Supported Ni catalysts are used in various industrially important chemical processes such as (de)hydrogenation,methanation,reforming,and hydrocracking [1, 2, 3, 4, 5]. Inorganic-pillared clays (PILCs) have been widely used as catalysts due to their high specific surface areas and pore volumes,and tunable pillars [6, 7, 8]. Therefore,PILC-supported Ni catalysts have been investigated [9, 10, 11, 12]. However,the use of organic-PILCs is limited because of their low thermal stability at high temperatures.
Recently,supported noble-metal catalysts with excellent performance have been designed and prepared,based on the organophilic characteristics of organic-PILCs [13,14],which improve catalyst dispersion in organic solvents [13] and enable tuning of the catalyst polarity [14]. The organophilic characteristics of the catalysts are destroyed by thermal decomposition of the organic pillars at high temperatures,so catalysts derived from organic-PILCs need to be prepared and used at low temperatures (usually below 100 °C) [13,14]. Ni catalysts are usually used at high temperatures,so they are not suitable for use in catalysts designed on the basis of the organophilic characteristics of organic clays. However,highly dispersed metal oxide catalysts can be obtained using as-prepared mesoporous materials because the templates in such materials can occlude and interact with introduced species [15]. Organic clays are very similar to the mesoporous materials; an organic pillar such as cetyltrimethylammonium bromide (CTAB) is usually used as a template in the preparation of MCM-41,and the basal spacing of the clay is enlarged by the large organic pillar molecules. Highly dispersed Ni catalysts could therefore be obtained using organic-PILCs as the supporting matrixes.
Hydrogenation of aromatic hydrocarbon is normally used for catalytic tests because it is not only a useful model reaction for evaluating the activity of metal catalysts,but is also of commercial importance in the upgrading of coal liquids and diesel fuels [16, 17, 18, 19]. Here,a highly active Ni/montmorillonite (MMT) catalyst for naphthalene hydrogenation was prepared via an impregnation method,using CTAB-pillared MMT as the supporting matrix. Ni supported on Al2O3-pillared MMT was also prepared for comparison. The mechanism of the promoting effect of CTAB pillars on the Ni/MMT catalyst is discussed.
Preparation of CTAB-pillared MMT (MMT-CTAB). Na-MMT (Zhejiang Sanding Group Co.,Ltd.) was used directly without further purification. Na-MMT (2.0 g) was dispersed in H2O (100 mL) and CTAB (2.0 g) was dissolved in H2O (50 mL). The CTAB aqueous solution was slowly added to the Na-MMT suspension at 80 °C. After ion exchanging for 8 h,the suspension was subjected to centrifugation. Excess CTAB was removed by washing the solid several times with 50% ethanol until no Br- ions were detected using AgNO3 solution. After drying at 80 °C for 12 h,MMT-CTAB was obtained.
Preparation of Al2O3-pillared MMT (MMT-Al2O3). Na-MMT (1.0 g) was dispersed in H2O (200 mL). The Al polycations as the pillar agent [20] were obtained by microwave-assisted hydrolysis of AlCl3 via decomposition of urea. The Na-MMT suspension was then equilibrated with the Al polycation solution (25 mmol) for 20 h,and the product was separated by centrifugation. The solid was washed several times with H2O until no Cl- ions were detected using AgNO3 solution. After drying at 120 °C for 12 h,and then calcined at 550 °C for 4 h,the MMT-Al2O3 was obtained.
Preparation of Ni/MMT,Ni/MMT-CTAB,and Ni/MMT-Al2O3. MMT,MMT-CTAB,or MMT-Al2O3 (1.0 g) was impregnated with an aqueous solution containing required amount of Ni(NO3)2·6H2O (0.3 mol/L) under stirring at 60 °C until dryness,and drying at 100 °C overnight. The supported Ni precursors were then reduced at 550 °C by H2 flow (30 mL/min) for 4 h,and passivated in 1.0% O2/N2 flow at room temperature for 3 h to obtain the supported Ni catalysts,i.e. Ni/MMT,Ni/MMT-CTAB,and Ni/MMT-Al2O3. The Ni loading on each catalyst was 10.0 wt%,determined by flame atomic absorption spectroscopy (TAS-990F,Beijing Purkinje General Instrument Co.,Ltd.).
Fourier-transform infrared (FT-IR) spectra were recorded using a Nicolet 6700 FT-IR spectrometer at a resolution of 4 cm-1. X-ray diffraction (XRD) data were obtained with a Shimadzu XD-3A diffractometer,using Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 40 mA. The specific surface areas and pore structure were determined by N2 adsorption-desorption at -196 °C using a Micromeritics ASAP 2420 analyzer. H2 temperature-programmed desorption (H2-TPD) experiments were carried out using a Quantachrome instrument (ChemBET 3000) as described previously [21,22]. Before the measurements,the catalyst (50 mg) was heated to 550 °C in 10% H2/Ar at a rate of 20 °C/min,and kept at 550 °C for 0.5 h. The catalyst was then cooled to 40 °C and kept at this temperature for 0.5 h. The H2/Ar flow was then switched to pure Ar (40 mL/min) for 0.5 h at 40 °C. Finally,the catalyst was heated from 40 to 500 °C at a rate of 10 °C/min,and the H2-TPD curve was recorded. The peak area was used to calculate the volume of chemisorbed H2. The peak area was calibrated by running a pure H2 pulse-titration experiment on a blank sample. The Ni surface area (S,m2/g),dispersion of metallic Ni (D,%) and average particle size of metallic Ni (d,nm) were calculated from the volume of H2 chemisorbed,using the reported method [21,22]. Ultraviolet diffuse reflectance spectroscopy (UV-DRS) was performed in the range 200-800 nm using a PE Lambda 750S spectrophotometer.
Naphthalene hydrogenation was performed in a 60 mL stainless-steel autoclave (Ordino CS340,Premex) equipped with a heating system and a magnetic-coupled paddle stirrer. In a typical run,a solution of naphthalene in n-dodecane (10.0 wt%; 10 g) and catalyst (0.12 g) were loaded into the reactor. The reaction conditions were 300 °C and p(H2) = 5.0 MPa. Each catalytic test was performed for 2.0 h. The reaction products were analyzed using a gas chromatograph (GC-7890II,TechCom) equipped with an OV-1 column (30 m × 0.25 mm × 0.33 μm) and a flame ionization detector.
MMT-CTAB was obtained by ion exchange of CTAB onto MMT. FT-IR spectra of MMT,MMT-CTAB,and CTAB were recorded between 4000 and 400 cm-1 to determine whether the organic cations were exchanged onto MMT (Fig. 1(a)). Unlike the spectrum of pristine MMT,the C-H bonds (2919,2849 cm-1) of methylene groups for CTAB are clearly observed in the MMT-CTAB. CTAB adsorbed on the external surface of MMT was washed out during preparation of MMT-CTAB,so it indicates CTAB intercalation into MMT interlayers. The basal spacing of MMT-CTAB was characterized using XRD in the small-angle range (Fig. 1(b)). Similar to the results obtained by Su group [23],the basal spacing of MMT was 1.24 nm,and the basal spacing of MMT-CTAB was enlarged to 2.26 nm as a result of incorporation of organic cations of CTAB into the MMT galleries by ion exchange.
The FT-IR spectrum of Ni/MMT-CTAB was recorded to determine whether CTAB remained in the as-prepared catalyst. In contrast to the case for MMT-CTAB,the C-H bonds of methylene groups disappeared over Ni/MMT-CTAB. This indicates that CTAB was thermally decomposed during the reduction of Ni/MMT-CTAB. So,although the Ni/MMT catalyst obtained using MMT-CTAB as the supporting matrix (Ni/MMT-CTAB),the CTAB pillars were decomposed during reduction and were not present in the Ni/MMT-CTAB catalyst.
XRD studies of Ni/MMT,Ni/MMT-CTAB,and Ni/MMT-Al2O3 catalysts were performed to determine the effect of the organic modification of MMT on Ni/MMT. The characteristic peaks of MMT (2θ = 18.4°,19.8°,27.8°,35.1°,62.0°,68.0°,and 73.3°) and Ni metal (2θ = 44.4°,52.0°,and 76.5°) were observed for all the catalysts (Fig. 2). This means that the crystal structure of MMT was not destroyed and the Ni species were reduced to Ni metal in all the catalysts during preparation. In comparison with the Ni/MMT catalyst,the peak intensity for Ni metal decreased and the widths of the peaks at half height increased significantly for Ni/MMT-Al2O3,and particularly for Ni/MMT-CTAB. This indicates that the Ni metal particles in Ni/MMT-CTAB are smaller than those in Ni/MMT and Ni/MMT-Al2O3,suggesting that Ni/MMT dispersion is improved by organic modification of MMT.
H2 chemisorption on all the catalysts was measured using H2-TPD to examine the effect of the organic modification of MMT on Ni/MMT dispersion further (Fig. 3). The peak area for Ni/MMT-CTAB is much bigger than those for Ni/MMT and Ni/MMT-Al2O3. The H2 chemisorption uptakes,surface areas,and dispersions were estimated based on H2-TPD profiles,assuming adsorption of one H atom per metal atom [21,22,24]; the results are shown in Table 1. The Ni dispersion for Ni/MMT-CTAB is 14.1%,which is much higher than those for Ni/MMT (6.0%) and Ni/MMT-Al2O3 (8.5%),suggesting that Ni dispersion of Ni/MMT is improved greatly by organic modification of MMT.
Figure 4(a) shows the N2 adsorption-desorption isotherms of MMT,Ni/MMT,and Ni/MMT-CTAB. The specific surface area,pore volume,and pore size are shown in Table 2. It can be seen that the N2 adsorption,pore volume,and specific surface area of MMT and Ni/MMT are very low. However,the specific surface area increases from 10 m2/g for Ni/MMT to 76 m2/g for Ni/MMT-CTAB,and the pore volume increases from 0.04 cm3/g for Ni/MMT to 0.17 cm3/g for Ni/MMT-CTAB. In contrast,the specific surface area of Ni/MMT-Al2O3 is 64 m2/g,although that of MMT-Al2O3 is 113 m2/g. The pore volume of Ni/MMT-Al2O3 (0.08 cm3/g) is only about half that of Ni/MMT-CTAB. In addition,the micropore volume decreases from 0.05 cm3/g for MMT-Al2O3 to 0.02 cm3/g for Ni/MMT-Al2O3,and the mesopore volume and mesopore surface area are almost unchanged. This indicates that some micropores are blocked by Ni metal on the Ni/MMT-Al2O3 catalyst. However,only mesopores are observed for Ni/MMT-CTAB with the narrowest pore size distribution (Fig. 4(b)). This means that the pore size distribution of Ni/MMT is also improved by organic modification of MMT.
We studied the activity of Ni/MMT,Ni/MMT-CTAB,and Ni/MMT-Al2O3 catalysts in the hydrogenation of naphthalene. The main hydrogenation products were tetralin and decalin,and no hydrocracking products were identified. Naphthalene conversion over the Ni/MMT-CTAB and Ni/MMT-Al2O3 catalysts was higher than that over the Ni/MMT catalyst (Table 3). The conversion of naphthalene obviously increases from 13.1% on Ni/MMT to 88.2% on Ni/MMT-CTAB,indicating the effect of organic modification of MMT. The use of Al2O3-pillared MMT as a support increases the activity of Ni/MMT by about 85% (the naphthalene conversion 24.2%). These results indicate that pillaring MMT with an organic compound (CTAB) or inorganic compound (Al2O3) increases the activity of the Ni/MMT catalyst,but the promoting of CTAB is much more effective than that of Al2O3. Moreover,we previously found that calcination in H2 increased the activity of Ni/SBA-15 and 68.2% of conversion was achieved [21],which is lower than that obtained over Ni/MMT-CTAB.
We have shown that organic modification of MMT (MMT pillared with CTAB) greatly enhances the catalytic activity of Ni/MMT. Specifically,we found that the effect on Ni/MMT with the organic modification of MMT is not based on the organophilic characteristics of CTAB,because the CTAB pillars are thermally decomposed during reduction of the catalyst. Therefore,the promoting effect of the organic modification of MMT is ascribed to interactions between the CTAB pillars and the Ni2+ ions introduced during impregnation. To investigate this interaction,we prepared a Ni(NO3)2·6H2O + CTAB mixture and compared it with Ni(NO3)2·6H2O/MMT-CTAB and Ni(NO3)2· 6H2O/MMT samples. These samples show obvious color differences. An orange color appears after manual mixing of Ni(NO3)2·6H2O and CTAB,the same color as the Ni(NO3)2· 6H2O/MMT-CTAB sample. However,the Ni(NO3)2·6H2O/MMT sample remains pale blue,which is derived from Ni(NO3)2· 6H2O. This means that CTAB may complex with Ni2+ ions to form orange Ni-CTAB complexes. We performed UV-DRS measurements on Ni(NO3)2·6H2O/MMT,Ni(NO3)2·6H2O/ MMT-CTAB,MMT-CTAB,and the Ni(NO3)2·6H2O + CTAB mixture (Fig. 5) to determine whether Ni-CTAB complexation occurred in Ni(NO3)2·6H2O/MMT-CTAB. The UV band at around 390 nm for Ni(NO3)2 is observed for Ni(NO3)2·6H2O/MMT. New UV bands at around 335 and 440 nm ascribed to the orange Ni-CTAB complex appear,but the band at around 390 nm disappears for both Ni(NO3)2·6H2O/MMT-CTAB and the Ni(NO3)2·6H2O + CTAB mixtu re. In contrast,the peaks at around 335 and 440 nm are not observed for MMT-CTAB and Ni(NO3)2·6H2O/MMT.
Based on these results,a mechanism for catalyst promotion by organic modification of MMT in Ni/MMT catalysts is proposed,as shown in Scheme 1. Compared with that of MMT,the basal spacing of MMT-CTAB is enlarged because the CTAB molecules are large. Nickel nitrate can then easily enter the MMT interlayers and the CTAB pillars can occlude Ni-CTAB complexes formed with the introduced Ni2+ ions during impregnation,which leads to greatly improved dispersion of Ni/MMT after reduction. The specific surface area and pore volume are greatly enhanced by the small Ni particles located in the MMT interlayers. Moreover,there are no micropores in the Ni/MMT catalyst derived from CTAB-pillared MMT,which favors mass transfer of naphthalene and its hydrogenation products. In contrast,Al2O3-pillared MMT has a high specific surface area as a result of the very small Al2O3 pillars located in the MMT interlayers. The Al2O3 pillars are small,so Al2O3-pillared MMT contains micropores. Although more nickel nitrate can enter the Al2O3-pillared MMT interlayers and Ni/MMT-Al2O3 dispersion is improved,some micropores are blocked by Ni metal particles. The specific surface area and pore volume of Ni/MMT-Al2O3 are therefore lower than those of MMT-Al2O3. As a result,the hydrogenation activity of Ni/MMT is significantly promoted by using CTAB-pillared MMT as the supporting matrix,and the resulting catalyst is superior to the Ni/MMT-Al2O3 catalyst.
MMT pillared with CTAB greatly enhances Ni/MMT catalytic activity for naphthalene hydrogenation. The CTAB pillars are thermally decomposed during the reduction of the catalyst,so this promotion is not caused by the organophilic characteristics of CTAB-pillared MMT,but by interactions between the CTAB pillars and the Ni2+ ions introduced during impregnation. The CTAB pillars enlarge the basal spacing of MMT,and occlude and complex with the introduced Ni2+ ions. After reduction,the dispersion,specific surface area,and pore volume of the Ni/MMT catalyst are greatly improved. The obtained Ni/MMT catalyst exhibits excellent catalytic activity for the hydrogenation of naphthalene,not only much higher than those of Ni/pristine MMT and Ni/MMT-Al2O3 catalysts,but even higher than that of Ni/SBA-15. These results provide a new perspective on the preparation of highly active metal or metal oxide catalysts supported on clays.
2014, Vol. 35 Issue (4): 546-552
PDF (832KB)
2014, Vol. 35 Issue (4): 546-552
PDF (832KB)