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Nanoreactors: An efficient tool to control the chainlength distribution in Fischer-Tropsch synthesis Vijayanand Subramanian, Kang Cheng, Christine Lancelot, Svetlana Heyte, Sebastien Paul, Simona Moldovan, Ovidiu Ersen, Maya Marinova, Vitaly Ordomsky, and Andrei Y Khodakov ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01596 • Publication Date (Web): 01 Feb 2016 Downloaded from http://pubs.acs.org on February 3, 2016
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ACS Catalysis
Nanoreactors: An efficient tool to control the chain-length distribution in Fischer-Tropsch synthesis Vijayanand Subramanian1, Kang Cheng2, Christine Lancelot1, Svetlana Heyte1, Sébastien Paul1, Simona Moldovan3, Ovidiu Ersen3, Maya Marinova4, Vitaly V. Ordomsky1* and Andrei Y. Khodakov1 1
Unité de Catalyse et de Chimie du Solide, UMR 8181 CNRS, Bât. C3, Université Lille 1, ENSCL, Ecole Centrale de Lille, 59655 Villeneuve d’Ascq, France 2 Collaborative Innovation Center of Chemistry for Energy Materials, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China 3 Department of Surfaces and Interfaces (DSI), 23, rue du Loess BP 43, F-67034 Strasbourg, France 4 Institut Chevreul, FR2638 CNRS, Bât. C6 Université Lille 1, F-59655 Villeneuve d’Ascq, France Keywords: Nanoreactor · cobalt · shape selectivity · Fischer-Tropsch · chain length distribution ABSTRACT: One of the main problems of the Fischer-Tropsch synthesis is too broad distribution of the produced hydrocarbons due to the Anderson-Schulz-Flory law. This work is directed on the solution of this problem by application of nanoreactors with incorporated metal nanoparticles. Our results show that encapsulation of Co nanoparticles in nanosized porous silica spheres results in higher activity per catalyst weight and stability with a shift of the chain length distribution of hydrocarbons to lower values in comparison with Fischer-Tropsch synthesis over impregnated catalysts. These effects are due to the presence of well dispersed isolated and stable Co nanoparticles inside of nanoreactors and shape selectivity effect which restricts the chain growth by the walls of nanoreactors. The proposed new strategy can be further extended to synthesis of olefins and alcohols with desired chain-length distribution from syngas by encapsulation of other metals (Fe, Cu, Rh etc).
Introduction Fischer-Tropsch (FT) synthesis produces hydrocarbons and oxygenates from syngas (H2/CO mixtures) which might be generated from biomass, coal and natural gas. Low temperature FT synthesis which occurs at 220-240°C, 20 bar and H2/CO molar ratio of 2 over cobalt-based catalysts, is very attractive because of high cobalt intrinsic activity, higher conversion per single pass and better catalyst stability than over iron catalysts [1, 2]. Selectivity control is however one of the most difficult challenges in FT synthesis. The hydrocarbon chain length distribution follows Anderson-Schulz-Flory (ASF) statistics and is usually rather broad and unselective [3, 4]. Low selectivity significantly limits the application of the FT process for synthesis of specific narrow hydrocarbon fractions. For example, diesel fuel is mostly constituted by C10-C20 hydrocarbons, while FT synthesis unselectively produces hydrocarbons from gaseous methane to C60-C80 solid waxes. Two strategies have been currently proposed to restrict broad FT hydrocarbon distribution to a specific hydrocarbon range. First, an additional stage of catalytic cracking can be added to FT technology to upgrade the reaction products to a specific fuel with a narrow hydrocarbon distribution [5]. This multistage process, however, significantly reduces the efficiency of synthetic fuel production. Second, bifunctional catalysts combining a FT active phase (e.g. cobalt or ruthenium) and an acid
catalyst (e.g zeolite) [6, 7, 8], zeolite seed-grafted SBA-15 [9] or aluminum promoted SBA-16 [10] have been proposed for direct selective synthesis of diesel fuel from syngas. The bifunctional cobalt-zeolite catalyst however suffers from deactivation and low selectivity. These problems are growing with time on-stream due to the different deactivation rates for FT reaction and hydrocarbon cracking. In heterogeneous catalysis, the molecular size of the reaction products can be controlled by shape selectivity. The shape selectivity concept is rather straightforward. It suggests that “the transformation of reactants into products depends on how the processed molecules fit the active site of the catalyst” [11]. This effect has been widely applied in zeolite catalysis for the synthesis of para-xylene by isomerization and toluene alkylation or disproportionation [12]. The narrow pores of zeolite favor formation of smaller para-isomers with suppression of the formation of larger meta- and orthoisomers. The effect of shape selectivity in FT synthesis has been first uncovered by Fraenkel in 1980 [13]. It has shown that cobalt metal clusters incorporated inside very small cages of A zeolite (1.1 nm) produces mainly propane and propylene [13]. This effect has been later confirmed over different bifunctional Co-zeolite catalysts [14, 15, 16] More recently the effect of shape selectivity was also observed in FT synthesis on Co/SBA-15 [17]. The decrease in the SBA-15 pore size from 11 to 5 nm led to a significant decrease in the
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C19+ selectivity from 15 to 4 %. Note that the selectivity phenomena on the catalysts with different pore sizes can be also attributed to different cobalt dispersion. The effect of SBA-15 pore sizes on the selectivity of FT synthesis was investigated by the group of Li [18, 19] et al. Higher C5+ hydrocarbon selectivity observed on large pore SBA-15 silicas was attributed to the larger size of cobalt clusters [19]. Nanoreactors represent a new generation of heterogeneous catalysts with excellent catalytic stability due to their specific structure, which consists of a core, commonly the active phase, surrounded by a porous shell [20]. A nanoreactor has a limited volume with at least one of the dimensions smaller than 100 nm. Most of previous works have been dedicated to the synthesis of encapsulated metal nanoparticles like Au, Pd, Pt, Ni inside the porous shell of different materials like silica, carbon, titania etc [21-25]. The principal approach used for the syntheses of this type of materials is application of organic or inorganic matrix as templates, which usually results in nanoreactors in the range from 30 to 150 nm [20-25]. The catalytic systems prepared on the basis of hollow nanospheres have shown to maintain catalytic activity even under severe reaction conditions [19] and during the recycling process [22]. This type of catalysts also shows high-activity due to the weak interactions between the surfactant inside of the cavity and material of the wall [23]. The group of Sun [26, 27] has recently developed core-shell Co/SiO2 catalysts for FT synthesis. Application of a solvothermal route using polyvinylpyrrolidone (PVP) as capping agent [26] and in-situ coating of Co nanoparticles by silica [27] led to the synthesis of Co nanoparticles inside of about 50 nm silica shells. The catalysts show higher methane and C5-C18 hydrocarbon selectivity in FT synthesis than the conventionally supported Co/SiO2 prepared by impregnation method [26, 27]. The authors have explained this effect by serious diffusion restrictions of carbon monoxide compared to hydrogen due to thick silica shell on the surface of Co nanoparticles leading to a higher H2/CO ratio inside the core-shell catalysts and retarded secondary reactions of olefins [27]. Because of very high selectivity to methane and light hydrocarbons, this method cannot be however, considered as highly efficient for the control of the products distribution in a broader hydrocarbon range. Our approach for the control of hydrocarbon distribution for selective production of diesel fuels in low temperature FT synthesis is founded on the encapsulation of Co metal nanoparticles inside thin walls of porous silica nanoreactors. In order to improve the selectivity to long-chain hydrocarbons we elaborated the synthesis of nano-sized silica spheres with thin walls using the microemulsion technique. This strategy results in an efficient control of the chain length distribution in FT reaction due to the steric restrictions for the growth of hydrocarbons, which sizes exceed the nanoreactor diameter.
Experimental Section The Co/SiO2 catalyst was synthesized via incipient wetness co-impregnation using aqueous solutions of cobalt nitrate (Co(NO3)2·6H2O) in order to obtain 10 wt. % Co in the final catalyst. The catalyst was dried and calcined at 500°C. The Co@SiO2-5 and Co@SiO2-48 catalysts were prepared as follows. Initially, 3 g of CTAB (cethyltrimetylammonium bromide) were dispersed in 5 g of hexanol under vigorous stirring. The mixture was turbid in nature. Upon addition of 2 g
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of water containing 0.5 M Co (NO3)2·6H2O a clear optically transparent microemulsion system was obtained. This emulsion has been mixed with a similar microemulsion containing 1.5 M of NaBH4. After stirring for 30 min, the appropriate amount (to obtain 90 wt.% of SiO2) of TEOS was added and allowed to hydrolyze during 5 h for Co@SiO2-5 or 48 h for Co@SiO2-48. To break the microemulsion, 20 mL of acetone were added. Further, the metal nanoparticles coated with silica were washed thoroughly using ethanol and water to remove residual sodium and surfactant. To get the final product, the dried precursor was calcined at 500°C. The scheme of the synthesis of Co@SiO2-5 and Co@SiO2-48 nanoreactor catalysts is shown in Figure 1. Co@/SiO2 was prepared in a similar manner to Co@SiO2-5 and Co@SiO2-48 but without addition of TEOS. The reduced cobalt nanoparticles were directly impregnated over silica support (CARiACT Q10 silica). TEM observations of the samples were performed on a Tecnai instrument, equipped with a LaB6 crystal, operating at 200 kV. The HAADF-TEM analyses were carried out on a Jeol 2100F (field emission gun) microscope operating at 200 kV by -1 using a spot size of 1.1 Å with a current density of 0.5 pA*Å . The cobalt content of the samples was determined by ICP analysis. The BET surface area, pore volume and average pore diameter were determined by N2 physisorption using a Micromeritics ASAP 2000 automated system. The reducibility of the catalysts was studied by hydrogen temperatureprogrammed reduction (H2-TPR). The H2-TPR was carried out on AutoChem II (Micromeritics) apparatus using 20 mg 3 -1 of the sample in 5 vol. % H2/Ar stream (50 cm min ). The temperature was increased from room temperature to 900°C -1 at a rate of 10°C min . The consumption of H2 was followed by a TCD detector.
Figure 1. Scheme of the synthesis of nanoreactor Co@SiO2 catalysts Carbon monoxide hydrogenation was carried out on the REALCAT platform using a Flowrence® high-throughput unit (Avantium) equipped with 16 parallel milli-fixed-bed reactors (d=2 mm) operating at a total pressure of 20 bar, H2/CO= 2 molar ratio and GHSV of 67 L/gCo·h. Prior to the catalytic test all the samples were activated in a flow of H2 at atmospheric pressure during 10 h at 400°C. After the reduction, the catalysts were cooled down to 180°C and a flow of premixed syngas was gradually introduced to the catalysts. When pressure attained 20 bar, the temperature was slowly increased to 240°C. Gaseous reaction products were analyzed by on-line gas chromatography. Analysis of permanent gases was performed using a Molecular Sieve column and a thermal conductivity detector. Carbon dioxide and C1-C4 hydrocarbons were separated in a PPQ column and analyzed by a thermoconductivity detector. C5-C12 hydrocarbons were analyzed using CP-Sil5 column and a flame-ionization detector. Highmolecular-weight products were collected at atmospheric pressure in vials heated at 70°C with subsequent analysis by SIMDIST technique. The samples after catalysis (prior to
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ACS Catalysis
Table 1. Catalyst characterization and catalytic performance (P=20 bar, H2/CO=2, T=240ºC, GHSV=67 L/gCo·h) after 60 h of reaction.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Catalyst sample
ICP analysis, wt. %
Co
B
Na
Size Co before/aft er catalysis, nm
Co reduci bility, %
N2 adsorption SBET, 2 m /g
Co/SiO2
10
0
0.01
9/16
72
220
15
0.18
0.02
8.2
1.6
7.3
82.8
Co@/SiO2
7.4
0.07
0.01
3/11
87
319
19
0.12
0.008
9.4
1.4
7.1
82.1
Co@SiO2-5
49.6
0.08
0.01
3/3
78
113
5
0.28
0.139
11.2
0.5
9.6
78.7
Co@SiO2-48
11
1.5
0.01
-
80
-
-
0.04
0.004
26
-
16
55
analysis) were treated in the flow of nitrogen for 1 h at atmospheric pressure and reaction temperature with subsequent slow cooling to the ambient temperature.
Results and Discussion Catalyst characterization The catalyst characterization results are shown in Table 1. Four cobalt catalysts were investigated in this work. A conventional Co/SiO2 catalyst (10 wt. % Co, CARiACT Q10 Silica) was prepared using incipient wetness impregnation [28]. The Co@SiO2-5 and Co@SiO2-48 catalysts were prepared using water in oil (W/O) microemulsion method with nanosized water droplets. Co (NO3)2·6H2O was dispersed in oil phase (hexanol) with the interface being stabilized by a surfactant CTAB (Figure 1). It has been shown earlier that porous silica spheres might be synthesized by TEOS hydrolysis in the presence of CTAB [29, 30]. The silica shell around cobalt nanoparticles in nanoreactors was built by hydrolysis of tetraethoxysilane (TEOS) during 5 h (Co@SiO2-5) or 48 h (Co@SiO2-48). Additionally, we evaluated the effect of steric constrains in the nanoreactor on the catalytic performance. For this purpose, the catalyst labelled as Co@/SiO2 was prepared by impregnating silica with cobalt nanoparticles synthetized using the microemulsion method. In Co@/SiO2 cobalt particle size was exactly the same as in Co@SiO2-5 and Co@SiO2-48. The cobalt particles in Co@/SiO2 however were located on silica support and not encapsulated in silica nanoreactors. The BET surface area of the conventional catalyst Co/SiO2 prepared using incipient wetness impregnation was 220 2 m /g, while the average pore diameter was 15 nm. Co3O4 nanoparticles were identified in the calcined catalyst by XRD (Figure S1, Supporting Information). As expected from the literature [1, 2, 28], TEM showed a broad non-uniform distribution of the metal nanoparticles in the conventional Co/SiO2 catalyst in the range from 2 to 20 nm with the average size of about 9 nm (Figure 2, Table 1). Synthesis of cobalt nanoparticles in microemulsions with their subsequent deposition on the silica surface via impregnation resulted in well dispersed cobalt uniformly distributed on the surface of silica support (Figure 2, Co@/SiO2 sample).
FT rate per catal. weight, mol/gcat h
SCH4 ,%
SCO2 ,%
SC2C4, %
SC5+, %
Pore diameter, nm
FT rate per cobalt weight, mol/gCo h
The size of deposed Co nanoparticles measured by TEM varied in the range 2-4 nm. This was also consistent with XRD, which showed very broad patterns relevant to cobalt metallic phase (Figure S1, SI). The catalyst exhibited textural properties similar to the parent silica support. The TEM images of Co@SiO2-5 and Co@SiO2-48 are displayed in Figure 3. Hydrolysis of TEOS on the surface of micelles leads to the formation of a silica layer around cobalt nanoparticles (Figure 1). The diameter of cobalt nanoparticles was between 2 and 4 nm. The amount of the silica in the shell has been controlled by the time of hydrolysis. The hydrolysis during 5 h resulted in the formation of spheres with diameter of about 5-15 nm (Figure 3) with very thin silica shells. The ICP elemental analysis (Table 1) indicates that the observed spherical nanoreactors are principally composed by cobalt. The cobalt content in the nanoreactor was about 50 wt. %. Cobalt nanoparticles inside of the silica spheres were detected using TEM (Figure 3). The chemical composition was further confirmed by EDX (Figure S2, SI). In order to get more information about nanoreactors, the
Figure 2. TEM images of catalysts prepared by impregnation over bulk silica (Co/SiO2 and Co@/SiO2) before (a) and after catalysis (b)
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Co@SiO2-5 catalyst was studied by direct analysis of contrasts from the 3D STEM-HAADF micrographs (Figure 4). Because of higher sensitivity to the Z atomic number, this mode is particularly suitable for the assessment of the distribution of metallic nanoparticles. Co nanoparticles might be observed by light spots with diameter in the range 2-4 nm. STEM-HAADF also shows that nanoparticles are surrounded by 1-2 nm thin layer of silica. The textural properties of the prepared Co@SiO2-5 catalyst are significantly different from the properties of the bulk silica support (Figure 5, Table 1). The surface area is signifi2 cantly lower (113 m /g) due to the higher concentration of metallic phase. The sharp step in isotherm at P/P0 ≈ 0.45 indicates the existence of uniform mesopores with a diameter close to 6 nm which correspond to the size of the nanoreactor. At the same time, the samples with amorphous silica support demonstrated mainly N2 adsorption at high pressures due to the filling of silica interglobular spaces (Figure 5). Longer TEOS hydrolysis time used in the synthesis of the Co@SiO2-48 sample leads to the increase of the nanoreactor wall thickness (Figure 3) up to 30-40 nm. The empty space of 4-5 nm is observed as a light spot in TEM image of the micelle core.
Figure 3. TEM images of catalysts prepared by synthesis of nanoreactors with encapsulated cobalt (Co@SiO2-5) and Co@SiO2-48) before (a) and after reaction (b). Cobalt reducibility inside of the silica shell in Co@SiO2-5 and Co@SiO2-48 and in the catalysts prepared by impregnation has been studied by H2-TPR analysis (Figure 6). Two peaks appear in the TPR profiles over the impregnated catalysts corresponding to reduction of Co3O4 to CoO (300-330°C) and CoO to metallic Co (360-380°C). The reducibility (Table 1) of Co/SiO2 was lower (72%) in comparison with supported cobalt nanoparticles (87 %). Encapsulation of Co nanoparticles inside of thin silica shell leads to significant increase in the hydrogen adsorption due to the high cobalt content (Figure 6). In Co@SiO2-5, the reduction peaks have been shifted to higher temperatures by 50ºC in comparison with reference catalysts. It might be explained by mild diffusion limitations during reduction of the catalyst. This effect has
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Figure 4. Slices redrawn from the volume reconstructed from the electron tomography analysis under STEMHAADF. The red and green arrows point to the Co particles and silica shell, respectively, same colours as for the modelled system. been observed earlier over core-shell Co/SiO2 catalysts [23]. It is interesting to note that in the case of thick wall Co@SiO2-48 sample there is only one reduction peak shifted to 700ºC (Figure 6). This indicates on the difficulties of cobalt oxide being reduced to metallic Co due to the very significant diffusion limitations in the silica shell. The catalyst reducibility and catalytic performance could be also affected by the presence of boron in the catalyst reduced by NaBH4 [29]. ICP analysis (Table 1) shows very low B content (about 0.08%) in Co@/SiO2 and Co@SiO2-5. Significantly higher amount of B is observed in Co@/SiO2-48 (Table 1) most probably due to the diffusion limitations during the washing of the sample with thick silica shell. Some influence of boron on the catalytic performance of Co@/SiO2-48 cannot be therefore excluded. Thus, the four catalysts prepared in this work have rather different sizes and localizations of cobalt nanoparticles. Co/SiO2 is a conventional silica supported cobalt catalyst with a broad cobalt particle size distribution. Co@/SiO2 is a silica-supported catalyst containing cobalt nanoparticles of 2-4 nm deposed on silica from micellar systems. The section below addresses evaluation of the performance of these catalytic materials in low temperature FT synthesis.
FT reaction rate and catalyst stability
Carbon monoxide hydrogenation on all the cobalt catalysts led to hydrocarbons and water. Only very small amounts of CO2 (SCO2
