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Selective hydrogenation of crotonaldehyde on Au supported on mesoporous titania
Microporous and Mesoporous Materials 168 (2013) 51–56
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Selective hydrogenation of crotonaldehyde on Au supported on mesoporous titania Hideaki Yoshitake ⇑, Naoki Saito Division of Materials Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e
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Article history: Received 17 July 2012 Received in revised form 28 August 2012 Accepted 27 September 2012 Available online 8 October 2012 Keywords: Mesoporous titania Gold catalyst Hydrogenation of crotonaldehyde Mesoporous anatase
a b s t r a c t We report here on how gold supported on mesoporous anatase work as a catalyst for the highly-selective hydrogenation of crotonaldehyde into crotyl alcohol. The gold was supported by deposition–precipitation method onto mesoporous titania prepared using hexadecylamine as the structure directing agent. This mesoporous titania provides a narrow pore size distribution and an amorphous framework, which can be converted to another pore size distribution and anatase framework by heating at 673 K. Gold was more highly dispersed on mesoporous anatase prepared by heating at 673 K than on anatase sample heated at 473–573 K or on commercially-available anatase. The structure of these catalysts was analysed by XRD, N2-adsorption/desorption, TEM and XPS. In the hydrogenation of crotonaldehyde at ambient pressure, the selectivity for crotyl alcohol reached 82% on gold supported on mesoporous anatase, while 49% selectivity was obtained for gold supported on commercially-available anatase under the same reaction conditions. In addition, the rate of hydrogenation is also four times greater on Au/mesoporous anatase than on Au/anatase. The selectivity for crotyl aldehyde on gold on mesoporous amorphous titania was 68% under the same reaction conditions. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Use as a catalyst support is a potentially important application for mesoporous materials, while the exploitation of new metal catalyses has always been an active motor for the chemistry of catalysis. Since Haruta and Hutchings showed that nanoparticles of gold can be excellent catalysts for oxidation, extensive studies have been undertaken to contribute to the preparations, structural analyses and catalytic reactions of gold, which had long been considered to be an inert metal as solid catalyst [1–20]. Unlike the oxidation reactions investigated extensively in earlier studies, the selectivities are usually the primary concern in catalytic hydrogenations [21,22] rather than the activity [23,24]. In fact, the various gold catalysts have been investigated for their chemoselectivities in the hydrogenation of double bonds, including 1,3-butadiene [25,26], acetylene [27] and chloronitrobenzene [28]. The exchange reaction between H2 and D2 during propylene hydrogenation has also been studied with respect to the mechanism of unsaturated hydrocarbon conversion [29]. The conversion of the carbonyl group into alcohol is an important process in the perfumery industry and in the treatment of waste water [30]. The selective hydrogenation of the carbonyl bonds in unsaturated carbonyl compounds has long been regarded as a valuable reaction, though the hydrogenation of –C@O is less thermodynamically favourable than that of –C@C– in simple unsaturated ⇑ Corresponding author. Fax: +81 45 339 4378. E-mail address: [email protected] (H. Yoshitake). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.09.031
aldehydes without other heteroatoms or stereochemical distortions. The hydrogenation of the simplest unsaturated aldehyde, acrolein, has been investigated over Au-supported on several oxides [21,31]. However, this type of substrate can be polymerized in the presence of acid sites and the existence of multiple paths to convert the unsaturated alcohol into aldehyde has been proposed from several sources of experimental evidence [32–35]. Furthermore, the selective hydrogenation of the carbonyl group in a,bunsaturated aldehydes is usually more important between inner – C@C– and –CH@O groups than between the terminal CH2@C– and –CH@O groups. For these reasons, crotonaldehyde or cinnamaldehyde have often been chosen as the molecule for investigation of the selectivities in hydrogenation of a,b-unsaturated aldehydes. In the studies into Au catalysts various selectivities have been reported for saturated aldehyde, unsaturated alcohol and saturated alcohol, depending on the support and the preparation conditions [26,36–49]. This fact suggests that hydrogenation of a,b-unsaturated aldehydes can be a good test reaction for unveiling the properties of the catalyst support materials. Mesoporous titania prepared with a primary amine as the structure directing agent has an extremely large surface area, uniform pore size and a large pore volume [50,51]. The crystalline structure of this solid can be changed simply by heating in air from amorphous into crystalline anatase [52] via an anatase–rutile mixed phase (confirmed by vibrational spectroscopy [53], which does not imply that nanocrystallites of anatase and rutile are mixed but that the local structure of the Ti atoms is either anatase or rutile). A transition of the mesoporous structure occurs, which is
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accompanied by the crystallisation into anatase [52,53]. The oxidation number of catalytically active elements on mesoporous titania can be considerably different from those on the conventional nonporous titania supports [54,55]. It is, therefore, natural that oxidation and acid catalyses behave completely differently on catalysts prepared using a mesoporous titania support [54,55]. Since catalyses by gold are often correlated with the oxidation state of the gold [4,37,38,40,41,44,45,56], the use of mesoporous titania as a support will be an interesting subject for gold catalysts. To the best of our knowledge, Au/mesoporous titania with a narrow pore size distribution has been studied only once, to investigate the water– gas shift reaction on Au/mesoporous anatase, where gold is loaded on mesoporous anatase prepared using a C16(EO)6-polyoxyethylene(6) tridecylether template [57]. In this study, Au/mesoporous titania catalysts are prepared by a deposition–precipitation (DP) method while varying the crystalline and porous structures in order to elucidate the properties of this structurally-unique material as a support material for metal catalysts. 2. Experimental 2.1. Materials 1-Hexadecylamine, tetraisopropyl orthotitanate, p-toluenesulfonic acid, chloroauric acid tetrahydrate, sodium carbonate and 1-butanol were obtained from Wako Pure Chemical Industries Ltd. Crotonaldehyde, crotyl alcohol and butylaldehyde were purchased from Tokyo Chemical Industry Co., Ltd. Anatase (specific surface area was 200 m2/g) was also commercially available from Aldrich.
2.4. Instrumental analyses The loading of Au was determined by ICP-AES using an ICPE9000 Multitype Inductively Coupled Plasma Emission Spectrometer (Shimadzu). The structural periodicity of the catalysts was confirmed by XRD using a RINT 2200 diffractometer (equipped with a Cu target, Rigaku Co.) operated at 40 kV and 40 mA. Nitrogen adsorption and desorption were carried out for the porous solids after drying at 453 K for 2 h using a Quantachrome NOVA 4200e sorptometer. X-ray photoelectron spectra of the catalysts were measured using a Quantera SXM spectrometer (Ulvac-Phi, Inc.). 2.5. Catalytic reactions Hydrogenation of crotonaldehyde was carried out in a flow reactor under ambient pressure. Typically, the catalyst (0.1 g) mounted in a quartz tube was oxidised at 473 K under flowing oxygen (Ar/O2 = 1/1, 4 L/h). Crotonaldehyde was vaporised in a temperature-controlled bath and mixed with the hydrogen flow (4 L/ h). The products were analysed using an on-line gas chromatograph equipped with packed columns and a TCD detector. 3. Results and discussion The nitrogen adsorption/desorption isotherms of the catalysts and the support mesoporous titanias are plotted in two frames in Fig.1. The pore size distributions are also shown in the other two frames in the same figure. The isotherms are assigned to the type IV curves. After the loading of gold for the MT200 and MT300, the uptake of nitrogen decreases significantly and the peak in the
2.2. Preparation of mesoporous titania 8.0 g of tetraisopropyl orthotitanate was added to a mixture of 3.4 g of 1-hexadecylamine and 40 g of water under vigorous stirring at 273 K. After the addition of 1.6 cm3 of 1 M hydrochloric acid, the stirring continued for 30 min at the same temperature and the mixture was then stood overnight at 298 K. It was then transferred to a Teflon container in an oven at 333 K. After 4 d, the solution was filtered off and the white solid was washed with ethanol. After being dried at 373 K for 24 h, the solid was transferred into a Pyrex tube in vacuum at 453 K and heated in vacuum for 2 h. The tube was then sealed. The powder in the tube was heated at 453 K for 10 d. (A part of the sealed tube must be kept below 373 K, in order to avoid pressurization due to the production of water.) The resulting solid was then treated twice in a 0.1 M ethanol solution of p-toluenesulfonic acid at room temperature, followed by washing with ethanol for 2 h, collected by filtration and drying at 373 K for 1 h. Mesoporous titania thus prepared was heated at 473 K for 2 h. This powder sample is denoted as MT200. MT300 and MT400 were prepared by heating at 573 and 673 K, respectively, instead of 473 K. 2.3. Loading of gold MT200, MT300 and MT400 were added to aqueous solutions of chloroauric acid with the same volumes as their mesopores, and then the pH was raised to 10 using sodium carbonate. The amount of gold ions in the solution was adjusted to be 3 wt.% of the support. After stirring at 313 K for 3 h, the powder was collected by filtration, washed with water and dried at 333 K for 1 h. Finally, the catalyst was heated at 473 K for 2 h to obtain Au/MT200, Au/ MT300 and Au/MT400. Au/anatase was prepared by the same procedure using anatase provided by Aldrich.
Fig. 1. Nitrogen adsorption/desorption isotherms and BJH pore size distributions of the catalysts, (a) Au/MT200, (b) Au/MT300 and (c) Au/MT400, and the supports, (d) MT200, (e) MT300 and (f) MT400.
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pore size distributions is slightly broadened. In contrast, no significant change is found in the isotherms between Au/MT400 and MT400. The BET specific surface areas, pore sizes and pore volumes calculated from the adsorption data are summarized in Table 1. The loading of Au is also listed in the same table. Although the decreases in surface area and pore volume are remarkable, the loading of Au does not significantly change the pore size in Au/MT200 and Au/MT300. Considering that the porous structure of mesoporous titania is likely to be composed of wormhole-like channels, this simultaneous change found in ABET and VP can be reasonably explained by pore blocking due to the growth of gold particles. On the other hand, none of these three parameters changes significantly in Au/MT400, though the loading of gold is almost the same as in Au/MT200 and Au/MT300. This result suggests that the pore blocking by the gold particles is negligible in Au/MT400. This can be attributed to the larger pore size (3.5 nm) in MT400 than those (ca. 2.0 nm) in MT200 and MT300. Once a pore is blocked, the metal particles inside the pore become inaccessible from the outside and the effective surface area of the gold surface inevitably decreases. Fig. 2 shows X-ray diffractions of the catalysts and the mesoporous titania supports. The single broad peak in the low angle region, being consistent with a wormhole-like structure, almost disappears after the loading of gold. This is probably due to the side reactions that occur during the deposition–precipitation. No low angle peak is found for MT400 and Au/MT400, which agrees with the results of previous studies [48,49]. No Bragg diffraction above 2h > 10 degree is observed for MT200 and MT300 (Fig. 2d and e), suggesting that the crystalline structure is not developed in MT200 and MT300. On the other hand, the diffraction pattern in Fig. 2f is assigned to that of anatase (2h = 25, 38, 47, 54, 62, 70, 74 and 82 degrees), which does not change after the loading of gold (Fig. 2c). In contrast, a new broad peak appears at 38 degree in Au/ MT200 and Au/MT300 ((a) and (b), respectively, in Fig. 2). The sizes of the crystallites as estimated by the Scherrer equation are 4.4 and 5.9 nm, respectively, which are larger than the pore sizes of MT200 and MT300, 2.0 nm, suggesting that most of the gold atoms form particles are deposited on the outer surfaces of MT200 and MT300. We measured the XRD of Au/anatase, as shown in Fig. 3. Unfortunately, the diffraction at 38 degree due to the gold lattice is masked by the diffraction of the anatase support. The particle size of the gold in Au/MT400 and Au/anatase was determined by averaging the sizes of particle imaged in the TEM photographs. These particle sizes are listed in Table 1. The loading is lower in Au/anatase (0.70%) than in Au/mesoporous titanias (2.1 ± 0.2%). The particle size of Au/anatase is larger than that of Au/MT400, though the crystalline phase is the same
Table 1 BET specific surface area, pore size, pore volume, Au loading and Au particle size of catalysts and support materials.
Au/MT200 MT200 Au/MT300 MT300 Au/MT400 MT400 Au/anatase Anatase a b c d e
ABET (m2/g)
2RPa (nm)
VPb (cm3/g)
Auc (wt.%)
Au particle size (nm)
775 1096 653 815 254 276 200 200
2.0 2.0 2.0 2.2 3.5 3.5
0.64 1.02 0.59 0.77 0.33 0.35
2.1
4d
1.9
6d
2.3
3e
0.70
5e
Peak top of BJH pore size distributions from adsorption branch. Calculated by BJH method of adsorption branch. Determined by ICP. Estimated using Scherrer equation. Average size in TEM photographs.
53
anatase. This may be explained by the confinement of Au3+ solution in the mesopores. The particle size of the gold in Au/MT400 (2.8 nm) is smaller than the mesopores (3.5 nm) and this is confirmed by the TEM photograph shown in Fig. 4, where the agglomeration of nanocrystals and the inter-crystallite mesopores can be clearly observed and the slightly stronger shadows with a size of 2–3 nm are attributed to the gold particles. It is, therefore, more probable that the interaction between solvated Au3+ and the surface of anatase is weak, though the Au3+ in the mesopore cannot be agglomerated to be larger than the pore size during the DP process. The size of solvated Au3+ is clearly smaller than that of Au in the form of gold particles, which can results in the formation of particles smaller than the size of the mesopores. The number of papers on Au/anatase in the literature is less than those on Au/rutile. This is probably because of the difficulty in the preparation of gold particles on a several tenths of nm scale on anatase while achieving high loading. On the other hand, the successful synthesis of gold catalysts has been reported for mesoporous anatase with a broad pore size distribution [57], nanotubes of anatase [58] and mesoporous anatase/rutile containing sulfur [59]. Although the mechanism has not been adequately discussed in these studies, partly because the pore size distributions are broader than in this study, the pore confinement of Au3+ solution is possibly the key to the preparation of highly-dispersed Au/anatase catalysts. Fig. 5 depicts the Au 4f X-ray photoelectron spectra of Au/mesoporous titania catalysts. The peak tops assigned to the 4f7/2 and 4f5/2 lines for Au/MT200 are found at 83.6 and 87.4 eV, respectively. These positions are not changed for Au/MT300 (83.7 and 87.3 eV, respectively), while they shift to higher binding energies for Au/MT400 (84.0 and 87.6 eV, respectively). These peaks are attributed to the emission from the Au0 species. The peaks observed for Au/MT300 and Au/MT400 are relatively well defined, suggesting that Au particles are more positively charged in Au/ MT400 than in Au/MT300. On the other hand, those of Au/MT200 broaden slightly, suggesting the coexistence of different Au species. The results of catalytic reactions at 413 K are listed in Table 2. The conversion of crotonaldehyde (CRAL) on Au/mesoporous titania increases with the oxidation temperature of the support, and decreases with the partial pressure of CRAL. The selectivity of crotyl alcohol (CROL) is greater than that of butylaldehyde (BUAL) for all runs. The rate of CROL formation is evaluated by the product of the conversion of CRAL and the selectivity for CROL. This product is also compared in Table 2; its largest value over all H2/CRAL ratios is found for Au/MT400 among the three Au/mesoporous titanias that we investigated. The selectivity for BUOL increases with H2/CRAL ratio for all mesoporous catalysts, as is expected. The amorphous support MT200 provides the catalyst that is least active, as well as being the least selective for CROL. The mechanisms related with the positively-charged Au atoms have been proposed for the high activities elucidated for CO oxidation on Au catalysts [4,10,60–62]. In Fig. 5 and in Table 2, a correlation is found between the shift of the Au 4f signals in Au/MT400 and its high activity and selectivity for CROL formation. However, further study is necessary to elucidate the mechanism of hydrogenation and the role of positively-charged gold. In the gaseous hydrogenations of CRAL on supported gold catalysts at ambient pressure, the selectivity of CROL has been reported to be 38.9% (at 6.5% conversion on Au/SiO2 [21]), 24.4% (at 29.6% conversion on Au/Degussa P-25 [26]), 29.7–54.1% (at 7.8–41.4% conversion on Au/ZnO [36]), 20.0–65.4% (at 26.8–44.5% conversion on Au/ZnO [36]), 32.1–50.9% (at 9.2–30.4% conversion on Au/ZrO2 [36]), 34.3–50.8% (at 7.5–21.3% conversion on Au/ZrO2 [36]), 0% (at 11.2–23.2% conversion on Au/SiO2 [36]), 62–70% (at 6.3– 22.8 10 6 mol g 1-Au s 1 on Au/Degussa P25 [38]), 70% (at 7%
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Fig. 2. X-ray diffractions of the catalysts, (a) Au/MT200, (b) Au/MT300 and (c) Au/MT400, and the supports, (d) MT200, (e) MT300 and (f) MT400.
Fig. 3. X-ray diffraction of Au/anatase. The data for anatase support is also shown.
Fig. 5. X-ray photo electron spectra of (a) Au/MT200, (b) Au/MT300 and (c) Au/ MT400.
Fig. 4. Transmission micropscopic image of Au/MT400. The scale bar is 10 nm.
conversion on Au/CeO2 [39]), 0% (at 2% conversion on Au/Nb2O5 [39]) and 55% (at 5% conversion on Au/HAS–CeO2 [40]). Considering the data in the literature shown above, the selectivity of CROL observed on Au/MT400 (82%) in this study is outstanding. In a pressurized reactor, higher selectivity has been reported than that
observed at the ambient pressure, e.g., 68–78% (at 98–99% conversion on Au/Fe2O3 at P(H2) = 2 MPa [44]), which is still lower than the result achieved at ambient pressure in this study. In contrast, the selectivity was only 39% (at 6% conversion) on Au/SiO2 even at Ptotal = 2 MPa [37]. In an aqueous solution, the 90% selectivity for CROL was achieved (at 50% conversion on Au/CeO2 at P(H2) = 1.0 MPa [43]). The conversion and selectivity for CROL on Au/anatase are also shown in Table 2. Although the amount of catalyst is three times larger than in the reaction on Au/mesoporous titania, the conversion is as low as Au/MT200. The selectivity for CROL is lower than on any of the other catalysts. On the other hand, that for BUOL is
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H. Yoshitake, N. Saito / Microporous and Mesoporous Materials 168 (2013) 51–56 Table 2 Results of hydrogenation of crotonaldehyde at 413 K. Catalyst
H2/CRAL
Conversion (%)
Au/MT200 Au/MT300 Au/MT400 Au/anatase Au/MT200 Au/MT300 Au/MT400 Au/anatase Au/MT200 Au/MT300 Au/MT400 Au/anatase
126
4.9 6.5 25 5.7 1.6 6.5 7.4 1.7 1.1 2.4 6.9 1.6
50
24
(Conversion) (selectivity) for CROLa
Selectivity (%) BUAL
CROL
BUOL
19 16 13 8 16 19 8 13 15 24 7 21
54 71 68 62 70 69 78 65 68 68 82 49
31 13 19 30 14 12 14 22 17 8 11 30
1 1.8 6.5 1.4 1 4.0 5.3 1.0 1 2.1 7.6 1.0
Catalyst: 0.1 g except Au/anatase (0.3 g). Flow rate of hydrogen: 4 L/h. CRAL: crotonaldehyde, BUAL: butylaldehyde, CROL: crotyl alcohol, BUOL: 1-butanol. a Normalized by the value calculated for Au/MT200.
Table 3 Results of hydrogenation of crotonaldehyde on the catalyst calcined at 573 K. Catalyst
Au/MT200 Au/MT300 Au/MT400 Au/anatase
H2/CRAL
126
Conversion (%)
0 1.9 13 0
Selectivity (%) BUAL
CROL
BUOL
n.d. 32 11 n.d.
n. d. 59 72 n. d.
n.d. 9 17 n.d.
Catalyst: 0.1 g except Au/anatase (0.3 g). Flow rate of hydrogen: 4 L/h. Reaction temperature: 413 K. CRAL: crotonaldehyde, BUAL: butylaldehyde, CROL: crotyl alcohol, BUOL: 1-butanol.
larger than or nearly equal to the results of Au/mesoporous titania when it is compared at the same H2/CRAL condition. After calcination of the catalyst at 573 K, the catalytic activity was lost in Au/MT200 and in Au/anatase as shown in Table 3. However, 29% and 52% of the activity remains in Au/MT300 and Au/ MT400, respectively. Considering the size of the Au particles of Au/MT400, then confinement in the mesopores is effective for resisting deactivation by agglomeration of the metal particles. The product distributions are almost identical with those on these catalysts oxidised at 473 K. Considering that deactivation most likely occurs by the growth of gold particles, this result suggests that the hydrogenation of CRAL is insensitive to the particle sizes of the gold. Since this tendency is completely different from that in CO oxidation [4,7,10,60,62–65], further investigation is necessary.
4. Conclusions We prepared gold supported on mesoporous titanias synthesized with a hexadecylamine template and, after the removal of the template, heated at 473, 573 and 673 K, which were denoted as MT200, MT300 and MT400, respectively. MT200 and MT300 showed an amorphous framework and narrow pore size distributions centred at 2.0 nm, while MT400 took the anatase structure and the median of the pore size distribution was 3.5 nm. After gold was supported by the deposition–precipitation method, the specific surface areas and pore volumes of MT200 and MT300 decreased considerably, while they did not change significantly for MT400. The difference in loading of gold was within 10% (i.e., 2.1 ± 0.2 wt.%) between MT200, MT300 and MT400 and the change in pore size was negligible for all these mesoporous silicas. These changes in porous structure suggest that the nitrogen adsorption is blocked after gold deposition on MT200 and MT300, while gold is highly dispersed on MT400. This structural model is consistent
with the estimation of the sizes of the gold particles; 4.4, 5.9 and 2.8 nm for Au/MT200, Au/MT300 and Au/MT400, respectively. In the hydrogenation of crotonaldehyde at ambient pressure, the selectivity for crotyl alcohol reached at 82% on Au/MT400, while a value of 49% was obtained under the same reaction conditions with gold supported on commercially-available anatase, Au/ anatase. In addition, the rate of hydrogenation was much higher on Au/MT400 than on Au/anatase. The selectivity for crotyl aldehyde on Au/MT200 and Au/MT300 was 68% under the same reaction conditions. The high selectivity is correlated with the position of the Au 4f peaks in the XPS spectra. Acknowledgement The authors thank Dr. M. Kondo (Instrumental Analysis Centre, YNU) for his assistance with the XPS measurements. References [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]
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Selective hydrogenation of crotonaldehyde on Au supported on mesoporous titania Hideaki Yoshitake ⇑, Naoki Saito Division of Materials Science and Chemical Engineering, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
a r t i c l e
i n f o
Article history: Received 17 July 2012 Received in revised form 28 August 2012 Accepted 27 September 2012 Available online 8 October 2012 Keywords: Mesoporous titania Gold catalyst Hydrogenation of crotonaldehyde Mesoporous anatase
a b s t r a c t We report here on how gold supported on mesoporous anatase work as a catalyst for the highly-selective hydrogenation of crotonaldehyde into crotyl alcohol. The gold was supported by deposition–precipitation method onto mesoporous titania prepared using hexadecylamine as the structure directing agent. This mesoporous titania provides a narrow pore size distribution and an amorphous framework, which can be converted to another pore size distribution and anatase framework by heating at 673 K. Gold was more highly dispersed on mesoporous anatase prepared by heating at 673 K than on anatase sample heated at 473–573 K or on commercially-available anatase. The structure of these catalysts was analysed by XRD, N2-adsorption/desorption, TEM and XPS. In the hydrogenation of crotonaldehyde at ambient pressure, the selectivity for crotyl alcohol reached 82% on gold supported on mesoporous anatase, while 49% selectivity was obtained for gold supported on commercially-available anatase under the same reaction conditions. In addition, the rate of hydrogenation is also four times greater on Au/mesoporous anatase than on Au/anatase. The selectivity for crotyl aldehyde on gold on mesoporous amorphous titania was 68% under the same reaction conditions. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction Use as a catalyst support is a potentially important application for mesoporous materials, while the exploitation of new metal catalyses has always been an active motor for the chemistry of catalysis. Since Haruta and Hutchings showed that nanoparticles of gold can be excellent catalysts for oxidation, extensive studies have been undertaken to contribute to the preparations, structural analyses and catalytic reactions of gold, which had long been considered to be an inert metal as solid catalyst [1–20]. Unlike the oxidation reactions investigated extensively in earlier studies, the selectivities are usually the primary concern in catalytic hydrogenations [21,22] rather than the activity [23,24]. In fact, the various gold catalysts have been investigated for their chemoselectivities in the hydrogenation of double bonds, including 1,3-butadiene [25,26], acetylene [27] and chloronitrobenzene [28]. The exchange reaction between H2 and D2 during propylene hydrogenation has also been studied with respect to the mechanism of unsaturated hydrocarbon conversion [29]. The conversion of the carbonyl group into alcohol is an important process in the perfumery industry and in the treatment of waste water [30]. The selective hydrogenation of the carbonyl bonds in unsaturated carbonyl compounds has long been regarded as a valuable reaction, though the hydrogenation of –C@O is less thermodynamically favourable than that of –C@C– in simple unsaturated ⇑ Corresponding author. Fax: +81 45 339 4378. E-mail address: [email protected] (H. Yoshitake). 1387-1811/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.micromeso.2012.09.031
aldehydes without other heteroatoms or stereochemical distortions. The hydrogenation of the simplest unsaturated aldehyde, acrolein, has been investigated over Au-supported on several oxides [21,31]. However, this type of substrate can be polymerized in the presence of acid sites and the existence of multiple paths to convert the unsaturated alcohol into aldehyde has been proposed from several sources of experimental evidence [32–35]. Furthermore, the selective hydrogenation of the carbonyl group in a,bunsaturated aldehydes is usually more important between inner – C@C– and –CH@O groups than between the terminal CH2@C– and –CH@O groups. For these reasons, crotonaldehyde or cinnamaldehyde have often been chosen as the molecule for investigation of the selectivities in hydrogenation of a,b-unsaturated aldehydes. In the studies into Au catalysts various selectivities have been reported for saturated aldehyde, unsaturated alcohol and saturated alcohol, depending on the support and the preparation conditions [26,36–49]. This fact suggests that hydrogenation of a,b-unsaturated aldehydes can be a good test reaction for unveiling the properties of the catalyst support materials. Mesoporous titania prepared with a primary amine as the structure directing agent has an extremely large surface area, uniform pore size and a large pore volume [50,51]. The crystalline structure of this solid can be changed simply by heating in air from amorphous into crystalline anatase [52] via an anatase–rutile mixed phase (confirmed by vibrational spectroscopy [53], which does not imply that nanocrystallites of anatase and rutile are mixed but that the local structure of the Ti atoms is either anatase or rutile). A transition of the mesoporous structure occurs, which is
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accompanied by the crystallisation into anatase [52,53]. The oxidation number of catalytically active elements on mesoporous titania can be considerably different from those on the conventional nonporous titania supports [54,55]. It is, therefore, natural that oxidation and acid catalyses behave completely differently on catalysts prepared using a mesoporous titania support [54,55]. Since catalyses by gold are often correlated with the oxidation state of the gold [4,37,38,40,41,44,45,56], the use of mesoporous titania as a support will be an interesting subject for gold catalysts. To the best of our knowledge, Au/mesoporous titania with a narrow pore size distribution has been studied only once, to investigate the water– gas shift reaction on Au/mesoporous anatase, where gold is loaded on mesoporous anatase prepared using a C16(EO)6-polyoxyethylene(6) tridecylether template [57]. In this study, Au/mesoporous titania catalysts are prepared by a deposition–precipitation (DP) method while varying the crystalline and porous structures in order to elucidate the properties of this structurally-unique material as a support material for metal catalysts. 2. Experimental 2.1. Materials 1-Hexadecylamine, tetraisopropyl orthotitanate, p-toluenesulfonic acid, chloroauric acid tetrahydrate, sodium carbonate and 1-butanol were obtained from Wako Pure Chemical Industries Ltd. Crotonaldehyde, crotyl alcohol and butylaldehyde were purchased from Tokyo Chemical Industry Co., Ltd. Anatase (specific surface area was 200 m2/g) was also commercially available from Aldrich.
2.4. Instrumental analyses The loading of Au was determined by ICP-AES using an ICPE9000 Multitype Inductively Coupled Plasma Emission Spectrometer (Shimadzu). The structural periodicity of the catalysts was confirmed by XRD using a RINT 2200 diffractometer (equipped with a Cu target, Rigaku Co.) operated at 40 kV and 40 mA. Nitrogen adsorption and desorption were carried out for the porous solids after drying at 453 K for 2 h using a Quantachrome NOVA 4200e sorptometer. X-ray photoelectron spectra of the catalysts were measured using a Quantera SXM spectrometer (Ulvac-Phi, Inc.). 2.5. Catalytic reactions Hydrogenation of crotonaldehyde was carried out in a flow reactor under ambient pressure. Typically, the catalyst (0.1 g) mounted in a quartz tube was oxidised at 473 K under flowing oxygen (Ar/O2 = 1/1, 4 L/h). Crotonaldehyde was vaporised in a temperature-controlled bath and mixed with the hydrogen flow (4 L/ h). The products were analysed using an on-line gas chromatograph equipped with packed columns and a TCD detector. 3. Results and discussion The nitrogen adsorption/desorption isotherms of the catalysts and the support mesoporous titanias are plotted in two frames in Fig.1. The pore size distributions are also shown in the other two frames in the same figure. The isotherms are assigned to the type IV curves. After the loading of gold for the MT200 and MT300, the uptake of nitrogen decreases significantly and the peak in the
2.2. Preparation of mesoporous titania 8.0 g of tetraisopropyl orthotitanate was added to a mixture of 3.4 g of 1-hexadecylamine and 40 g of water under vigorous stirring at 273 K. After the addition of 1.6 cm3 of 1 M hydrochloric acid, the stirring continued for 30 min at the same temperature and the mixture was then stood overnight at 298 K. It was then transferred to a Teflon container in an oven at 333 K. After 4 d, the solution was filtered off and the white solid was washed with ethanol. After being dried at 373 K for 24 h, the solid was transferred into a Pyrex tube in vacuum at 453 K and heated in vacuum for 2 h. The tube was then sealed. The powder in the tube was heated at 453 K for 10 d. (A part of the sealed tube must be kept below 373 K, in order to avoid pressurization due to the production of water.) The resulting solid was then treated twice in a 0.1 M ethanol solution of p-toluenesulfonic acid at room temperature, followed by washing with ethanol for 2 h, collected by filtration and drying at 373 K for 1 h. Mesoporous titania thus prepared was heated at 473 K for 2 h. This powder sample is denoted as MT200. MT300 and MT400 were prepared by heating at 573 and 673 K, respectively, instead of 473 K. 2.3. Loading of gold MT200, MT300 and MT400 were added to aqueous solutions of chloroauric acid with the same volumes as their mesopores, and then the pH was raised to 10 using sodium carbonate. The amount of gold ions in the solution was adjusted to be 3 wt.% of the support. After stirring at 313 K for 3 h, the powder was collected by filtration, washed with water and dried at 333 K for 1 h. Finally, the catalyst was heated at 473 K for 2 h to obtain Au/MT200, Au/ MT300 and Au/MT400. Au/anatase was prepared by the same procedure using anatase provided by Aldrich.
Fig. 1. Nitrogen adsorption/desorption isotherms and BJH pore size distributions of the catalysts, (a) Au/MT200, (b) Au/MT300 and (c) Au/MT400, and the supports, (d) MT200, (e) MT300 and (f) MT400.
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pore size distributions is slightly broadened. In contrast, no significant change is found in the isotherms between Au/MT400 and MT400. The BET specific surface areas, pore sizes and pore volumes calculated from the adsorption data are summarized in Table 1. The loading of Au is also listed in the same table. Although the decreases in surface area and pore volume are remarkable, the loading of Au does not significantly change the pore size in Au/MT200 and Au/MT300. Considering that the porous structure of mesoporous titania is likely to be composed of wormhole-like channels, this simultaneous change found in ABET and VP can be reasonably explained by pore blocking due to the growth of gold particles. On the other hand, none of these three parameters changes significantly in Au/MT400, though the loading of gold is almost the same as in Au/MT200 and Au/MT300. This result suggests that the pore blocking by the gold particles is negligible in Au/MT400. This can be attributed to the larger pore size (3.5 nm) in MT400 than those (ca. 2.0 nm) in MT200 and MT300. Once a pore is blocked, the metal particles inside the pore become inaccessible from the outside and the effective surface area of the gold surface inevitably decreases. Fig. 2 shows X-ray diffractions of the catalysts and the mesoporous titania supports. The single broad peak in the low angle region, being consistent with a wormhole-like structure, almost disappears after the loading of gold. This is probably due to the side reactions that occur during the deposition–precipitation. No low angle peak is found for MT400 and Au/MT400, which agrees with the results of previous studies [48,49]. No Bragg diffraction above 2h > 10 degree is observed for MT200 and MT300 (Fig. 2d and e), suggesting that the crystalline structure is not developed in MT200 and MT300. On the other hand, the diffraction pattern in Fig. 2f is assigned to that of anatase (2h = 25, 38, 47, 54, 62, 70, 74 and 82 degrees), which does not change after the loading of gold (Fig. 2c). In contrast, a new broad peak appears at 38 degree in Au/ MT200 and Au/MT300 ((a) and (b), respectively, in Fig. 2). The sizes of the crystallites as estimated by the Scherrer equation are 4.4 and 5.9 nm, respectively, which are larger than the pore sizes of MT200 and MT300, 2.0 nm, suggesting that most of the gold atoms form particles are deposited on the outer surfaces of MT200 and MT300. We measured the XRD of Au/anatase, as shown in Fig. 3. Unfortunately, the diffraction at 38 degree due to the gold lattice is masked by the diffraction of the anatase support. The particle size of the gold in Au/MT400 and Au/anatase was determined by averaging the sizes of particle imaged in the TEM photographs. These particle sizes are listed in Table 1. The loading is lower in Au/anatase (0.70%) than in Au/mesoporous titanias (2.1 ± 0.2%). The particle size of Au/anatase is larger than that of Au/MT400, though the crystalline phase is the same
Table 1 BET specific surface area, pore size, pore volume, Au loading and Au particle size of catalysts and support materials.
Au/MT200 MT200 Au/MT300 MT300 Au/MT400 MT400 Au/anatase Anatase a b c d e
ABET (m2/g)
2RPa (nm)
VPb (cm3/g)
Auc (wt.%)
Au particle size (nm)
775 1096 653 815 254 276 200 200
2.0 2.0 2.0 2.2 3.5 3.5
0.64 1.02 0.59 0.77 0.33 0.35
2.1
4d
1.9
6d
2.3
3e
0.70
5e
Peak top of BJH pore size distributions from adsorption branch. Calculated by BJH method of adsorption branch. Determined by ICP. Estimated using Scherrer equation. Average size in TEM photographs.
53
anatase. This may be explained by the confinement of Au3+ solution in the mesopores. The particle size of the gold in Au/MT400 (2.8 nm) is smaller than the mesopores (3.5 nm) and this is confirmed by the TEM photograph shown in Fig. 4, where the agglomeration of nanocrystals and the inter-crystallite mesopores can be clearly observed and the slightly stronger shadows with a size of 2–3 nm are attributed to the gold particles. It is, therefore, more probable that the interaction between solvated Au3+ and the surface of anatase is weak, though the Au3+ in the mesopore cannot be agglomerated to be larger than the pore size during the DP process. The size of solvated Au3+ is clearly smaller than that of Au in the form of gold particles, which can results in the formation of particles smaller than the size of the mesopores. The number of papers on Au/anatase in the literature is less than those on Au/rutile. This is probably because of the difficulty in the preparation of gold particles on a several tenths of nm scale on anatase while achieving high loading. On the other hand, the successful synthesis of gold catalysts has been reported for mesoporous anatase with a broad pore size distribution [57], nanotubes of anatase [58] and mesoporous anatase/rutile containing sulfur [59]. Although the mechanism has not been adequately discussed in these studies, partly because the pore size distributions are broader than in this study, the pore confinement of Au3+ solution is possibly the key to the preparation of highly-dispersed Au/anatase catalysts. Fig. 5 depicts the Au 4f X-ray photoelectron spectra of Au/mesoporous titania catalysts. The peak tops assigned to the 4f7/2 and 4f5/2 lines for Au/MT200 are found at 83.6 and 87.4 eV, respectively. These positions are not changed for Au/MT300 (83.7 and 87.3 eV, respectively), while they shift to higher binding energies for Au/MT400 (84.0 and 87.6 eV, respectively). These peaks are attributed to the emission from the Au0 species. The peaks observed for Au/MT300 and Au/MT400 are relatively well defined, suggesting that Au particles are more positively charged in Au/ MT400 than in Au/MT300. On the other hand, those of Au/MT200 broaden slightly, suggesting the coexistence of different Au species. The results of catalytic reactions at 413 K are listed in Table 2. The conversion of crotonaldehyde (CRAL) on Au/mesoporous titania increases with the oxidation temperature of the support, and decreases with the partial pressure of CRAL. The selectivity of crotyl alcohol (CROL) is greater than that of butylaldehyde (BUAL) for all runs. The rate of CROL formation is evaluated by the product of the conversion of CRAL and the selectivity for CROL. This product is also compared in Table 2; its largest value over all H2/CRAL ratios is found for Au/MT400 among the three Au/mesoporous titanias that we investigated. The selectivity for BUOL increases with H2/CRAL ratio for all mesoporous catalysts, as is expected. The amorphous support MT200 provides the catalyst that is least active, as well as being the least selective for CROL. The mechanisms related with the positively-charged Au atoms have been proposed for the high activities elucidated for CO oxidation on Au catalysts [4,10,60–62]. In Fig. 5 and in Table 2, a correlation is found between the shift of the Au 4f signals in Au/MT400 and its high activity and selectivity for CROL formation. However, further study is necessary to elucidate the mechanism of hydrogenation and the role of positively-charged gold. In the gaseous hydrogenations of CRAL on supported gold catalysts at ambient pressure, the selectivity of CROL has been reported to be 38.9% (at 6.5% conversion on Au/SiO2 [21]), 24.4% (at 29.6% conversion on Au/Degussa P-25 [26]), 29.7–54.1% (at 7.8–41.4% conversion on Au/ZnO [36]), 20.0–65.4% (at 26.8–44.5% conversion on Au/ZnO [36]), 32.1–50.9% (at 9.2–30.4% conversion on Au/ZrO2 [36]), 34.3–50.8% (at 7.5–21.3% conversion on Au/ZrO2 [36]), 0% (at 11.2–23.2% conversion on Au/SiO2 [36]), 62–70% (at 6.3– 22.8 10 6 mol g 1-Au s 1 on Au/Degussa P25 [38]), 70% (at 7%
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Fig. 2. X-ray diffractions of the catalysts, (a) Au/MT200, (b) Au/MT300 and (c) Au/MT400, and the supports, (d) MT200, (e) MT300 and (f) MT400.
Fig. 3. X-ray diffraction of Au/anatase. The data for anatase support is also shown.
Fig. 5. X-ray photo electron spectra of (a) Au/MT200, (b) Au/MT300 and (c) Au/ MT400.
Fig. 4. Transmission micropscopic image of Au/MT400. The scale bar is 10 nm.
conversion on Au/CeO2 [39]), 0% (at 2% conversion on Au/Nb2O5 [39]) and 55% (at 5% conversion on Au/HAS–CeO2 [40]). Considering the data in the literature shown above, the selectivity of CROL observed on Au/MT400 (82%) in this study is outstanding. In a pressurized reactor, higher selectivity has been reported than that
observed at the ambient pressure, e.g., 68–78% (at 98–99% conversion on Au/Fe2O3 at P(H2) = 2 MPa [44]), which is still lower than the result achieved at ambient pressure in this study. In contrast, the selectivity was only 39% (at 6% conversion) on Au/SiO2 even at Ptotal = 2 MPa [37]. In an aqueous solution, the 90% selectivity for CROL was achieved (at 50% conversion on Au/CeO2 at P(H2) = 1.0 MPa [43]). The conversion and selectivity for CROL on Au/anatase are also shown in Table 2. Although the amount of catalyst is three times larger than in the reaction on Au/mesoporous titania, the conversion is as low as Au/MT200. The selectivity for CROL is lower than on any of the other catalysts. On the other hand, that for BUOL is
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H. Yoshitake, N. Saito / Microporous and Mesoporous Materials 168 (2013) 51–56 Table 2 Results of hydrogenation of crotonaldehyde at 413 K. Catalyst
H2/CRAL
Conversion (%)
Au/MT200 Au/MT300 Au/MT400 Au/anatase Au/MT200 Au/MT300 Au/MT400 Au/anatase Au/MT200 Au/MT300 Au/MT400 Au/anatase
126
4.9 6.5 25 5.7 1.6 6.5 7.4 1.7 1.1 2.4 6.9 1.6
50
24
(Conversion) (selectivity) for CROLa
Selectivity (%) BUAL
CROL
BUOL
19 16 13 8 16 19 8 13 15 24 7 21
54 71 68 62 70 69 78 65 68 68 82 49
31 13 19 30 14 12 14 22 17 8 11 30
1 1.8 6.5 1.4 1 4.0 5.3 1.0 1 2.1 7.6 1.0
Catalyst: 0.1 g except Au/anatase (0.3 g). Flow rate of hydrogen: 4 L/h. CRAL: crotonaldehyde, BUAL: butylaldehyde, CROL: crotyl alcohol, BUOL: 1-butanol. a Normalized by the value calculated for Au/MT200.
Table 3 Results of hydrogenation of crotonaldehyde on the catalyst calcined at 573 K. Catalyst
Au/MT200 Au/MT300 Au/MT400 Au/anatase
H2/CRAL
126
Conversion (%)
0 1.9 13 0
Selectivity (%) BUAL
CROL
BUOL
n.d. 32 11 n.d.
n. d. 59 72 n. d.
n.d. 9 17 n.d.
Catalyst: 0.1 g except Au/anatase (0.3 g). Flow rate of hydrogen: 4 L/h. Reaction temperature: 413 K. CRAL: crotonaldehyde, BUAL: butylaldehyde, CROL: crotyl alcohol, BUOL: 1-butanol.
larger than or nearly equal to the results of Au/mesoporous titania when it is compared at the same H2/CRAL condition. After calcination of the catalyst at 573 K, the catalytic activity was lost in Au/MT200 and in Au/anatase as shown in Table 3. However, 29% and 52% of the activity remains in Au/MT300 and Au/ MT400, respectively. Considering the size of the Au particles of Au/MT400, then confinement in the mesopores is effective for resisting deactivation by agglomeration of the metal particles. The product distributions are almost identical with those on these catalysts oxidised at 473 K. Considering that deactivation most likely occurs by the growth of gold particles, this result suggests that the hydrogenation of CRAL is insensitive to the particle sizes of the gold. Since this tendency is completely different from that in CO oxidation [4,7,10,60,62–65], further investigation is necessary.
4. Conclusions We prepared gold supported on mesoporous titanias synthesized with a hexadecylamine template and, after the removal of the template, heated at 473, 573 and 673 K, which were denoted as MT200, MT300 and MT400, respectively. MT200 and MT300 showed an amorphous framework and narrow pore size distributions centred at 2.0 nm, while MT400 took the anatase structure and the median of the pore size distribution was 3.5 nm. After gold was supported by the deposition–precipitation method, the specific surface areas and pore volumes of MT200 and MT300 decreased considerably, while they did not change significantly for MT400. The difference in loading of gold was within 10% (i.e., 2.1 ± 0.2 wt.%) between MT200, MT300 and MT400 and the change in pore size was negligible for all these mesoporous silicas. These changes in porous structure suggest that the nitrogen adsorption is blocked after gold deposition on MT200 and MT300, while gold is highly dispersed on MT400. This structural model is consistent
with the estimation of the sizes of the gold particles; 4.4, 5.9 and 2.8 nm for Au/MT200, Au/MT300 and Au/MT400, respectively. In the hydrogenation of crotonaldehyde at ambient pressure, the selectivity for crotyl alcohol reached at 82% on Au/MT400, while a value of 49% was obtained under the same reaction conditions with gold supported on commercially-available anatase, Au/ anatase. In addition, the rate of hydrogenation was much higher on Au/MT400 than on Au/anatase. The selectivity for crotyl aldehyde on Au/MT200 and Au/MT300 was 68% under the same reaction conditions. The high selectivity is correlated with the position of the Au 4f peaks in the XPS spectra. Acknowledgement The authors thank Dr. M. Kondo (Instrumental Analysis Centre, YNU) for his assistance with the XPS measurements. References [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]
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