum-based fuels used in heavy transport (aviation, maritime transport, etc.) where the shift from conventional engines to electric ones is still far away from being viable. According
∗ Corresponding author at: Thermochemical Processes Unit, IMDEA Energy Institute, Avda. Ramón de la Sagra 3, Móstoles, Madrid, Spain. Tel.: +34 91 737 11 20; fax: +34 91 737 11 40. E-mail addresses: [email protected], [email protected] (P. Pizarro).
to the International Energy Agency (IEA), conventional biofuels, commonly referred to as first generation biofuels, are those whose production process has already reached technological maturity and are implanted at the commercial level [1]. It is the case of bioethanol, biodiesel and biogas obtained through processes of fermentation, transesterification and anaerobic digestion, respectively. However, there are some controversies about their real environmental and socio-economic impacts, including the competition between the provision of raw materials for the production of biofuels and food supply, the risk of deforestation in tropical areas and a positive net balance of CO2 emissions by some biofuels revealed by several recent Life Cycle Assessment (LCA) studies [2]. Resulting from these drawbacks, one of the goals set by the European Union for 2020 is limiting the contribution of first generation biofuels while promoting the development of other and sustainable biofuels [1]. These advanced biofuels will be obtained from
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raw materials that do not compete with the food industry, mainly from plants cultivated in non-agricultural or marginal lands, as well as from lignocellulosic wastes. Lignocellulose, which is composed by cellulose, hemicellulose and lignin, can be transformed by either fast or flash pyrolysis, yielding up to 70% of a liquid product known as bio-oil. This bio-oil has a high potential as a liquid fuel since it retains up to 70% of the initial energy of the starting biomass and contains less nitrogen and sulphur than fossil fuels [3]. However, it possesses an excessively high content in water (15–25 wt%) and oxygen (>40 wt%) for being directly employed as a high quality fuel for transportation [4,5]. Likewise, the untreated bio-oil has other multiple deficiencies to be used as fuel: (a) relatively low calorific value, (b) high corrosiveness due to its low pH and (c) high viscosity and limited chemical stability in comparison with conventional fuels. Therefore, in order to use the pyrolysis bio-oil as liquid fuel in conventional engines, it is necessary to carry out additional processing [6,7]. Among the catalytic paths for upgrading the pyrolysis bio-oils, hydrodeoxygenation (HDO) seems to be one of the most promising [7,8]. This process consists in the treatment of bio-oil at temperatures between 250 and 450 ◦ C with hydrogen at high pressure (20–300 bar) in the presence of a catalyst, so that the oxygen contained in organic molecules is extracted in the form of water. Sulfided CoMo and NiMo catalysts have been widely studied for HDO processes, as they are conventional catalysts employed in petroleum hydroprocessing [9–15]. Other catalysts based on oxides [16–18], transition metals, supported noble metals (Pt, Pd and Ru) [19,20], metallic phosphides [21,22] or zeolites (ZSM-5) [23] are being investigated. In the present study non-conventional catalysts based on supported metallic Ni or Co are explored as potential catalysts for HDO. The effect of both metal phase and support properties, as well as their interactions, on their performance as HDO catalysts has been investigated. With this aim, three supports with different textural and acidic properties have been selected: hierarchical ZSM-5 (h-ZSM-5) with Si/Al = 47, mesostructured pure silica (SBA-15) and mesostructured Al-SBA-15 with Si/Al = 70. For the catalytic assays, anisole has been chosen as model compound because it contains a methoxy-phenyl group, which is characteristic of lignin depolymerisation fragments present in pyrolysis bio-oils obtained from lignocellulosic biomass [24].
2. Experimental 2.1. Catalysts preparation 2.1.1. Synthesis of supports Pure silica SBA-15 was synthesized by a hydrothermal method described by Zhao et al. [25]. Typically, 8 g of Pluronic 123 (Aldrich) was dissolved in 250 ml of 1.9 M HCl at room temperature. Then, it was heated to 40 ◦ C and the silica source, 16.2 g of tetraethylorthosilicate (TEOS, Aldrich), was added. After stirring for 20 h at 40 ◦ C, the mixture was aged at 110 ◦ C for 24 h under autogenous pressure. The solid product was recovered by filtration, dried overnight and calcined at 550 ◦ C for 5 h using a heating rate of 1.8 ◦ C/min under static air. Al-containing SBA-15 (Al-SBA-15) was synthesized according to a procedure described in literature [26], using aluminum isopropoxide (AIP, Aldrich) as Al source. In a typical synthesis, 8.5 g of TEOS and 0.125 g of AIP (SiO2 /Al2 O3 mol ratio = 135) were added to 10 ml of an aqueous HCl solution (pH = 1.5). After stirring for 3 h, the homogenous mixture was added to other solution containing 4 g of Pluronic 123 dissolved in 150 ml of HCl at pH = 1.5. The final mixture was stirred for 20 h at 40 ◦ C and aged under static
conditions and autogenous pressure at 110 ◦ C for 24 h. The solid product obtained was filtered, dried overnight and calcined in static air at 550 ◦ C for 5 h with a heating rate of 1.8 ◦ C/min. Hierarchical ZSM-5 zeolite was obtained by a procedure based on the silanization of protozeolitic units previously to the hydrothermal crystallization treatment [27]. Firstly, a clear solution containing tetraethylorthosilicate, tetrapropylammonium hydroxide, aluminum isopropoxide and distilled water, with the following molar composition: 1 Al2 O3 :80 SiO2 :14.4 TPAOH:2000 H2 O, was precrystallized under reflux with stirring at 90 ◦ C for 20 h. Subsequently, 8 mol% (referred to the total silica content) of the seed silanization agent (phenylaminopropyltrimethoxysilane, PHAPTMS) was added and left reacting at 90 ◦ C for 6 h. The final solution was crystallized in an autoclave at 170 ◦ C for 7 days. The solid products were separated by centrifugation, washed several times, dried overnight at 110 ◦ C and calcined in air at 550 ◦ C for 5 h. 2.1.2. Metal incorporation In all cases the catalysts were prepared by incipient wetness impregnation of aqueous solutions containing Ni(NO3 )2 ·6H2 O or Co(NO3 )2 ·6H2 O (Aldrich) in the amount required to get a final metal loading of 5 wt%, followed by calcination at 550 ◦ C for 5 h. 2.2. Catalysts characterization Porous properties were measured by gas physisorption under isothermal conditions (77 or 87 K, respectively). SBA-15 and Al-SBA-15 materials were analyzed in a Quadrasorb system (N2 adsorption–desorption) while h-ZSM-5 was measured in an Autosorb equipment (Ar adsorption–desorption). The surface area was calculated by the BET method. The pore size distribution was determined by applying the BJH method to the adsorption branch of the isotherm in the case of mesostructured materials, while the nonlocal density functional theory (NLDFT) was used for the zeolitic catalysts. Metal content was determined by ICP-OES on a Perkin Elmer Optima 7300AD instrument. X-ray diffraction (XRD) patterns of the catalysts were obtained using a Philips X’Pert PRO ˚ The phases were diffractometer with Cu-K␣ radiation ( = 1.542 A). identified by matching the peaks appearing in the XRD pattern of the test sample with JCPDS (Joint Committee on Powder Diffraction Standards) data files. TEM images of the catalysts were taken using a PHILIPS TECNAI 20T instrument, working at 200 kV and equipped with an EDX spectrometer for measuring X-ray energy dispersive spectra. The acidity of the supports was measured by temperature programmed desorption (TPD) of NH3 using an AUTOCHEM 2910 system (Micromeritics). The standard procedure for the TPD measurements involved the activation of the sample by flowing He at 600 ◦ C (1 h), cooling to 100 ◦ C, adsorbing NH3 from a He–NH3 (15%) mixture, removing the weakly adsorbed NH3 by flowing He at 100 ◦ C for 30 min, and finally carrying out the TPD experiment by raising the temperature of the catalyst sample with a ramp of 10 ◦ C/min. H2 chemisorption (H2 -TPD) was carried out using the same setup as in the NH3 -TPD measurements. In a first step, the sample (ca. 100 mg) was degassed and reduced with 50 ml of 10% H2 in Ar at 500 ◦ C (10 ◦ C/min) for 30 min. Then it was cooled down to 60 ◦ C and kept for 60 min. Subsequently, the H2 /Ar mixture was changed to 50 ml of Ar and left flowing for 30 min. Finally the dynamic chemisorption was measured by heating from 60 to 550 ◦ C at a rate of 10 ◦ C/min. The dispersion (D%), metallic surface area (S, m2 /g-M) and metal average particle size (dP , nm) were calculated from the volume of chemisorbed H2 (TPD curve up to 450 ◦ C) using an earlier established method [28]. Temperature programmed reduction (TPR) characteristics of the oxide catalysts were obtained using the same instrument as in the
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Fig. 1. Ar and N2 adsorption–desorption isotherms at 87 K (A) and 77 K (B and C), pore size distribution applying NLDFT (D) and BJH methods (E and F).
above described TPD tests (AUTOCHEM 2910, Micromeritics). Prior to the measurements, the catalyst (ca. 100 mg) was dried in the TPR cell for 1 h at 500 ◦ C in the presence of an Ar stream to remove water. The TPR profiles were obtained by passing a 10% H2 /Ar flow (50 ml/min) through the sample at temperatures between 60 and 1000 ◦ C. The temperature was increased at a rate of 10 ◦ C/min and the amount of H2 consumed was determined by calibration of the output of the thermal conductivity detector (TCD). A cooling trap (liq. N2 + isopropanol) placed between the sample and the TCD was used to retain the water produced during the reduction process. 2.3. Catalytic activity tests The anisole HDO catalytic tests were carried out in a 100 ml stainless steel (SS) high-pressure stirred batch reactor. Initially, 50 ml of a solution containing 3 wt% anisole (Sigma–Aldrich) in decalin (Sigma–Aldrich) was loaded into the reactor. Before the reaction tests, the calcined samples were pelletized, crushed, sieved with 60–40 mesh (250–420 m) and reduced in a tubular furnace using the following treatment: flowing H2 (∼60 ml/min) at 500 ◦ C for 3 h under a heating rate of 1.8 ◦ C/min, cooling down to room temperature under argon flow, and finally passivizing by progressively feeding synthetic air at room temperature. The passivation treatment forms an oxide layer that avoids the sudden re-oxidation during the transfer to the reactor. Then, the H2 reduced catalyst was introduced in the reaction system. The sealed autoclave was purged by flowing both pure N2 and H2 at room temperature (three times each). The reactor was then pressurized with pure H2 up to 50 bar and rapidly heated up
(around 24 min) to the reaction temperature (220 ◦ C) under stirring (800 rpm). The heating causes that the pressure increases up to 60 bar, approximately. After 2 h, the reactor was cooled down rapidly with cold water and ice. Reactant and products were analyzed off-line by gas chromatography (Agilent, 7890A) using a FID detector and a HP-INNOWAX column, as well as by GC–MS (Bruker, BP-5 ms; 5% diphenyl/95% dimethyl polysiloxane column). The gas products were collected and analyzed separately in other GC Agilent chromatograph provided with a TCD detector and molecular sieve 5 A˚ and Poropack QS columns. In preliminary tests, the effect of stirring was investigated in order to select a rate value (800 rpm) appropriated to avoid external mass transfer limitations. Regarding internal diffusion phenomena, their possible influence is strongly determined by the particle diameter and the pore size. Pellets of the different catalysts have been prepared by pressing the samples in powder form, being subsequently sieved to get particles with diameters in the range 250–420 m. This small particle size has been selected to decrease as much as possible the length of the intraparticle diffusional pathway, avoiding that the diffusion through the macropores generated during pelletization may influence the overall reaction rate. This fact has been also checked in previous experiments carried out varying the pellet size. On the other hand, in the case of the transport of molecules through the meso- and micropores of the catalysts, it can be assumed that its effect on the overall reaction rate is not relevant due to the large size of the mesopores in the SBA-15 type supports (about 10 nm) and the high contribution of the secondary porosity in the hierarchical ZSM-5 sample, which is formed by aggregates of really small nanounits (5–10 nm).
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3. Results and discussion 3.1. Catalysts characterization Both supports and metal-supported catalysts (calcined form) were characterized by different techniques in order to assess the main properties affecting their catalytic activity: textural properties, metal dispersion and acidity. Fig. 1 displays the N2 (mesostructured-based catalysts) or Ar (zeolite-based catalysts) isotherms and the corresponding pore size distributions. As mentioned in Section 2, the pore size distribution was determined by using the BJH method in the case of mesostructured materials and a NLDFT-based model for the zeolitic catalysts. In addition, Table 1 summarizes the corresponding BET surface area and pore volume calculated from those measurements, as well as the real metal contents measured by ICP-OES. Both SBA-15- and Al-SBA-15-based samples are mesoporous materials with ordered cylindrical pores interconnected by micropores, having average mesopore sizes of 8.3 and 10.1 nm, respectively. Hierarchical ZSM-5 zeolite is characterized by having an enhanced surface area compared to conventional ZSM-5 samples due to the generation of a secondary porosity by silanization of the protozeolitic units prior to their crystallization stage [27]. Then, additionally to the micropores associated to the MFI structure, it presents mesopores in the range 2–10 nm. More details concerning the textural properties of these three types of supports can be found in the literature [27,29,30]. For all samples, loading Ni or Co into the supports caused an evident drop on both the surface area and total pore volume, indicating that, at least partially, deposition of metal nanoparticles has occurred within the channels. Concerning the pore size distribution a shift toward lower diameters is observed when Al-SBA-15 was used as support, the mean pore diameter decreasing from 10 nm down to about 7.5 nm for the Ni- and Co-containing samples. The acid properties of the parent supports were investigated by TPD analyses using NH3 as probe molecule. As expected, NH3 chemisorption was negligible in SBA-15 (0.05 mmol NH3 /g) denoting very low acidity. Al-SBA-15 presented higher acidity (0.123 mmol NH3 /g) due to the presence of Al3+ ions with Si/Al = 70 [30–32] and exhibited a broad desorption peak centered at around 270 ◦ C. Finally, h-ZSM-5 support revealed the highest acidic features, with an ammonia release of 0.353 mmol/g, which is in agreement with its higher Al content (Si/Al = 47). As earlier reported, this acidity in hierarchical ZSM-5 zeolites comprises not only Brönsted acid sites (H+ ) but also Lewis centers, which are generated during the calcination of the as-synthesized zeolite and are probably associated to the large share of external/mesopore surface area in these materials [33,34].
X-ray diffraction analyses of both supports and metal-supported catalysts are illustrated in Fig. 2. The low angle diffraction patterns (Fig. 2A) reveal the presence of an ordered mesoporous structure before and after metal impregnation for both the SBA-15 and AlSBA-15 based materials. The well-defined peaks corresponding to (1 0 0), (1 1 0) and (2 0 0) reflections, which are characteristics of a 2D hexagonal mesostructure p6mm, are clearly observed for SBA15. Nevertheless, incorporation of Al into the silica framework leads to a slight distortion of the structure that causes a decrease on the intensity of the two last signals, and even disappearing after both metals are deposited. Since characterization was applied to calcined samples, the existence of Ni and Co oxides is expected to be detected through wide-angle XRD patterns. For comparison purposes, the resultant patterns corresponding to supports and supported materials are presented in Fig. 2B and C, respectively. The oxide phases have been examined with the help of JCPDS data. However, oxide phases are not visible in the case of h-ZSM-5, which may be attributed to the formation of highly dispersed oxide particles not detectable by this technique. On the contrary, these oxide phases are quite visible in the case of SBA-15 and Al-SBA-15 supports. TEM images of NiO and Co3 O4 supported catalysts are presented in Fig. 3, revealing different distribution of the oxides depending on the supports. In the case of NiO and Co3 O4 over h-ZSM-5, particles are smaller and more difficult to distinguish, which is in accordance with the absence of XRD reflections related to these phases, as commented above. On the other hand, large particles of up to 100 nm in size can be appreciated over SBA-15, whereas smaller metal oxide agglomerates (around 50 nm) were attained over Al-SBA-15. Indeed, primary nanoparticles of around 10 nm can be distinguished forming these aggregates. In summary, the overall dispersion of the oxides over the supports decreases in the following order: h-ZSM-5 > Al-SBA-15 > SBA-15. Fig. 4 shows the H2 -TPR profiles of the impregnated materials. Clear differences in the reduction profiles of Ni and Co over h-ZSM5, SBA-15 and Al-SBA-15 are observed, denoting different spreading behavior and interaction of the active phases with each support. Ni/h-ZSM-5 shows a quite broad temperature range of reduction indicating the presence of different NiO species. The shoulder at 370 ◦ C is attributed to NiO particles that are more easily reduced to Ni. Correspondingly, it is also distinguished the presence of a broad peak at 530 ◦ C associated with strong metal–support interactions. In addition, there is a small H2 uptake at ∼708 ◦ C, which probably corresponds with trace amounts of Ni introduced in the zeolite by ion exchange. In the case of Ni/SBA-15, the presence of a broad peak can be considered as two overlapping contributions at around 300 ◦ C and 402 ◦ C, respectively. Comparing to the zeolitic support,
Table 1 Textural properties, surface acidity and H2 chemisorption of supports and metal supported catalysts. Sample name
Metal contenta [%]
SBET b [m2 /g]
VPORE c [cc/g]
Dd [%]
Metallic surface area, S [m2 /g-M]
Metal crystallite size, dP e [nm]
h-ZSM-5 Ni/h-ZSM-5 Co/h-ZSM5 SBA-15 Ni/SBA-15 Co/SBA-15 Al-SBA-15 Ni/Al-SBA-15 Co/Al-SBA-15
47* 4.41 4.67 – 4.37 4.57 70* 4.55 4.55
563 469 507 772 544 534 922 697 760
0.601 0.545 0.579 1.38 1.10 1.10 1.25 0.82 0.91
– 11.9 16.5 – 2 1.8 – 7.6 8.2
– 79.7 111.9 – 14 12.7 – 50.4 55.9
– 8.5 6 – 48 53 – 13.3 12
* a b c d e
Si/Al ratio from ICP analyses. Actual metal content from ICP analyses. Surface area determined by BET method. Total pore volume measured at P/Po ≈ 0.98. Metal dispersion from H2 -chemisorption method [stoichiometric factor, Ni(Co):H2 ,1]. Metal crystallite size from H2 -chemisorption.
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Fig. 2. Low- (A) and wide-angle (B, C) XRD analyses of supports and supported catalysts prepared.
these lower temperatures are in accordance with the lower dispersion of the metal nanoparticles, possessing larger sizes and leading to weaker interactions with the support. For the Ni/Al-SBA-15 sample, the H2 consumption peak could be decomposed into three overlapping contributions at around 320 ◦ C, 400 ◦ C and 530 ◦ C. In
contrast to SBA-15, the highest temperature peak denotes the generation of stronger metal–support interactions caused by the acidic sites created by the incorporation of Al3+ cations. The reduction profile for Co/h-ZSM-5 shows three peaks at around 293 ◦ C, 420 ◦ C and 742 ◦ C. The first one is assigned to the
Fig. 3. TEM images of supports and supported catalysts.
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Fig. 4. TPR profiles measured for all the supported catalysts.
transition of Co3 O4 to CoO, while the second, at 420 ◦ C, corresponds with the reduction of CoO phase to Co [29]. Furthermore, the large peak observed at high temperature suggests the existence of Co species with nanosizes and really strong metal–support interactions. This peak represents the largest contribution to the H2 -TPR profile in the Co/h-ZSM-5 and could indicate that a significant part of the Co species are not reduced when this catalyst is loaded into the reactor, being instead in the form of Co oxides. Regarding to the Co/SBA-15 sample, two strong hydrogen consumption peaks are observed at 232 ◦ C and 293 ◦ C, respectively. In addition, a very small peak at ∼820 ◦ C reveals the presence probably of Co silicate-type species. Finally, comparing to the pure silica support, the reduction profile of the Co/Al-SBA-15 sample shows a shift to higher temperatures with peaks at about 360 ◦ C and 660 ◦ C, respectively. This variation can be clearly assigned to the existence of specific and relatively strong interactions between the Co species and the aluminum incorporated into the mesopore walls. On the other hand, it must be taken into account that the H2 TPR measurements are carried out in dynamic conditions, so the equilibrium is not reached in any of the points obtained along the temperature ramp period. However, prior to the reaction tests the catalysts are reduced at 500 ◦ C by passing a H2 stream and holding these conditions for a period of 3 h, which increases significantly the amount of metal reduced if compared with the corresponding value in the H2 -TPR test. By performing the H2 -TPR experiments on the reduced samples, it has been possible to check that for most catalysts the reduction degree was very high (over 95%) after this treatment. Just in the case of the Co/Al-SBA-15 and Co/h-ZSM5 samples, reduction degrees significantly lower were obtained (about 86% and 71%, respectively), which may affect negatively their catalytic activity as further commented. In summary, the results of the H2 -TPR tests indicate how the interaction of the metallic species (Ni or Co) with the porous support is strongly affected by the nature of the latter and by the
Fig. 5. Reaction network in anisole transformation: (1) dealkylation and demethylation, (2) direct deoxygenation, (3) hydrogenation, (4) Isomerization, (5) dehydration, (6) alkylation, (7) ring opening reaction.
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Fig. 6. Conversion of anisole over Ni and Co supported catalysts.
presence of Al, being enhanced in the case of the hierarchical zeolite. Accordingly, the strength of the metal–support interactions varies as follows: h-ZSM-5 > Al-SBA-15 > SBA-15. 3.2. Catalytic activity In general HDO processes of oxygenated organic compounds proceed through several parallel/consecutive transformations. The sequence of reactions taking place during the hydrogenation and hydrodeoxygenation of anisole has been reported in previous works [4,5,21,30]. Fig. 5 illustrates the possible reaction pathways and involved products proposed by those studies. The main identified products are saturated compounds such as cyclohexane and methylcyclopentane, along with intermediates like phenol, benzene, cyclohexanol, cyclohexanone, cyclohexene, ethers and transalkyl products. In the case of gaseous products, methane and trace amounts of CO, ethylene and propyl/butyl isomers have been detected. The results of the anisole HDO tests over Ni and Co supported on h-ZSM-5, SBA-15 and Al-SBA-15 are presented in this section. Identification of the main reaction pathway of anisole as a function of both the nature of support and metal is pursued. The supported metal catalysts were evaluated at 220 ◦ C and 50 bar of pure hydrogen for 2 h. As indicated in Section 2, all the prepared catalysts were pre-reduced at 500 ◦ C for 3 h before reaction and protected by controlled passivation. The anisole conversion is presented in Fig. 6, showing that high values are obtained with all the catalysts. In fact, except for Co/Al-SBA-15 and Co/h-ZSM-5, which exhibit conversions around 70%, the rest of catalysts lead to almost total conversion of anisole. For both Co/Al-SBA-15 and Co/h-ZSM-5 catalysts, a strong metal–support interaction is present, as concluded from the H2 -TPR experiments, which suggests that the high strength of this interaction affects negatively to the catalytic activity for anisole conversion. In addition, the lower activity of these catalysts can be related to the fact that Co is not completely reduced when the catalyst is loaded into the reactor (71% and 86% reduction degree for Co/h-ZSM-5 and Co/Al-SBA-15 samples, respectively). The yields of the main reaction products are shown in Fig. 7 for all the assayed catalysts. The significant differences observed in product distribution reveal the strong influence of the support nature and, specially, of their acidity. The major product of both Ni and Co supported on h-ZSM-5 was found to be cyclohexane followed by methylcyclopentane. The conversion over Ni/h-ZSM-5 was 99.8%, while it was 69% over Co/h-ZSM-5. The product yield of Ni/h-ZSM-5 was 90.3% cyclohexane, 2.1% methylcyclopentane and trace amounts of benzene,
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phenol and cyclohexene. In addition, around 5.6% of dimethyl ether was observed. As a whole, these results suggest that the transformation of anisole on this catalyst occurs preferentially through the following route: anisole → benzene → cyclohexane (Fig. 5). The major products obtained with the Co/h-ZSM-5 were cyclohexane (64.6% yield) and minor amounts of methylcyclopentane and benzene. In both cases, the ratio of isomerization versus hydrogenation products is fairly similar (0.023 and 0.026). In the case of Ni and Co supported on SBA-15, the anisole conversions were 99.8% and 98.5%, respectively. Interestingly, the product distribution is entirely different compared to that obtained when using h-ZSM-5 as support. The product yields over Ni/SBA-15 were 53.5% phenol, 35.2% cyclohexanol, 8% cyclohexane, 2% cyclohexanone and trace amounts of benzene and 2 cyclohexyl-phenol. These results denote that the demethylation/ring hydrogenation route (Fig. 5) is the main pathway with this catalyst. In contrast, the major product obtained over Co/SBA-15 was methylcyclohexyl ether (around 72%), accompanied by lower amounts of cyclohexane (12%), cyclohexanone (1%) and cyclohexene (0.5%). This product distribution corresponds with the ring hydrogenation/demethylation route: anisole → methylcyclohexyl ether → cyclohexanol → cyclohexane. Therefore, HDO is significantly lower in the case of the metal supported on the non-acidic SBA-15 material, similar conclusions being earlier reported in the literature when comparing acidic and non-acidic supports [35]. Ni and Co supported on Al-SBA-15 led to anisole conversions rather similar to those of the h-ZSM-5-based materials. Ni/Al-SBA15 produced mainly cyclohexane (95%) and marginal amounts of methylcyclopentane (0.9%) and dimethyl ether (1.6%). In the case of Co/Al-SBA-15 the product yields were 69.8% of cyclohexane, 1.3% of benzene, 0.7% methylcyclopentane and 0.5% of dimethyl ether. Hydrodeoxygenation (HDO) and hydrodearomatization (HDA) activity were calculated based on the yields of both oxygenated and non-aromatics (hydrogenated products) compounds. The comparative data are presented in Fig. 8A and B. It can be seen that Co/SBA-15, Ni/Al-SBA-15 and Ni/h-ZSM-5 catalysts exhibit the highest HDA activity. However, only the last two catalysts present very high HDO performances, evidencing that Ni, combined with acidic supports, is a highly appropriated catalytic system for upgrading compounds containing methoxy-phenyl groups. Nevertheless, it must be taken into account that this comparison is made with lower conversion levels for the Co/h-ZSM-5 and Co/Al-SBA15 samples, due to their incomplete reduction degrees (71% and 86%, respectively) compared to the rest of the catalysts. Therefore, it could be expected that the anisole conversion over both materials, and likely their HDO activity, would increase if higher reduction degrees could be achieved. Dispersion values of metallic phases deposited over the different supports were determined by H2 -chemisorption (see Table 1). The degree of dispersion was in the order: Co/h-ZSM-5 > Ni/h-ZSM5 > Co/Al-SBA-15 > Ni/Al-SBA-15 > Ni/SBA-15 > Co/SBA-15. Due to the existence of abundant surface hydroxyl groups and strong acid centers, the metal interaction is stronger with the h-ZSM-5 support compared to Al-SBA-15 and SBA-15. These factors influence the dispersion and the spreading behavior of the metal during the reduction. The lowest dispersion was observed in the case of Ni/SBA-15 (D, 2.0%) and Co/SBA-15 (D, 1.8%), although significant anisole conversion via HDA was obtained with these catalysts. This result indicates that the main HDA activity is associated to the metallic phases, acting as pure hydrogenation catalysts, while the combination of metal–acid sites is required for favoring HDO reactions. Finally, the reason for the slightly higher HDO activity of Ni/Al-SBA-15 (D, 7.6%), in spite of its lower dispersion compared to Ni/h-ZSM-5 (D, 11.9%), could be derived from a larger electron
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Fig. 7. Product distribution attained over Ni and Co supported catalysts.
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Fig. 8. (A) Anisole conversion, HDO and HDA and (B) HDO versus HDA for all the catalysts tested.
density in the metal nanoparticles of the former catalyst, due to size and geometrical factors, facilitating the Ni-mediated hydrogenolysis reaction. Therefore, from the results presented and discussed above, it can be concluded that the nature of the support strongly influences the product distribution, as well as, that metallic sites associated with acid centers of the supports exhibit a synergistic role for promoting the HDO activity. In this way, it can be envisaged that the intermediates formed on the metal sites are then transferred to neighboring acid sites of the support, where they undergo further transformations. Nevertheless, a direct interaction of anisole with the acid sites may also occur in a significant extension. Moreover, it is interesting to point out that the order of catalytic activity does not agree with the degree of dispersion of the metal phases. This result can be related to the high hydrogenation ability of relatively large metal particles, as well as to changes in the intrinsic catalytic activity of the metal sites when the metal particles show a high dispersion and strong interaction with the support. In general, the cracking of anisole takes place through hydrogenolysis of the Ar OC bond or demethylation of the ArO C bond. Cleavage of the ArO C bond requires ∼385 kJ/mol whereas breaking the Ar OC bond is more difficult since it needs to supply ∼422 kJ/mol [36]. In this work, the formation of DME over the Ni/h-ZSM-5 catalyst suggests that cleavage of the Ar OCH3 bond is likely occurring through hydrogenolysis. Although the HDO of phenolic compounds has been extensively studied during the last five years, limited information exists on the ether formation. This compound has suitable Cetane number (∼55) and it can be blended up to 30% with diesel. This fact confirms the suitability of the Ni/h-ZSM-5 catalyst for the production of synthetic second generation biofuels from the phenolic fractions of lignocellulosic-derived bio-oils.
The acidity of the supports associated with the metals strongly favors the HDO conversion of anisole, which has an important effect on the product distribution, suggesting the existence of a synergetic effect between both metallic and acid sites. In the case of the acidic supports, Ni-based catalysts show the highest anisole conversion compared to Co-based materials. The strong interaction with the supports of the latter, which hinders the total Co reduction prior to the reaction, is probably the reason of their lower activity. Interestingly, the order of catalytic activity does not agree with the degree of dispersion of the metal phases. This result can be related to the high hydrogenation ability of relatively large metal particles, as well as to changes in the intrinsic catalytic activity of the metal sites when the metal particles show a high dispersion and strong interaction with the support. Hydrodeoxygenation, hydrodearomatization and isomerization reactions take place in high extension over the Ni/h-ZSM-5 sample, revealing it is a suitable catalyst for bio-oil processing in order to attain high quality fuels. This can be assigned to an improved dispersion of the Ni particles over the mesoporosity present in hierarchical ZSM-5. In addition, it is remarkable that over Ni/h-ZSM-5 significant amounts of isomerization products, as well as of dimethyl ether coming from anisole hydrogenolysis, were obtained. Acknowledgments The authors thank to the Spanish “Ministry of Economy and Competiveness” for their financial support through the LIGCATUP (ENE2011-29643-C02-01) and INNPACTO (IPT-20120219-120000) projects, as well as to the “Regional Government of Madrid” for the RESTOENE (P2009/ENE-1743) project. References
4. Conclusions Ni and Co supported on h-ZSM-5, SBA-15 and Al-SBA15 were synthesized and evaluated as HDO catalysts using anisole as biooil model component. The results of H2 -TPR and H2 chemisorption tests indicate how the interaction of the metallic species (Ni or Co) with the porous support, as well as their dispersion, is strongly affected by the support nature and the presence of Al. In particular, Co species evidence strong interactions with the acidic Al-SBA-15 and h-ZSM-5 supports.
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Please cite this article in press as: T.M. Sankaranarayanan, et al., Hydrodeoxygenation of anisole as bio-oil model compound over supported Ni and Co catalysts: Effect of metal and support properties, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.09.004