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Impact of Zr incorporation into Ni/AlSBA-15 catalyst on its activity in cellulose conversion to hydrogen-rich gas Jacek Grams, Joanna Goscianska, Natalia Potrzebowska, Robert Ryczkowski, Beata Michalkiewicz, and Agnieszka Malgorzata Ruppert Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02561 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on December 3, 2017
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Impact of Zr incorporation into Ni/AlSBA-15 catalyst on its activity in cellulose conversion to hydrogen-rich gas Jacek Grams a,*, Joanna Goscianska b, Natalia Potrzebowska a, Robert Ryczkowski a, Beata Michalkiewicz c, Agnieszka M. Ruppert a a
Institute of General and Ecological Chemistry, Faculty of Chemistry, Lodz University of Technology, Zeromskiego 116, 90-924 Lodz, Poland b Adam Mickiewicz University in Poznan, Faculty of Chemistry, Laboratory of Applied Chemistry, Umultowska 89b, 61-614 Poznan, Poland c West Pomeranian University of Technology, Szczecin, Faculty of Chemical Technology and Engineering, Institute of Inorganic Chemical Technology and Environment Engineering, Pulaskiego 10, 70-322 Szczecin, Poland Abstract
This work was focused on the investigation of the effect of zirconium incorporation into the structure of Ni/AlSBA-15 catalyst on its performance in high temperature conversion of cellulose (the main component of lignocellulosic feedstock) to hydrogen-rich gas. The modified supports were prepared by direct incorporation of zirconium and impregnation methods. The obtained results exhibited that introduction of zirconium into the structure of Ni/AlSBA-15 allowed for considerable increase in the amount of hydrogen produced in the studied process in comparison to unmodified Ni/AlSBA-15 material. The characterization of physicochemical properties of the investigated materials (XRD, SEM-EDS, ToF-SIMS, TPR, TPD-NH3, etc.) showed that the preparation of mesoporous Ni/ZrAlSBA-15 with the use of direct synthesis method led to obtain the catalyst with higher surface area, pore volume and smaller crystallites of an active phase in comparison to the material containing nickel supported on ZrAlSBA-15 with zirconium introduced by impregnation. Despite that mesoporous catalyst prepared by impregnation possessed higher acidity its structure underwent partial collapse during preparation procedure.
Keywords: AlSBA-15; Zirconia, Nickel; Catalyst; Hydrogen; Lignocellulosic biomass conversion
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1. Introduction
Lignocellulosic biomass is considered one of the most promising feedstock for the production of energy and industrially important compounds. Its high temperature treatment can lead to the formation of hydrogen which has been mainly produced from fossil fuels until now1,2. However, biomass conversion is a complex process consisting of a large number of consecutive reactions and the yield of formed desirable products is usually unsatisfactory3,4. One of the ways leading to the increase in the efficiency and selectivity of hydrogen production in high temperature treatment of lignocellulosic feedstock is an application of heterogeneous catalysts. Although, it is known that heterogeneous catalysts may undergo deactivation during the mentioned process due to poisoning or carbon deposition5,6. The literature data demonstrates that nickel based systems are the most popular catalysts for thermal conversion of lignocellulosic biomass7. Nevertheless, their activity strongly depends on the kind of the used support, which can hinder coke formation and deterioration of the catalyst structure in high temperature8,9. Our previous studies exhibited that mesoporous silicas are promising supports for Ni catalyst used for high temperature treatment of cellulose towards hydrogen-rich gas10. One of the most popular of them is SBA-15. This material is known for its high hydrothermal stability (even more than 700°C) and large surface area11–13. Moreover, it was noted that aluminum substitution in silica structure can enhance the performance of the catalyst due to generation of Brönsted acidity, among others14. It was demonstrated that SBA-15 and AlSBA-15 were used as catalysts or catalyst supports in various processes related to the conversion of lignocellulosic biomass, biomass derived chemicals, waste materials or model compounds towards biofuels12,15–21. However, the catalytic properties of mesoporous silicas can be further improved by the modification of their structure by selected transition metals22–24. It turned out that an addition of zirconium to Ni/SBA-15 can result in the increase in the performance of the catalyst. Arslan et al.25 used Ni/ZrSBA-15 catalyst in steam reforming of ethanol and suggested that the increase in the activity of this system was associated with high thermal and chemical stability of dopant, possibility of the formation of stronger interactions between active phase and the support, and owing to that higher capability to the adsorption of water molecules which can dissociate into reactive OH groups taking part in the removal of deposited carbon. The similar effects were reported by Biswas et al.26. It was demonstrated that the modification of NiMo/SBA-15 with zirconium allowed for improvement of textural properties of the catalyst, surface acidity, stability of the system and dispersion of active 2 ACS Paragon Plus Environment
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phase particles. The literature data shows that ZrSBA-15 materials were used as catalysts for hydrotreating reactions27, biodiesel production from low grade oils and fats28 or production of γ-valerolactone from biomass-derived levulinic acid in the vapor phase29. Taking that into account we decided to focus in this work on the studies of the effect of zirconium incorporation into Ni/AlSBA-15 catalyst on its activity in cellulose conversion to hydrogen-rich gas. The influence of the physicochemical properties of the modified catalyst on their activity in the mentioned process was investigated. Zirconium was introduced to AlSBA-15 support via impregnation and direct synthesis methods. It was found that the modification of catalysts allowed for the enhancement of the efficiency of the formation of gaseous products and especially increase in the yield of hydrogen originated during high temperature treatment of cellulose (model biomass compound).
2. Experimental
2.1. Preparation of supports Mesoporous silica AlSBA-15 was prepared using triblock copolymer poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123, SigmaAldrich) as a structure directing agent and aluminum isopropoxide (Sigma-Aldrich) as aluminum source. In a typical synthesis, 0.5 g of Pluronic P123 was added to 19 mL of 1.6 mol/L HCl (Chempur). When the copolymer was dissolved, 1.1 g of tetraethyl orthosilicate (TEOS, Aldrich, 98%) with an appropriate amount of aluminum precursor, according to the desired Si/Al ratio (Si/Al = 30) in the initial gel, was inserted dropwise to the solution. The final mixture was stirred at 35°C for 6 h and then kept in this temperature for 24 h and subsequently heated to 100°C and kept in this temperature for 6 h. The product was filtered without washing and dried at 100°C for 24 h. Finally, the sample was calcined at 550°C in air in order to remove the template. In the further step AlSBA-15 material was modified by zirconium twofold. The direct synthesis (DS) and impregnation (IMP) methods were used for incorporation of Zr4+ ions into the support structure, as well as for introduction of ZrO2 on its surface, respectively. ZrAlSBA-15 (DS) was synthesized by the same method as AlSBA-15. Aluminum isopropoxide and zirconium (IV) oxynitrate hydrate (ZrO(NO3)2·6H2O, Sigma-Aldrich) were used as metals precursors. The Si/Al and Si/Zr atomic ratios in the gels were 30 and 10, respectively.
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For the impregnation method (IMP) zirconium (IV) oxynitrate hydrate (SigmaAldrich) was used as zirconium source. Calculated amount of the precursor (in order to obtain 15% wt. of Zr) was dissolved in water and then added to the known amount of AlSBA-15. Then the sample was aged for 24 h, water was evaporated on heating mantle, drying took place for 2 h in 120oC and the final step was calcination in air flow at 500°С for 4 h with the temperature ramp of 20°C/min. Commercial SiO2 (Merck) was used as a support for reference catalyst. 2.2. Preparation of catalysts The supported 20 wt% Ni catalysts (about 5 g) were prepared by the impregnation method. Nickel was introduced from aqueous solution of Ni(NO3)2⋅6H2O (Chemland, pure for analysis (≥99.5%)) on the surface of earlier prepared materials. Then the samples were aged for 24 h at room temperature. After evaporation of water, the catalysts were dried at 110°С for 2 h, then calcined in air flow at 500°С for 4 h with the temperature ramp of 20°C/min. However, our previous investigations10 confirmed that NiO crystallites underwent reduction to the metallic nickel during the performed reactions. The grain size of the prepared catalysts was in the range 0.075-0.2 mm.
The synthesized catalysts were designated as follow: Ni/AlSBA-15 - (nickel catalyst supported on unmodified AlSBA-15), Ni/ZrAlSBA-15 (DS) - (nickel catalyst supported on ZrAlSBA-15 prepared by direct synthesis method), Ni/ZrAlSBA-15 (IMP) - (nickel catalyst supported on ZrAlSBA-15 prepared by impregnation method).
Some parts of the samples were subjected to the characterization after the treatment at the temperature of 700°C for 4h which was used in order to simulate the reaction conditions. An analysis of the catalysts directly after the reaction was impossible due to their mixing with the substrate and carbon deposit or tars formed during the investigated process.
2.3. Characterization methods Sorptometer Quantachrome Autosorb iQ was used to perform surface area measurements with N2 as adsorbent at -196°C, with prior outgassing at 200°C for 3 h in order to desorb the impurities or moisture. Surface area and pore size distribution were calculated 4 ACS Paragon Plus Environment
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by BET (Brunauer–Emmett–Teller) and BJH (Barret, Joyner, Halenda) methods, respectively. Total pore volume and average pore diameter were determined as well. Acidity of the catalysts was determined by temperature-programmed desorption of ammonia (TPD-NH3) in a dynamic mode. First, in order to remove water and impurities each sample was heated at 500°С for 1 h in argon atmosphere. After cooling the samples to 100°С an adsorption of ammonia (pure) was performed for 15 minutes with flow rate of 30 mL/min. Then, flushing of the samples with argon was carried out for 15 minutes. After that the samples were cooled down to 40°C. The measurements were performed by heating the samples from 40oC to 500°С with ramp rate of about 25°С/min and registering amount of desorbed ammonia using thermal conductivity detector. Powder X-ray diffractograms (XRD) were collected using a PANalytical X’Pert Pro MPD diffractometer. The X-ray source was a copper long fine focus X-ray diffraction tube operating at 40 kV and 30 mA. Data were collected in the 5–90° 2Θ range with 0.0167° step. Crystalline phases were identified by references to ICDD PDF-2 (ver. 2004) database. All calculations were performed with X’Pert HighScorePlus computer program. The calculation of NiO crystallite size was based on the Scherrer equation. The small angle X-ray diffraction measurements were performed using a D8 Advance Diffractometer made by Bruker with the copper Kα1 radiation (λ = 1.5406 Å). The XRD patterns were recorded at room temperature with a step size 0.02°. Temperature-programmed reduction (TPR) was performed on AMI1 system from Altamira Instruments equipped with a thermal conductivity detector and used for examining the reducibility of the catalysts (0.1 g) calcined at 500oC. In the experiments, mixture of 5 vol% H2 and 95 vol% Ar at flow rate of 30 mL/min and linear temperature ramp of 10oC/min was used. Scanning electron microscope (SEM) UHR FE-SEM Hitachi SU8020 equipped with a secondary electron (SE) and backscattered electron (BSE) detector and attached energy dispersive X-ray spectroscopy (EDS) system were used for the investigation of the morphology and composition of the catalysts surface. The measurements were performed at the acceleration voltages of 5.0 kV, and the current of about 10 µA. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) was applied to the characterization of the surface composition of prepared catalysts. The measurements were performed using the ION-TOF GmbH instrument (TOF-SIMS IV) equipped with 25 kV pulsed Bi+ primary ion gun in the static mode. The analyzed area corresponded to a square of 500 µm x 500 µm. Before the measurements the samples were pressed into pellets and 5 ACS Paragon Plus Environment
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attached to the sample holder using a double-sided tape. Moreover, pulsed electron flood gun was used for charge compensation. This technique is not fully quantitative, however it enables a determination of the changes in the relative concentrations of the main constituents of the uppermost layer of the studied catalysts. This can be done by a comparison of the intensities of particular secondary ions recorder for the investigated materials.
2.4. Catalytic activity measurements
The activity of the investigated catalysts was tested in stirred steel reactor (250 mL) under atmospheric pressure at 700°C for 4 h. The flow of Ar was used in order to direct the obtained gaseous mixture from the reactor to the gas chromatograph and maintain the total gas flow at 15 mL/min. The conversion of the model biomass sample - α-cellulose (Sigma– Aldrich, pure) was conducted in the presence of catalysts. In each case 5 g of α-cellulose and 0.2 g of the catalyst were mixed together. An amount of permanent gases such as hydrogen, methane, carbon oxide and carbon dioxide was determined using gas chromatograph (GCHF 18.3, Chromosorb 102 column) equipped with a thermal conductivity detector (TCD). The gaseous products were collected every 30 min, the measurements were repeated three times. An analysis of the formation of solid residue demonstrated that independently from the type of the used catalyst the contribution of the carbonaceous material remaining after reaction was about 20%.
3. Results and discussion
3.1. Characterization of catalysts
3.1.1. Surface area and acidity An analysis of the physicochemical properties of the prepared catalysts showed that their surface area ranged between 287 m2/g and 495 m2/g and was higher than that noticed for reference Ni/SiO2 material (Table 1), which is typical for metal supported AlSBA-15 systems30,31. The highest surface area was observed in the case of Ni/ZrAlSBA-15 with the support synthesized by direct synthesis method. This catalyst possessed also larger pore volume in comparison to Ni/ZrAlSBA-15 (IMP) and Ni supported on unmodified AlSBA-15. On the other hand, the obtained results showed that incorporation of zirconium into the support structure resulted in a decrease in the pore size of the investigated materials. 6 ACS Paragon Plus Environment
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The mesoporous character of the obtained catalysts was indicated by the nitrogen adsorption/desorption isotherms, presented in Figure 1. All materials display the typical type IV isotherms, according to the classification of IUPAC, with the hysteresis loop in a relative pressure (p/p0) range of 0.45-0.80 for Ni/ZrAlSBA-15 (DS) and 0.45-0.96 for Ni/ZrAlSBA-15 (IMP) and Ni/AlSBA-15 due to the capillary condensation and evaporation within the mesopores and on the external surface. The lowest surface area, pore volume and pore diameter observed in the case of Ni/ZrAlSBA-15 (IMP) sample was probably connected with a partial collapse of the structure of the support by formed zirconia crystallites.
Table 1. Physicochemical properties of synthesized catalysts (the values in brackets refer to samples submitted to temperature treatment in the reaction conditions – 700°C).
Ni/SiO2 Ni/AlSBA-15 Ni/ZrAlSBA-15 (IMP) Ni/ZrAlSBA-15 (DS)
BET surface [m2/g]
Pore volume [cm3/g]
Pore size [nm]
Acidity [µmol NH3/g]
Average crystallite size [nm]
219 386 (358)
0.55 (0.58)
5.6 (6.3)
81 542 (267)
15 21 (27)
287 (201)
0.31 (0.31)
4.4 (6.0)
884 (562)
33 (36)
495 (511)
0.61 (0.71)
4.9 (5.4)
570 (497)
26 (32)
Characterization of the catalysts after the treatment at 700°C showed that Ni/ZrAlSBA15 (DS) is the most stable in the reaction conditions (Table 1). On the other hand, the highest drop in the surface area at high temperature was observed for Ni/ZrAlSBA-15 (IMP) sample. Its size decreased from 287 to 201 m2/g. The low stability of this material is also connected with the highest growth in the pore size which increased to 6.0 nm after high temperature treatment. In this case the noticed value exceeds the average size of the pores of the catalyst prepared by direct synthesis method. The shape of the nitrogen adsorption/desorption isotherms collected from the surface of the studied catalysts remained almost unchanged after their treatment at higher temperature (see – Supporting Information).
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400
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Ni/ZrAlSBA-15 (DS) Ni/ZrAlSBA-15 (IMP) Ni/AlSBA-15
-1 3
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Volume adsorbed [cm g , STP]
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300
200
100
0 0,2
0,4
0,6
0,8
1,0
Relative pressure [p/p0]
Figure 1. The nitrogen adsorption/desorption isotherms obtained for the investigated catalysts.
The results of TPD-NH3 measurements revealed considerably higher surface acidity of the catalysts based on AlSBA-15 in comparison to Ni supported on silica. The modification of the catalysts by zirconium led to the increase in the amount of adsorbed ammonia only for ZrAlSBA-15 prepared by impregnation method (884 µmol NH3/g). On the other hand, the acidity of Ni/ZrAlSBA-15 (DS) (570 µmol NH3/g) was only slightly higher than that observed for unmodified Ni/AlSBA-15 sample (542 µmol NH3/g). Figure 2 shows that the TPD-NH3 profiles obtained for both zirconium modified catalysts are similar. In spite of that, for Ni/ZrAlSBA-15 (IMP) only one broad desorption peak is observed while in the case of Ni/ZrAlSBA-15 (DS) broad signal with two overlapping maxima can be distinguished, the strength of the acid sites formed on the surface of these catalysts is comparable (maximum of ammonia desorption between 170°C and 200°C). In the case of Ni/SiO2 two maxima of NH3 desorption rate were also noticed (120°C and 190°C), however their intensity was noticeably lower than that observed in previous cases. It is worth noting that for Ni supported on SiO2 only weak acid sites were found on the surface while for Zr modified catalysts the contribution of small amount of medium acid sites was also observed (right part of the peak shoulder in the temperature range above 300°C). The
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strength of acidic centers observed on TPD-NH3 profiles obtained for the investigated catalysts was consistent with that reported before in Refs.32,33. An amount of acid sites on the surface of catalysts after their treatment in the reaction temperature decreased in comparison to as prepared samples. However, still the same order of the acidity was observed for the studied materials. The most stable was the catalyst prepared by direct synthesis method. In spite of a decrease in the amount of acid sites their strength remained unchanged (the same course of the TPD-NH3 profiles – see Supporting Information)
Figure 2. TPD-NH3 profiles obtained for the investigated catalysts. 3.1.2. X-ray diffraction (XRD) X-ray diffractograms collected for unmodified and Zr modified Ni/AlSBA-15 catalysts (Figure 3) revealed the presence of two diffraction lines at 37.2° and 43.2° 2ϴ confirming the formation of NiO crystals ((110) and (111) planes) on the surface of the investigated samples after calcination step31. According to the results of the calculations of NiO crystallites size performed with the use of Scherrer equation (Table 1) it can be seen that the largest nickel oxide nanoparticles were formed in the case of Ni/ZrAlSBA-15 catalysts (33 nm and 26 nm for the material prepared by impregnation and direct synthesis methods, respectively). The NiO crystallites present on the surface of unmodified Ni/AlSBA-15 were slightly smaller (21
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nm) while the smallest particles of nickel oxide were formed in the case of reference Ni/SiO2 sample (15 nm). The small angle XRD patterns obtained for the investigated catalysts are illustrated in Figure 4. In the case of Ni/ZrAlSBA-15 (DS) sample a highly intense reflection centered at 2Ɵ ∼1.0°, which is assigned to the crystallographic plane (100), together with two additional weaker reflections at 2Ɵ 1.7° and 1.9°, corresponding to (110) and (200) planes, were observed33. These are characteristic of the ordered hexagonal 2D structures of P6mm symmetry of the support. On the other hand, an introduction of zirconium onto the surface of AlSBA-15 via impregnation resulted in a considerable collapse of its framework (no welldeveloped peaks were noticed). .
Figure 3. X-ray diffraction analysis of the investigated catalysts. The treatment of the catalysts in the reaction temperature (700°C) did not changed XRD patterns (see Supporting Information) obtained for the analyzed materials. Ni/ZrAlSBA-15 (DS) maintained its ordered hexagonal structure. The only difference was a slight increase in the size of NiO crystallites (Table 1). However, the largest nickel oxide nanoparticles were still observed in the case of zirconium modified catalysts (especially Ni/ZrAlSBA-15 (IMP) sample).
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Figure 4. Small angle diffractograms obtained for the investigated catalysts.
3.1.3. Temperature programmed reduction (TPR) Temperature programmed reduction profiles of the investigated catalysts are shown in Figure 5. In the case of Ni/SiO2 sample only one peak of the reduction of NiO nanoparticles not interacting with the support with the maximum of hydrogen uptake at 350°C was observed, which is in agreement with the literature data34. Different shape of the reduction curves was noted to AlSBA-15 based system. In this case two reduction peaks with the maxima at 300°C and 570°C were noticed. The first of them can be ascribed to nickel oxide with the limited contact with the support, while the second can be attributed to NiO strongly interacting with the mesoporous material as previously reported in the literature32,35–37. The presence of at least two reduction peaks was also reported for Ni/ZrAlSBA-15 catalysts (320°C, 400°C and 550°C for Ni/ZrAlSBA-15 (IMP), and 455°C and 590°C for Ni/ZrAlSBA-15 (DS), respectively). However, the contribution of Ni species more strongly interacting with the support observed for Zr modified Ni/AlSBA-15 was considerably higher than that noted for unmodified mesoporous catalyst. This was consistent with previous studies
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which demonstrated that the presence of zirconium enhanced the strength of the interaction between the support and nickel clusters25,35,38
Figure 5. TPR profiles of the investigated catalysts.
3.1.4. Scanning electron microscopy (SEM-EDS) Scanning electron microscopy measurements (Figure 6) confirmed an introduction of nickel oxide phase on the surface of zirconia modified AlSBA-15 material. Since the BSE detector was applied simultaneously with SE, heavy elements backscatter electrons more strongly than light elements, and thus nickel oxide appear brighter in the micrographs. In both cases - Ni/ZrAlSBA-15 (DS) and Ni/ZrAlSBA-15 (IMP) - the presence of NiO crystallites evenly distributed on the support was observed. Figure 7 demonstrated that they usually formed shapes similar to pyramids with triangular walls. On the other hand the results obtained with the use of energy-dispersive X-ray spectroscopy exhibited the differences in the composition of the surface layer of both mentioned catalysts (Table 2). It turned out that the concentration of zirconium in the case of Ni/ZrAlSBA-15 (IMP) was more than twice higher in comparison to that observed for Ni/ZrAlSBA-15 (DS) sample. This phenomenon was connected with the kind of method of Zr incorporation in the catalyst structure. In the first 12 ACS Paragon Plus Environment
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case this metal remained on the surface of the support, while in the second, was introduced also into the bulk.
Figure 6. SEM images of AlSBA-15, ZrAlSBA-15 and Ni/ZrAlSBA-15 catalysts prepared by impregnation and direct synthesis methods.
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Figure 7. Shape of NiO crystallites distributed on the surface of Ni/ZrAlSBA-15 (DS) catalyst.
Table 2. Surface composition of Ni/ZrAlSBA-15 catalysts prepared by impregnation and direct synthesis methods determined by SEM-EDS method [wt. %]. 20%Ni/ZrAlSBA-15 (IMP)
20%Ni/ZrAlSBA-15 (DS)
Al
0.7
0.5
Si
21.1
15.6
Ni
18.6
12.7
Zr
2.0
0.8
3.1.5. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) The results of ToF-SIMS analysis confirmed successful introduction of zirconium on the surface of the modified catalysts (Table 3). Moreover, the changes in the values of relative Zr+/Si+ and Zr+/Al+ ratios proved an agglomeration of a larger amount of Zr in the case of Ni/ZrAlSBA-15 (IMP) sample. The comparison of the intensity of Ni+ with the intensities of Si+, Al+ and Zr+ ions exhibited that for Ni/ZrAlSBA-15 (DS) more nickel is exposed on the catalyst surface at the expense of zirconium while in the case of Ni/ZrAlSBA-15 (IMP) slightly larger contentration of silicon and aluminum was observed. It is connected with the fact that in the case of Ni/ZrAlSBA-15 (DS) zirconium atoms are also located into the bulk of the material and their concentration on the surface of the catalyst can be lower.
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Table 3. Relative intensity of selected ions calculated on the basis of ToF-SIMS spectra collected from the surface of the investigated catalysts.
Ni/AlSBA-15
Ni/ZrAlSBA-15 (IMP)
Ni/ZrAlSBA-15 (DS)
Zr+/Si+
-
9.3.10-3
7.3.10-3
Zr+/Al+
-
2.1.10-1
1.9.10-1
Ni+/Si+
1.4.10-1
2.0.10-1
1.7.10-1
Ni+/Al+
3.5
4.5
4.4
Ni+/Zr+
-
21
24
3.2. Effect of Zr incorporation into Ni/AlSBA-15 catalyst on its catalytic performance
The catalytic performance of the investigated catalysts was tested in high temperature conversion of cellulose (Figure 8). It was demonstrated that the modification of AlSBA-15 support by zirconium considerably enhanced the activity of nickel catalyst. An application of Ni/ZrAlSBA-15 (DS) and Ni/ZrAlSBA-15 (IMP) catalysts allowed for formation of 12.0 and 11.7 mmol H2/g cellulose, respectively while the use of unmodified Ni/AlSBA-15 material led to the production of 8.5 mmol H2/g cellulose. The last value was even a bit lower than that noted for reference Ni/SiO2 sample (9.7 mmol H2/g cellulose). The differences in a catalytic behavior of unmodified and Zr modified catalysts during high temperature conversion of cellulose are strictly connected with their surface properties. The cellulose decomposition is a very complex process consisted of high number of consecutive reactions including cracking, reforming, water-gas shift, dehydration, partial oxidation, decarboxylation, decarbonylation or oligomerization, among others2,39–41. The exemplary of them are demonstrated below: CxHyOz → (x-1)CO + ((y-4)/2)H2 +CH4
(1)
CxHyOz → (x-1)CO + (y/2)H2 + C(solid)
(2)
CxHyOz + (1/2)O2 → xCO + (y/2)H2
(3)
CxHyOz + O2 → (x-1)CO + CO2 + (y/2)H2
(4)
CxHyOz + H2O → xCO + (1+y/2)H2
(5)
CxHyOz + nH2O → qCO + (x-q)CO2 + (2n+y/2)H2 (6) CO + H2O → CO2 + H2
(7)
CO + 3H2 → CH4 + H2O
(8)
CH4 + CO2 → 2CO + H2
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CH4 + H2O → CO + 3H2
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C(solid) + CO2 → 2CO
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Complexity of the process causes that the catalytic activity of the investigated materials depends on several factors. The literature data demonstrated that one of the most important is the presence of acidic centers on the catalyst surface10,42. A higher acidity of the catalyst can favor biomass decomposition process and formation of a larger amount of gaseous products. However, a comparison of the activity of two modified Ni/ZrAlSBA-15 catalysts prepared by different methods revealed that in spite of higher acidity Ni/ZrAlSBA-15 (IMP) did not allowed for the production of a larger amount of hydrogen than Ni/ZrAlSBA-15 (DS) sample. This is connected with different surface structure of these two catalysts. Nickel supported on ZrAlSBA-15 synthesized by direct synthesis method possessed higher surface area, pore volume and smaller crystallites of an active phase than Ni/ZrAlSBA-15 (IMP) catalyst (Table 1). It is also worth noting that small angle XRD results exhibited partial collapse of the structure of ZrAlSBA-15 mesoporous support prepared by impregnation method which could be responsible for the limitation of accessibility of mesopores for the reactants. A larger surface area and pore volume provide better contact between reaction intermediates and the catalysts. This led to the higher efficiency of the formation of gaseous products which can dominate on the effect of higher surface acidity10. On the other hand ToF-SIMS and SEM-EDS measurements showed a lower zirconium content on the surface of Ni/ZrAlSBA-15 (DS) sample. It could be also important taking into account that Zr plays an essential role in maintaining the catalyst activity43. The TPR measurements showed that the presence of Zr increases the strength of the interaction between nickel species and the support. This in turn enhances the capability of the catalyst to the adsorption of water and its further dissociation which facilitates the formation of active OH groups taking part in the gasification of deposited tar. The similar phenomenon was observed during the investigations of steam reforming of ethanol conducted with the use of Ni/ZrSBA15 and Ni/MCM-41 catalysts25. The results of activity tests revealed that the catalysts containing zirconium were active even below set temperature of the process and were able to produce hydrogen at 600 °C, while nickel deposited on silica was considerably less active in those conditions (3-4 and 0.5 mmol H2/g cellulose, respectively). It was also observed that an application of Ni/ZrAlSBA15 (DS) allowed to obtain higher H2/CO ratio than in the case of Ni/ZrAlSBA-15 (IMP) sample (2.4 and 1.6, respectively).
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Energy & Fuels
Figure 8. Catalytic performance of the investigated catalysts in high temperature conversion of cellulose (700°C, 4 h).
Furthermore, a comparison of the amount of hydrogen formed in the presence of Ni/AlSBA-15 modified by zirconium with the efficiency of mesoporous catalysts described in the literature showed that in the case of Ni/ZrAlSBA-15 system H2 yield was higher than that observed for Ni/MCM-41 (10.5 mmol H2/g cellulose)44, Co supported on MCM-41, SBA-15 and Al2O345 or Ni/MCF46 (not more than 9.2 mmol H2/g cellulose). On the other hand, slightly higher hydrogen yield was reported for bimetallic Ni-Co catalyst supported on SBA-15 (13.2 mmol H2/g cellulose), however high activity of this system can be related to synergy effect between the introduced metals44.
4. Conclusions The obtained results exhibited that incorporation of zirconium into the structure of Ni/AlSBA-15 allowed for considerable increase in the production of hydrogen-rich gas in high temperature conversion of cellulose. The highest H2 yield was obtained in the presence of nickel supported on mesoporous ZrAlSBA-15 prepared by direct synthesis method. In spite of lower zirconium content and lower acidity this material possessed higher surface area, pore 17 ACS Paragon Plus Environment
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volume and smaller crystallites of an active phase than the catalyst supported on ZrAlSBA-15 synthesized by impregnation. Moreover, the XRD experiment showed that an impregnation of AlSBA-15 by zirconium precursor resulted in partial collapse of the mesoporous structure of this material. A comparison of the obtained results with the literature data suggests that an introduction of zirconia to Ni/AlSBA-15 resulted in the increase in the strength of the interaction between nickel species and the mesoporous support. It leads to the enhancement of the formation of OH groups on the catalyst surface taking part in the increase in the efficiency of the formation of gaseous products.
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