Ruifeng Wangab,
Lungang Chenb,
Xinghua Zhang*b,
Qi Zhangb,
Yuping Lib,
Chenguang Wangb and
Longlong Mab
aNano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, P. R. China
bGuangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, P.R. China. E-mail: zhangxh@ms.giec.ac.cn; Fax: +86 20 8705 7789; Tel: +86 20 8705 7790
First published on 7th December 2018
Novel catalytic material with high catalytic activity and hydrothermal stability plays a key role in the efficient conversion of levulinic acid (LA) to γ-valerolactone (GVL) in water. In this study, mixed oxides Al2O3–TiO2, Al2O3–MoO3 and Al2O3–Co3O4 were synthesized by co-precipitation using aqueous solution of NaOH as precipitant. Ru catalysts supported on mixed oxides were prepared by impregnation method and their catalytic performances were tested in the hydrogenation of LA to GVL on a fixed bed reactor. The physicochemical properties of the catalysts were characterized by XRD, H2-TPR, NH3-TPD, and BET techniques. The TiO2 component significantly affected the acidity of the catalyst, and thus its catalytic activity for the GVL yield was affected. The desired product GVL with a yield of about 97% was obtained over the Ru/Al2O3–TiO2 catalyst under mild conditions (WHSV = 1.8 h−1, T = 80 °C). Moreover, the catalyst Ru/Al2O3–TiO2 exhibited excellent thermal stability in the test period of time.
In the past years, extensive research had been conducted on the catalytic conversion of LA to GVL.5,8 Selective hydrogenation of LA to GVL was found to be an efficient method. Typically, base metal catalysts, such as Cu, Ni and Co, often show lower catalytic activity and relatively high temperature is required.9 To improve the catalytic activity, base metal catalysts modified by noble metal were explored. Results demonstrated that hydrogenation of LA to GVL can be achieved over the bi-metallic catalysts supported on neutral supports like active carbon, TiO2, ZrO2 and acid support like γ-Al2O3 at the temperature of 180–250 °C.10–13 It can be seen that high temperature is required in the conversion of LA to GVL over the base metal catalyst, which inevitably incur an energy cost for viable applications.
Fortunately, the conversion of LA to GVL can also be catalysed efficiently by noble metal catalysts under mild condition because of their excellent catalytic activity for hydrogenation.14–16 It had been reported that GVL yields are more than 90% over the catalysts of Ru, Pt and Pd at 90 °C.17,18 Sudhakar et al.19 had reported that hydroxyapatite (HAP)-supported Ru catalyst exhibited 99% yield of GVL from LA by hydrogenation at 70 °C and 0.5 MPa H2 pressure. However, LA conversion decreased to below 90% when the Ru/HAP was repeatedly used for 5 times. It is found that about 47% of Ru were leached into solution when Ru/Al2O3 was used as catalyst in the conversion of LA to GVL.18 Deteriorated hydrothermal stability of the catalyst has become a bottleneck restricting the production of GVL from LA. To improve the hydrothermal stability of catalyst, hydrogenation catalysts supported on water-tolerant materials, such as carbon20 and silica,21 had been tested. However, these catalysts had a low yield and selectivity for GVL. Chia et al.22 developed a new conversion method for the production of GVL from alkyl levulinates over ZrO2 catalysts. Rapid deactivation of the catalyst was also observed during a 100 h period. Arguably, more studies are required to develop a novel catalyst with longer service life.
The mixed oxides are usually attractive for the improvement of catalytic performance.23 To develop a novel material with high catalytic activity and hydrothermal stability, mixed oxides Al2O3–TiO2, Al2O3–Co3O4 and Al2O3–MoO3 were prepared in this study, and were used as carrier for Ru-based catalyst. Also, the catalytic activity was tested in the conversion of LA to GVL in water. It was found that Ru/Al2O3–TiO2 catalyst is capable of the efficient synthesis of GVL from LA in a continuous fixed bed reactor under mild condition. TiO2 is pristine material with excellent hydrothermal stability.23 The screening investigations have indicated that Ru/TiO2 catalyst can offer catalytic activity in the conversion of LA to GVL.24 For comparison purposes, the pure oxide TiO2 and Al2O3 were also used as carrier for Ru-based catalyst and were tested in the conversion of LA to GVL.
Al2O3–MoO3: solutions of (NH4)6Mo7O24 and Al (NO3)3 were mixed. With continuous stirring, sodium hydroxide solution was added into the mixed solution at 70 °C until the pH of the solution reached an approximate value of 8. The obtained precipitate was filtered and washed with distilled water. Then the obtained solid was dried at 80 °C for 12 h and calcined at 500 °C for 5 h, by which the complex oxide Al2O3–MoO3 (mole ratio of Al:Mo = 1:1) was prepared and was designated as Al–Mo.
Similar to that of Al2O3–MoO3, complex oxide Al2O3–Co3O4 (mole ratio of Al:Co = 1:1) was prepared using Al(NO3)3·9H2O and Co(NO3)2·6H2O as precursor and designated as Al–Co. Al2O3–TiO2 (mole ratio of Al:Ti = 1:1) was prepared using TiCl4 and Al(NO3)3 as precursor and designated as Al–Ti.
Ru-based catalysts with 5 wt% Ru loadings were prepared by impregnation. Specifically, support was dipped into the RuCl3 solution (0.1 mol l−1), followed by treating with ultrasonic irradiation for 2 h. After impregnation for 12 h, the solution was evaporated to dryness. Then, the obtained solid was dried at 120 °C overnight and calcined at 550 °C for 4 h. For comparison, Ru catalysts supported on Al2O3 and TiO2 were also prepared and designated as Ru/Al, Ru/Ti, respectively.
The prepared catalysts were crushed, sieved through 0.83–0.38 mm mesh before use.
Chemicals used in this study are C.P. reagents and purchased from Aladdin's Reagent Company.
The crystal structure of the catalysts were analyzed by X-ray diffraction radiation (X Per Pro MPD with Cu Kα (λ = 0.15406 nm) radiation, Philip). The transmission electron microscope (TEM) images of the catalysts were obtained on an instrument (JEM-2100F).
The H2-TPR (H2 temperature-programmed reduction) analysis of catalyst was conducted on a chemisorption apparatus. The catalyst samples (100 mg) were pretreated at 300 °C for 1 h under an Ar flow and subsequently cooled to 50 °C. The reduction temperature increased to 800 °C with 10 °C min−1 in the flow of reduction gas (H2 + N2, 40 ml min−1). The H2 consumption was monitored by a thermal conductivity detector (TCD).
The XPS (X-ray photoelectron spectroscopy) analysis of Ru catalyst was carried out on the instrument of Thermo Scientific ESCALAB 250Xi employing a monochromated AlKα X-ray source.
The NH3-TPD (NH3 temperature-programmed desorption) analysis was also conducted on a chemisorption apparatus equipped with a TCD detector. Before ammonia adsorption, the catalyst samples (100 mg) were in situ reduced for 2 h at 300 °C in the flow of hydrogen and purged at 100 °C for 1 h with helium. The detailed method was described in a literature.25 The catalyst was pretreated at 200 °C under vacuum (1 × 10−4 Pa) for 1 h. Pyridine was introduced in the IR cell at room temperature. Desorption was carried out at 200 °C, and the catalyst was evacuated for 10 min prior to recording the IR spectra.
(1) |
(2) |
The gas product obtained from the hydrogenation process was analyzed on a Gas Chromatogram (Agilent 6980) with a TCD (Thermal Conductivity Detector) and a FID (Flame Ionization Detector) using the external standard method.
The concentration of Al and Ru ions remaining in the liquid samples was determined by ICP-AES (Inductively coupled plasma-atomic emission spectroscopy, IRIS Advantage 1000, Thermo Electron Corporation).
Catalyst | SBET (m2 g−1) | Pore volume (cm3 g−1) | Pore size (nm) |
---|---|---|---|
Ru/Al–Mo | 243 | 0.65 | 10.7 |
Ru/Al–Co | 77 | 0.27 | 14 |
Ru/Al–Ti | 98 | 0.24 | 9.8 |
Ru/Al | 176 | 0.18 | 4.1 |
Ru/Ti | 11 | 0.04 | 14.5 |
XRD patterns of samples were gathered in Fig. 2. The peaks assigned to Ru were not significant for all samples. Only one weak diffraction peak positioned at the 2θ of 38.4° was observed for all fresh catalysts, which were assigned to the (1 0 0) plane of Ru. This result suggests good dispersion of Ru metal. The size of Ru particles supported on different mixed oxides was determined by TEM. As shown in Fig. 3, most of Ru particle are in the range of approximately 2–5 nm. Especially for Ru/Al–Ti catalyst, the particle size of Ru are all in the range of 1–3 nm and the average particle size is 1.76 nm. Primo et al.24 had reported that the Ru particle with the size of about 2 nm exhibited the highest catalytic activity for hydrogenation.
Fig. 4 gathered the H2-TPR profiles of the Ru-based catalysts. The peaks, positioned in the range of 90–250 °C, were observed for all of catalysts, which are attributed to the reduction of RuO2. The effect of support on the reduction of active metal is minor. Noticeably, two broad peaks centred at 316 °C and 485 °C were clearly observed in the profile of Ru/Al–Co catalyst. The former peak can be assigned to the reduction of Co3O4 to CoO and the latter can be attributed to the reduction of CoO to Co.26
The acidity analysis of the supported Ru catalysts was carried out by NH3-TPD, and the results were presented in Fig. 5 and Table 2. Two clear peaks were observed in the desorption curves of NH3 for Ru/Al–Mo, Ru/Al–Co and Ru/Al catalysts. The first peak positioned in the range of 200–250 °C is assigned to medium acid sites, and the high-temperature desorption peak positioned in the range of 440–490 °C is assigned to strong acid sites.27 The acidity of TiO2 is very weak as expected and the amount of acid sites is little.8 It should be noted that the NH3-TPD profile of Ru/Ti–Al is rather close to that of Ru/Ti catalyst. A possible reason is that TiO2 was coated on the surface of Al2O3,28,29 leading to the less acidic sites of Ru/Ti–Al catalyst. The type of acidic sites is determined by infrared spectroscopy of pyridine adsorption. It can be found that Lewis acid sites (1450 cm−1) is dominant for all tested catalyst as shown in Fig. 6.
Catalyst | 120–345 °C, mmol g−1 | 345–787 °C, mmol g−1 | Total acid sites, mmol g−1 |
---|---|---|---|
Ru/Al–Mo | 0.277 | 0.291 | 0.568 |
Ru/Al–Co | 0.124 | 0.189 | 0.313 |
Ru/Al–Ti | 0.040 | 0.063 | 0.103 |
Ru/Al | 0.279 | 0.053 | 0.332 |
Ru/Ti | 0.013 | 0.042 | 0.055 |
The Ru 3d XPS spectrum of Ru catalyst is presented in Fig. 7. The spectrum in the region of 283–287 eV is complex due to the overlapping of C 1s signal and Ru 3d doublet (5/2 and 3/2).30 The regions of 281.5–283 eV are assigned to RuOx, suggesting that ruthenium oxides remain at the catalyst surface even after reduction. Normally, the Ru 3d XPS spectrum should show a signal at 280.0 eV, which is attributed to Ru0.16 However, the signals of Ru0 were observed at 280.2 eV for Ru/Al–Mo, 280.4 eV for Ru/Al–Ti and 280.6 eV for Ru/Al–Co, respectively. The small shift of the BE (binding energy) may have been caused by the interaction between the metallic Ru0 and its support.31
Fig. 8 Conversion of LA to GVL over different Ru catalysts. Reaction conditions. PH2 = 4.0 MPa, H2 flow: 40 ml min−1; WHSV: 0.6 h−1. |
As shown in Fig. 9(a), the components of CO, CO2, CH4, C2+ were detected in the gas product. These gas products were derived from LA, GVL and intermediates via aqueous phase reforming with Ru catalyst. For example, apart from the decarboxylation,32 the formation of CO2 may also be considered to be a catalytic decomposition of the reactants to form CO, combined with the conversion of CO and H2O to produce CO2. Cleavage of C–C and C–O bonds can be completed by hydrogenolysis over Ru catalyst in the presence of H2, thereby producing the smaller hydrocarbon molecule.33 Similar reactions had been reported.34,35 Arguably, higher temperature can promote the aqueous phase reforming (Fig. 9(b)). This is the reason that LA conversion increases at higher temperature, but the product yield does not increase in synchronization, as shown in Fig. 8.
To explore the effect of mixed oxides on the catalytic activity, the hydrogenation of LA over Ru catalyst supported on pure oxide was also conducted at 80 °C. As shown in Fig. 10, the GVL yields were only 41.6% and 21.8% over Ru/Al and Ru/Ti catalysts, respectively. For the Ru/Ti catalyst, the deteriorated porous and small surface area might be the root cause for the lower GVL yield of the Ru/Ti catalyst. As for the Ru/Al catalyst, it is similar to that of Ru/Al–Co and Ru/Al–Mo catalysts, the strong acidity of the catalyst results in the lower yield of desired product. On the contrary, the GVL yield drastically increased to 80% when Ru/Al–Ti was used as catalyst. This result suggests that mixed oxides have significant advantages in improving catalytic activity.
Fig. 10 Comparison for the effect of mixed oxides and pure oxide. Reaction conditions: T = 80 °C; PH2 = 4.0 MPa; H2 flow: 40 ml min−1; WHSV = 0.6 h−1. |
Fig. 11 Effect of WHSV on the conversion of LA to GVL over Ru/Al–Ti catalyst. Reaction conditions: T = 80 °C; PH2 = 4.0 MPa; H2 flow: 40 ml min−1. |
Fig. 12 The stability test for Ru/Al–Ti catalyst. Reaction conditions: WHSV = 1.8 h−1; T = 80 °C; PH2 = 4.0 MPa; H2 flow: 40 ml min−1. |
Catalyst | Time on stream (h) | Total leached Al (wt%) | Total leached Ru (wt%) |
---|---|---|---|
a Reaction conditions: T = 80 °C; PH2 = 4.0 MPa; H2 flow: 40 ml min−1; WHSV = 1.8 h−1. | |||
Ru/Al | 30 | 2.1 | 35 |
60 | 3.6 | 44 | |
Ru/Ti | 30 | — | 0.6 |
120 | — | 0.8 | |
Ru/Al–Ti | 30 | 0.2 | 1.4 |
120 | 0.4 | 2.6 |
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