9706
J. Phys. Chem. C 2008, 112, 9706–9709
Preparation and Catalytic Activity for Fischer-Tropsch Synthesis of Ru Nanoparticles Confined in the Channels of Mesoporous SBA-15 Haifeng Xiong,† Yuhua Zhang,† Shuguo Wang,‡ Kongyong Liew,‡ and Jinlin Li*,†,‡ School of Chemistry and Chemical Engineering, Suzhou UniVersity, Suzhou, China, and Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, South-Central UniVersity for Nationalities, Wuhan, China ReceiVed: January 21, 2008; ReVised Manuscript ReceiVed: April 14, 2008
Ru nanoparticles confined in the channels of mesoporous SBA-15 with different pore sizes were prepared and investigated as catalysts for the Fischer-Tropsch synthesis. The deviation distribution from Anderson-Schulz-Flory rule was observed. By incorporating Ru nanoparticles into the pore of SBA-15 using surface modification, the catalyst activity was found to be significantly affected by pore size instead of particle size or other parameters. 1. Introduction Fischer-Tropsch synthesis (FTS) is a complicated heterogeneously catalyzed reaction for the production of paraffins, olefins, and oxygenates with different carbon numbers. The molecular weight distribution of the products is dominated by achaingrowthmechanism,describedbytheAnderson-Schulz-Flory (ASF) distribution. Obviously, such a distribution is unselective and attempts should be made to control the size of FTS product molecules.1 The catalyst pore size was found to have a significant effect on the activity and selectivity of FTS.2–4 Over cobalt catalysts supported on SBA-15 with pore size >3 nm and on MCM-41 with pore size M2 (7 nm). More recently, we found that the porosity of alumina resulted from different calcination temperatures affected the catalytic activity and selectivity as well as the reducibility of cobalt catalysts.7 Although the effect of the pore size on the performance of FTS catalysts have been investigated extensively,2–8 there are still doubts on the precise role of the pore size because the effect of pore size on the FTS selectivity can be masked by the presence of other parameters, such as active metal particle size, reducibility, surface structure of catalysts and certain metal-oxide interface. In the literatures described above, the catalysts were prepared by simple impregnation from solution, ion exchange or deposition processes. By these means, structurally well-defined FTS catalysts are rarely obtained. SBA-15 is a silica-based mesoporous material with uniform hexagonal channels ranging from 3 to 30 nm and very narrow pore size distribution.9 It is one of the most attractive support for catalysts with high hydrothermal stability and large surface area (600-1000 m2/g), allowing the dispersion of a large number of catalytically active components.10–13 It has been demonstrated that several types of surface silanol groups (SiOH) * Corresponding author. Tel: 086 027 67843016. Fax: 086 027 67842752. E-mail:
[email protected]. † Suzhou University. ‡ South-Central University for Nationalities.
on SBA-15 surface can be used as the anchor in modification of SBA-15.14–17 Recently, Zhang et al.18,19 investigated the oxygen sensing properties of functionalized mesoporous SBA15 and MCM-41 with a covalently linked ruthenium(II) complex and found that the functionalized mesoporous sensing materials appeared to be superior to those of the physically incorporated ones. These mesoporous sensing materials exhibited high sensitivity to the O2 concentration in N2 and short response times. The oxygen quenching result showed that the homogeneity and the sensitivity of the covalently assembled samples are also superior to those of the physically incorporated ones. Furthermore, a greatly minimized leaching effect of the sensing molecules could be observed in the covalently grafted systems. In the present study, modification on the exterior of SBA-15 matrix with different pore sizes by -SiMe3 groups (TMS) was first carried out. Subsequently, grafting on the inner surface of the pore walls of SBA-15 with -Si(CH2)3-NH2 groups (APTS) was used to introduce Ru ion via complexation to the internal pore after removing the template agent in the interior of material. The obtained catalyst was used to study the effect of pore confinement and size on FT synthesis. 2. Experimental Section SBA-15 with different pore sizes was obtained using Pluronic P123 (EO20PO70EO20, MAV ) 5800, BASF) and tetraethyl orthosilicate (TEOS, AR) under acidic conditions following the method reported in literature.9 The narrow pore size silica, labeled as SBA-(1), was synthesized without any hydrothermal treatment. The detailed synthesis route was as follows: P123 (10 g) was dissolved in a solution of 2 M HCl (350 mL) under stirring at 35 °C. Subsequently, TEOS (21 g) was gradually added to the solution and continuously stirred for 48 h. The precipitated material was then obtained by filtration and washed with deionized water. Similar to the preparation of SBA-(1), SBA-(2) and SBA-(3) were obtained after a hydrothermal treatment for 24 h and at 100 and 120 °C, respectively. The fourth silica, labeled as SBA-(4), was obtained with an addition of 1,3,5-trimethylbenzene (TMB, 5 mL) prior to TEOS. The pretreatment condition was the same as that of SBA-(2). For each of the as-synthesized SBA-15, a sample of 1.0 g was dispersed in 80 mL of dehydrated toluene. Then, 5.0 mL (CH3)3SiCl (TMCS, Alfa Aesar) was added dropwise to the
10.1021/jp800579v CCC: $40.75 2008 American Chemical Society Published on Web 06/06/2008
Fischer-Tropsch Synthesis of Ru Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9707
SCHEME 1: Preparation of the Pore-Confined Ruthenium Nanoparticle Catalysts
toluene solution while stirring. The resulting mixture was stirred at 80 °C for 24 h under N2 atmosphere and, the silica derivative was recovered by filtration. The solid was successively washed with toluene and ethanol. The template was then removed using ethanol extraction at 78 °C for 4 days. After filtration, ca. 1.0 g of template-extracted SBA-15 (denoted as TMS-SBA-15) was degassed in a vacuum system (less than 0.1 Torr) at 200 °C for 6 h. Subsequently, 30 mL of anhydrous toluene was added to the TMS-SBA-15 (0.5 g) followed by the addition of 3-aminopropyltriethoxysilane (APTES, ACROS). The suspension (denoted as APTS-TMS-SBA-15) was stirred at 60 °C for 24 h under N2 atmosphere, filtered, and successively washed with toluene and ethanol. The immobilization of Ru3+ onto the channel surface of SBA15 was carried out by dissolving Ru(NO)(NO3)3 (0.05 g) in ethanol (40 mL) and introducing the APTS-TMS-SBA-15 solid (0.2 g) in suspension at 30 °C and stirring for 24 h. The yellowish solid thus formed during stirring was filtered and washed with ethanol and dried under vacuum. The catalyst formation mechanism is described in Scheme 1. A portion of the resulting composites were reduced with hydrogen at 450 °C for 10 h for the characterization, while the rest of sample was stored for use in FTS. All the Ru-loaded samples prepared by functionalization were labeled as x% Ru/SBA-(n) with x standing for the ruthenium weight percent in the samples and n stands for 1, 2, 3 or 4. For the purpose of comparison, a catalyst (3.99% Ru/SBA-(2)-i) was prepared by incipient wetness impregnation. The prepared SBA-(2) was first calcined at 550 °C for 6 h in flowing air. Then, Ru salt (Ru(NO)(NO3)3) was dissolved in ethanol and directly impregnated onto the support. The 3.97% Ru3+/SBA-(2) precursor was reduced by ethylene glycol at 160 °C for 3 h and denoted as 3.97% Ru/SBA-(2)-E. The obtained suspension was directly transferred to a fixed bed reactor for FTS. The quantities of C, H, and N on the silylated samples were determined with a Perkin-Elmer 2400. Quantitative analysis of Ru was performed with inductively coupled plasma-atomic
emission spectroscopy (ICP-AES, Optima 4300DV, PerkinElmer). A Brukers D8 powder diffractometer with monochromatic Cu KR radiation and Ni filter equipped with a VANTEC-1 detector was used for the X-ray diffraction (XRD) measurements. The spectra were recorded from 20 to 80 ° with a step interval of 0.016 °. Crystallite phases were determined by comparing the diffraction patterns with those in the standard powder XRD file compiled by the Joint Committee on Powder Diffraction Standards (JCPDS) published by the International Center for Diffraction Data. N2 adsorption and desorption experiments were conducted at -193 °C using a Quantachrome Autosorb-1. Prior to the experiment, the sample was degassed at 200 °C for 6 h. The surface area was obtained using Brunauer-Emmett-Teller model for adsorption data in a relative pressure range from 0.05 to 0.30. The pore volumes were calculated from the amount of N2 vapor adsorbed at a relative pressure of 0.99, assuming that the pores are filled with the condensate in the liquid state. The pore size distribution was evaluated from the desorption branches of the isotherms using the Barrett-Joyner-Halenda method. To obtain the transmission electron microscopy (TEM) images using a FEI Tecnai G20 instrument, the samples were crushed and ground to a fine powder in an agate mortar. Then, the samples were prepared directly by suspending them in ethanol with ultrasonication. A copper microscope grid covered with perforated carbon was dipped into the solution. At least five TEM images were adopted to evaluate the particle distribution. H2 chemisorption was carried out in a U-tube quartz reactor with a Zeton Altamira AMI-200 unit. The sample weight was about 0.06 g. The catalyst was reduced at 450 °C for 12 h using a flow of high purity hydrogen and then cooled to 100 °C under the hydrogen stream. The sample was held at 100 °C for 1 h under flowing argon to remove weakly bound physisorbed species prior to increasing the temperature slowly to 450 °C. At that temperature, the catalyst was held under flowing argon to desorb the remaining chemisorbed hydrogen, and the thermal conductivity detector began to record the signal until the signal returned to the TABLE 1: Surface and Porous Characteristics of Mesoporous Materials
Figure 1. XRD patterns of 3.97% Ru/SBA-(2) (a), 3.99% Ru/SBA(2)-i (b), and small angle powder XRD (insert).
materials
surface area (m2/g)
total pore volume (cm3/g)
pore size (nm)
particle size (nm) from TEMa
TMS-SBA-(1) TMS-SBA-(2) TMS-SBA-(3) TMS-SBA-(4) 3.25% Ru/SBA-(1) 3.97% Ru/SBA-(2) 3.95% Ru/SBA-(3) 3.72% Ru/SBA-(4) 3.99% Ru/SBA-(2)-i 3.97% Ru/SBA-(2)-E
536.6 676.8 536.6 576.9 402.5 447.8 442.2 538.2 628.7 420.9
0.53 1.18 1.05 1.94 0.38 0.66 0.53 1.78 1.08 0.50
3.7 6.4 7.4 13.3 3.6 6.1 7.3 13.1 6.4 5.9
2.8(3.1) 3.5(3.8) 3.3(4.3) 3.6(4.2) 4.1(4.9) 5.4(5.7)
a
The values in parenthesis were obtained from H2 chemisorption assuming hemispherical crystallites.
9708 J. Phys. Chem. C, Vol. 112, No. 26, 2008
Figure 2. TEM image recorded for 3.97% Ru/SBA-(2).
baseline. The amount of desorbed hydrogen was determined by comparing to the mean areas of calibrated hydrogen pulses. Prior to the experiments, the sample loop was calibrated with pulses of nitrogen in helium flow, comparing with the signal produced from a gastight syringe injection (100 µL) of nitrogen under helium flow. The ruthenium metal particle size was calculated assuming spherical ruthenium metal particles.20,21 Fischer-Tropsch synthesis was conducted in a quartz tube fixed bed reactor (id ) 12 mm). The Ru3+/SBA-(n) sample (0.1 g) was mixed with 1.0 g carborundum and reduced in flow of high purity H2 (6 SL h-1 g-1) at atmosphere pressure. The reactor temperature was increased from ambient to 100 °C and held for 60 min and then increased to 450 °C in 2 h and held at that temperature for 10 h. Subsequently, the reactor was cooled down to 150 °C prior to switching to the syngas (CO/ H2 ) 1:2, 0.5 SL h-1 g-1). The reactor pressure was then increased to 1.0 MPa and temperature was raised to 235 °C at 1 °C/min. The reaction was carried out at 235 °C. The reaction products were collected in a hot trap (130 °C) and a cold trap (-2 °C) in sequence. The outlet gases were analyzed online with an Agilent 3000 GC, and the oil collected at -2 °C was analyzed using an Agilent 6890 GC. The analysis of solid wax collected at 130 °C was performed using an Agilent 4890 GC. 3. Results and Discussion N2 adsorption desorption isotherms and pore size distribution curves for SBA-15 and reduced Ru catalysts in hydrogen are shown in Figure S1 (see Supporting Information). According to the IUPAC classification,22 they are all of type IV isotherms and exhibit the condensation and evaporation steps characteristic of periodic mesoporous materials.23 The fact that the periodic mesoporous structure was retained after Ru insertion was also evidenced by small-angle powder XRD measurement as shown in the insert of Figure 1. The mesoporous structure parameters are summarized in Table 1. After Ru incorporation, the total pore volume decreases to some extent while the average pore size is essentially unchanged. This suggests that the majority of the nanometer-scaled void space of the host silica was still open, although a small portion of the channels may be occupied by the Ru nanoparticles leading to the reduction in the surface area and the pore volume.24
Xiong et al. The presence of Ru can be evidenced by energy dispersive microscopy (EDX, shown in Figure S2 in Supporting Information). Representative powder X-ray diffraction patterns of the selected catalysts are shown in Figure 1, where the broad and weak reflection peak (40-48 °) corresponding to the contribution of metal Ru (JCPDS 00-006-0663) is observed. On the basis of the width of the diffraction peaks, the size of the ruthenium particle is on the nanometer scale. The representative morphology and distribution of Ru nanoparticles inside the pore of SBA-15 are shown by the TEM image in Figure 2, as reflected by the TEM image. The image shows that the pore structure is regular with well-ordered arrays, and the metallic nanoparticles can be seen within the channels of SBA-15. The Ru nanoparticles have a narrow size distribution in the range of 1-3 nm. In this case, the density of sufaceattached APTS groups (molecules per nm2 surface area of derivatized silica) on the supports with different pore size is 2.80-2.84 nm-2. It has been found that the reduction conditions and ligand concentration were the parameters controlling the sizes of Ru nanoparticles.25,26 In this work, for example, after reducing the Ru3+/SBA-(2) precursor by ethylene glycol at 160 °C for 3 h, the nanoparticles were distributed uniformly in a narrow range of 5-6 nm, as revealed in the Figure S3 in the Supporting Information. However, decreasing the APTS surface density on the SBA-(2) matrix from 2.82 to 1.18 nm-2 resulted in the Ru particle with size so small that it can not be discerned by TEM. Consequently, the produced ruthenium nanoparticle confined in the supports with different pore sizes have almost the same particle size (Figure S3 in Supporting Information), possibly caused by the cap effect of the modified aminopropyl groups.27 It should be noted that the pyrolysis of aminopropyl group may occurred due to the detachment of the carbon silicon bond when the catalyst was reduced at 450 °C in flowing H2.28 This can be confirmed by in situ X-ray photoelectron spectroscopy (XPS) technique that N 1s peak disappeared completely after reduction (see Figure S4 in Supporting Information). Table 2 shows the FTS activities and selectivities of these catalysts evaluated under the same processing conditions. The ruthenium time yields of the pore-confined 3.97% Ru/SBA-(2) catalyst and the impregnated 3.99% Ru/SBA-(2)-i catalyst were 36.71 and 50.33 mmol CO/g Ru metal · h, respectively. Generally, the rate with the pore-confined Ru catalyst is lower than that of the reference catalyst. This can be due to the hindrance of the diffusion of syngas in the pore of SBA-15. On increasing the pore size of the catalyst from 3.6 to 13.1 nm, it is found that the catalytic activity increased and then decreased with further increase in the pore size. The Ru particle size may affect the catalytic rate. To demonstrate clearly the effect of different Ru particle sizes in this study, we measured the activity of 3.97% Ru/SBA-(2)-E in the same reaction conditions as 3.97% Ru/ SBA-(2). The results are displayed in Table 2. As can be seen, the average Ru particle sizes of the two pore-confined catalysts
TABLE 2: The Activity and Selectivity of FTS hydrocarbon selectivity (mol %)a catalyst 3.25% 3.97% 3.95% 3.72% 3.99% 3.97% a
Ru/SBA-(1) Ru/SBA-(2) Ru/SBA-(3) Ru/SBA-(4) Ru/SBA-(2)-i Ru/SBA-(2)-E
Ru time yield (mmol CO/g · h)
CH4
C2
C3
C4
C5+
28.89 36.71 37.22 30.15 50.33 36.24
22.85 23.28 22.56 29.97 38.62 23.01
3.26 2.89 2.91 4.06 3.60 2.75
6.82 6.11 6.14 9.02 6.81 6.12
8.77 7.47 7.51 11.1 7.78 7.46
57.53 60.06 60.08 44.86 43.15 60.30
Products were collected and analyzed after reaching steady state; carbon mass balance remained within experimental error ((3%).
Fischer-Tropsch Synthesis of Ru Nanoparticles
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9709 Acknowledgment. This work was supported by National Natural Science foundation of China (20590360, 20773166). Supporting Information Available: N2 adsorption desorption isotherms, EDX spectra, TEM images, and in situ XPS of pore-confined Ru catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 3. Hydrocarbon product distribution over 3.97% Ru/SBA-(2) (b) and 3.99% Ru/SBA-(2)-i (O) catalysts.
are different (3.8 and 5.7 nm, respectively), and the catalytic activity are similar. Thus, the effect of Ru particle sizes (3.8∼5.7 nm) in this study can be ruled out.29 Nevertheless, two factors should be considered to explain this phenomenon. One is that the larger pore facilitated the diffusion of reactants and products in and out of the pores and hence enhanced the reaction rate. On the other hand, further increase of the pore size reduced the suface area per unit weight and decreased the contact between the surface active sites and reactants and hence decreased the FT activity. It is interesting to note that the catalysts with Ru nanoparticles confined in SBA-15 channels exhibited an anti-ASF product distribution, as shown in Figure 3 and Figure S5 in Supporting Information. Comparison of the product distributions of 3.97% Ru/SBA-(2) and 3.99% Ru/SBA-(2)-i (Figure 3) suggests that more wax were produced with the former catalyst. Similar shape-selectivities product distribution has been reported by Fujimoto et al.30–32 Usually, the deviation from ASF distribution were considered to be caused by the occurrence of product accumulation33 or secondary reactions, such as olefin reinsertion into the chain growth process, hydrogenation, and hydrogenolysis.34,35 It has been reported that the deviation from ASF distributioncaused byproduct accumulation was obvious in continuously stirred tank reactors.33 In our studies, almost all the ruthenium composite on the 3.99% Ru/SBA-(2)-i catalyst was dispersed on the outer surface of the support while that on the poreconfined catalyst was completely within the inner surface of SBA-15. This confinement could result in the olefins produced, especially the light olefins, being prevented from rapid exiting the pore of the catalyst. This mass transport limitation effect has been claimed to be responsible for the repeated readsorption of the R-olefins which enhanced the chain growth process.36 4. Conclusions Pore-confined Ru nanoparticles catalysts were prepared for FTS and the pore confinement caused the occurrence of obvious anti-ASF product distribution, which is attributed to the repeated readsorption of R-olefin in the pore. By incorporating active component Ru into the pore of SBA-15 using surface modification, the catalyst activity was found to be significantly affected by pore size instead of particle size or other parameters. This contribution will help future studies on catalyst design and reaction mechanism.
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