Chemoselective hydrogenation of citral by Pt and Pt-Sn catalysts supported on TiO2 nanoparticles and nanowires

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Chemoselective hydrogenation of citral by Pt and Pt-Sn catalysts supported on TiO2 nanoparticles and nanowires Anne-Riikka Rautio a,∗ , Päivi Mäki-Arvela b , Atte Aho b , Kari Eränen b , Krisztian Kordas a a

Microelectronics and Materials Physics Laboratories, Department of Electrical Engineering, University of Oulu, P.O. Box 4500, FI-90014, Finland Laboratory of Industrial Chemistry and Reaction Engineering, Department of Chemical Engineering, Process Chemistry Centre Åbo Akademi University, Åbo, FI-20500, Finland b

a r t i c l e

i n f o

Article history: Received 30 September 2013 Received in revised form 29 November 2013 Accepted 1 December 2013 Available online xxx Keywords: Hydrogenation Citral Titanium dioxide Nanowire Nanoparticle

a b s t r a c t Chemoselective hydrogenation of citral is studied over platinum catalyst supported on titanium dioxide nanoparticles and nanowires. The support is found to have considerable effect on both selectivity and activity. Because of the presence of weak basic sites on the nanowire support, the platinum catalyst shows significantly higher activity and better selectivity towards citronellal than that obtained by the catalyst based on TiO2 nanoparticles having mainly medium strong basic sites. Furthermore, addition of tin promoter to the platinum catalyst decreases the initial rate of reaction and increases the selectivity towards unsaturated alcohols such as nerol and geraniol. The catalytic activity studies are complemented by chemical composition, surface adsorption and structural analyses of the applied catalyst materials assessed by the means of X-ray photoelectron spectroscopy, energy-dispersive X-ray analysis, temperature programmed reduction, H2 and CO2 desorption, CO chemisorption, electron microscopy and X-ray diffraction, respectively. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Chemoselective hydrogenation of citral has been intensively studied due to the versatile and industrially relevant applications of the products in e.g. fragrances, pharmaceutical and fine chemical intermediates. The reaction is also interesting from the academic point of view, since it is thermodynamically easier to hydrogenate an ethylenic double bond than a carbonyl bond [1]. There are three possible places where hydrogenation can occur on a structure of citral molecule (Scheme 1). Selective hydrogenation of C O bond on citral leads to the double unsaturated alcohols, nerol and geraniol (route 1) while hydrogenation of conjugated C C bond results citronellal (CAL) (route 2) which can continue hydrogenation to citronellol (COL). For perfumes, these two routes are desirable due to the pleasant odor of products; meanwhile routes 3 and 4 in Scheme 1 are less favored because of the unpleasant smell of dimethyloctanal (DNAL) and dimethyloctanol (DMOL). Factors that affect to which route citral hydrogenation proceeds are the nature of metallic phase and support [2,3], particle size of catalyst [4] and polarity of solvent [5] i.e. very similar to those in general reactions. The selection of solvent has great effect on the product distribution because for instance alcohols can react with

∗ Corresponding author. Tel.: +358 503505176. E-mail address: [email protected].fi (A.-R. Rautio).

citral and form side products such as acetals [6]. Also the choice of metal is crucial. For example, Pd is active in hydrogenating the ethylenic double bond in linear unsaturated aldehydes [7], whereas Ni and Ru are more selective towards the hydrogenation of carbonyl bond [6,8]. Furthermore, the application of co-catalyst metals (such as tin and iron) can also promote the selectivity towards nerol and geraniol (route 1) (Table 1) [9–14]. Although, subject of citral hydrogenation is closely examined, only a very few reports are available that apply TiO2 nanoparticles as a support material [15,16]. Typically, hydrogenation of citral produces unsaturated alcohols with high selectivity, when platinum decorated TiO2 is used as a catalyst [15,16]; however DMOL has also been reported as the main product [12] (Table 1). The reduction temperature of the catalyst was reported to have effect on the product distribution [15,16] and activity [17] due to the formation of strong metal-support interaction [18]. To the best of our knowledge, TiO2 nanowires have not been applied as catalyst support in liquid phase hydrogenation, although their usability has been demonstrated in gas phase hydrogenation reactions [19] and photocatalytic dehydrogenation reactions [20–24]. Since 1-dimensional nanostructures (nanowires and nanotubes) have a number of different practical advantages over their 0-dimensional counterparts (e.g. easier filtration, preparation of films and porous composites) [21,23] in this article, our aim is to study potential differences between nanoparticles and nanowires of TiO2 in their catalytic behavior when using as a

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Scheme 1. Reaction paths of citral hydrogenation [1].

support for metal catalyst nanoparticles. Here, we report on a comparative study on the catalytic activity and selectivity of Pt and Pt-Sn catalyst particles supported both on TiO2 nanoparticles and nanowires in the hydrogenation of citral.

2. Materials and methods 2.1. Catalyst preparation The TiO2 nanowires were synthesized from titanate nanowires by annealing in air at 600 ◦ C for 12 h [23,25]. The monometallic catalyst was prepared in following way. First, a stable mixture of TiO2 (nanoparticle, anatase <25 nm, Aldrich) and water–acetone (1:1) was made by ultrasonic mixing for 3 h. Pt(acac)2 (platinum acetylacetonate, Aldrich 99.99%) dissolved in water–acetone mixture was added to the mixture and then stirred overnight. After evaporating the solvent under N2 flow, the solids were dried at

100 ◦ C overnight and then annealed at 300 ◦ C for 1 h to decompose the Pt-complex followed by reduction in 15% H2 /Ar flow at 500 ◦ C for 5 min. The bimetallic catalyst was prepared by wet impregnation of the reduced monometallic sample in 1 M HCl with SnCl2 (tin chloride, Sigma). After 3 h stirring at room temperature the solvent was evaporated and the solids were dried overnight at 120 ◦ C in order to remove any volatiles. The as-obtained powder samples were annealed at 500 ◦ C for 3 h in air, and then reduced at 500 ◦ C under 15% H2 /Ar flow for 3 h. 2.2. Citral hydrogenation In a typical reaction, a pre-reduced catalyst (m = 200 mg) was reduced in situ in a Parr reactor (300 mL) at 250 ◦ C in pure H2 (AGA, 99.999%) for 1 h. Followed by introduction of the deoxygenated citral (Aldrich, >95%, m = 200 mg) mixed in toluene (J.T. Baker, 99.5%) or 2-pentanol (Aldrich, 98%) (100 mL) having an initial citral

Table 1 TiO2 based catalysts and reaction parameters reported in the contemporary literature for citral hydrogenation. Pressure (bar)

X (%)

Products and corresponding selectivities (%)

TOF (s−1 )

Ref.

90

100

92

N + G 45

–

[15]

2-Propanol

90

100

95

N + G 68

–

[15]

2-Propanol

70

70

>95

–

[16]

2-Propanol

70

70

>95

–

[16]

2-Propanol Hexane 2-Propanol 2-Propanol

70 100 70 70

12 20 1 12

>95 >75 99 >95

COL 30 N + G 25 Other 20 N + G 58 COL 30 DMOL 65 N + G 88a N + G 79 N + G 90

Catalyst

Solvent

Pt/TiO2 (375 ◦ C) Pt/TiO2 (575 ◦ C) Pt/TiO2 (300 ◦ C)

2-Propanol

Pt/TiO2 (500 ◦ C) Pt/TiO2 Pt/TiO2 Pt-Sn/TiO2 Pt-Sn/TiO2 a

Temp. (◦ C)

0.93 0.95 – 0.009

[12] [17] [11] [12]

At conversion <20%.

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concentration of 0.013 M. The reaction was carried out at 70 ◦ C under a total pressure of 10 bar in H2 for 180 min. We assume the experiments are in the kinetic regime, because vigorous stirring (1200 rpm) was applied and the size of the catalyst particles was well below 2 ␮m. Small volumes of the liquid from the reaction vessel were collected, filtered and diluted with the solvent before analyzing with gas chromatography. 2.3. Analysis of the liquid phase components The samples were analyzed with a gas chromatograph equipped with a capillary column (HP-Wax, 30 m, 250 ␮m, 0.25 ␮m) and a flame ionization detector. Helium as a carrier gas was used with the following temperature program: 100 ◦ C (10 min) – 5 ◦ C/min – 160 ◦ C (10 min) – 13 ◦ C/min – 190 ◦ C (10 min). The split ratio was 100:1. The components were calibrated with citronellal (Aldrich, >95%), citronellol (Aldrich, 95%), nerol (Aldrich, 97%), geraniol (Aldrich, 98%), (−)-isopulegol (Aldrich, 99%), and 3,7-dimethyl-1octanol (Aldrich, >98%). Furthermore, the unknown peaks were identified with GC–MS (Agilent Technologies 6890N). 2.4. Catalyst characterization Metal particle size and dispersion were examined with energy filtered transmission electron microscopy (EFTEM, Leo 912 Omega, acceleration voltage of 120 kV). Average particle size was determined by measuring at least 240 particles from TEM micrographs for each sample. Elemental concentrations of the samples were determined with scanning electron microscopy and energydispersive X-ray spectroscopy (FESEM-EDX, Zeiss Ultra plus, Inca) from five different sample locations. Phase of catalyst crystals were determined with X-ray diffraction (XRD, Bruker D8 Discover, Cu K␣ – radiation) with a scanning speed of 0.06◦ /min. Temperature programmed reduction (TPR) and desorption of hydrogen (TPD) of the catalysts were performed on pre-reduced catalysts (see Section 2.1) using a Micromeritics Autochem 2910 apparatus. Hydrogen was analyzed by mass-spectrometry (Balzers Instrument, Omnistar). For TPR the following temperature program was set: 10 ◦ C/min – 650 ◦ C (60 min) using 5 vol.% H2 /Ar (AGA). TPD measurements were carried out as follows: first, the sample (100 mg) was pre-reduced at 250 ◦ C for 1 h (heating rate of 10 ◦ C/min, H2 99.999% AGA) followed by cooling and flushing with argon for 20 min to remove any H2 physisorbed on the surface. The desorption measurements were performed by heating each sample to 600 ◦ C (heating rate of 10 ◦ C/min) and keeping the temperature for a 30 min. Metal dispersion and average particle size was measured with CO pulse chemisorption with Micromeritics Autochem 2910 apparatus. The catalyst was pre-reduced in situ under H2 with the following temperature program: 5 ◦ C/min – 350 ◦ C (180 min). Thereafter the catalyst was flushed with He for 90 min at 350 ◦ C and cooled down to room temperature. CO chemisorption was performed at 25 ◦ C using 10 vol.% CO in helium (AGA). The stoichiometry of Pt on CO was assumed to be 1:1 [26]. X-ray photoelectron spectroscopy (XPS) analysis was performed with a Perkin-Elmer PHI 5400 spectrometer (monochromatized Al K␣ X-ray source, 14 kV, 300 W). The specific surface area and pore volume was determined with methods based on the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) theories, respectively, by using nitrogen physisorption (−196 ◦ C, Micromeritics ASAP 2020 Surface Analyzer). The alkalinity of the support materials was determined two ways: acid titration and temperature programmed desorption of CO2 (TPD-CO2 , Micromeritcis Autochem 2910). Support material (∼70 mg) was sonicated in deionized water (50 mL) for 90 min and titrated with 1 mM HCl. The titration was repeated five times and

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the color change was monitored by using phenolphthalein indicator. In the TPD-CO2 measurements the support material (100 mg) was dried for 60 min at 400 ◦ C under He atmosphere (heating rate 10 ◦ C/min) followed by cooling down and introduction of CO2 at 25 ◦ C for 30 min. The physisorbed CO2 was flushed with helium for 30 min and then desorbed in He atmosphere by heating the sample to 800 ◦ C with a rate of 10 ◦ C/min. 3. Results and discussion 3.1. Catalyst characterization results 3.1.1. Metal particle size, phase and dispersion The monometallic platinum samples show small <2 nm particle size on both nanoparticle and nanowire supports, whereas the average metal particle size of bimetallic nanowire catalysts is slightly higher, around 2 nm (Fig. 1, Table 2). The slight increase in the average metal particle size is most probably caused by the fusion of platinum and tin nanoparticles. On the surface of the nanowires, we can also observe small voids with a diameter of ∼10 nm or so, similar to the samples synthesized earlier for photocatalytic studies [23–25]. XRD patterns (Fig. 2) of the catalyst powders show reflections from Pt, except the sample prepared with TiO2 nanoparticles, whose Pt particle size was found the smallest (by TEM) explaining the weak scattering [27,28]. The diffraction pattern of Pty Snx alloy (marked as ⊗ in Fig. 2) is partly overlapping with that of the support, however reflections emerging at ∼31.8◦ , ∼35.6◦ and ∼45.5◦ clearly indicate the formation of alloys in the samples [29]. Surprisingly, reflections of alloys are not more intensive for the sample where Sn:Pt is 1:1 than for the one with the ratio of 1:2 and the reflection of platinum is still existing indicating that a considerable fraction of the metal is alloyed. 3.1.2. XPS and BET results and basicity of supports The surface composition assessed by XPS gives slightly different results than EDX analysis (Table 3). XPS results clearly show an increased Sn to Pt ratio with increasing Sn loading. According to pore size measurements the nanowire powders have pores of ∼7.4 nm diameter, which is consistent with TEM observations. The specific surface area for the nanowires determined by BET (52 m2 /g) is on the same range than that of the nanoparticles (45–55 m2 /g) [30]. The measured pore volumes are ∼0.1 cm3 /g. The oxidation states of the metals in the catalysts were investigated using pre-reduced catalysts. The resolved Pt 4f7/2 peak shows binding energies at ∼71.9 eV and at ∼73.5 eV, that could be caused by the metallic Pt (71.2 eV) [9] and Pt(OH)2 (73.1 eV) [31], respectively. Although, those are the most likely states of Pt, because of the shifted energies, possibility of other states like Pt(II) (72.5 eV) [32] should not be ignored. The Sn 3d5/2 peak close to 486.5 eV indicates the presence of SnO, while the [32,33] in the catalyst, while the Ti 2p3/2 peak at 458.8 eV shows that titanium in the support is in the Ti4+ oxidation state [34]. When comparing the surface oxidation states of the metals to those achieved with XRD showing the bulk composition as well as the comparing these results to obtained by TPR (in Section 3.1.3), we can conclude that Pt is mainly in metallic state during the hydrogenation, since the catalyst was pre-reduced in situ at 250 ◦ C prior to the reaction. Furthermore, XRD revealed that part of Pt was also in alloy form. The alkaline nature of the supporting materials was examined by two independent methods; titration and CO2 chemisorption. The acid–base titration of support materials showed a major difference in supports; the pH of TiO2 NW-water dispersion was slightly alkaline with Brønsted base site concentration of 17 ␮mol/g, whereas the NP based colloid was practically neutral. On the other

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Fig. 1. TEM micrographs and size histograms of catalyst particle decorating the surface for (a) Pt/TiO2 (NP), (b) Pt/TiO2 (NW), (c) 1 wt.% Sn-2 wt.% Pt/TiO2 NW and (d) 2 wt.% Sn-2 wt.% Pt/TiO2 NW.

Table 2 Comparison of the average metal particle sizes measured by TEM and CO chemisorption. Catalyst

Average metal particle size by TEM (nm)

2 wt.% Pt/NP 2 wt.% Pt/NW 1 wt.% Sn-2 wt.% Pt/NW 2 wt.% Sn-2 wt.% Pt/NW

1.5 1.8 2.0 2.1

± ± ± ±

0.3 0.4 0.6 0.5

Average apparent metal particle size by CO chemisorption (nm)

Dispersion by CO chemisorption (%)

n.m. 5.9 26 57

n.m. 19.3 4.3 2

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Table 3 Mass fractions of different elements in the catalysts supported on TiO2 nanoparticles and nanowires measured by XPS and EDX (the values for this latter are in parenthesis). Catalyst

O (wt.%)

Ti (wt.%)

Pt (wt.%)

Sn (wt.%)

Na (wt.%)

Sn/Pt

Pt/Ti

Sn/Ti

2 wt.% Pt/NP 2 wt.% Pt/NW 1 wt.% Sn-2 wt.% Pt/NW 2 wt.% Sn-2 wt.% Pt/NW

(47.7) 45.3 (38.7) 45.8 (44.4) 44.8 (40.3)

(50.6) 50.0 (56.1) 48.4 (50.3) 52.0 (54.0)

(1.7) 4.8 (1.9) 5.3 (1.0) 1.9 (1.4)

0 0 0.6 (0.2) 1.3 (0.3)

(0) (3.3) (3.2) (3.1)

0 0 0.11 0.67

n.m. 0.10 0.11 0.04

0 0 0.012 0.025

hand, according to the CO2 -TPD, the total density of (Lewis and Brønsted) basic sites of NP and NW were similar, 3.7 ␮mol/m2 (i.e. 186 ␮mol/g) and 2.8 ␮mol/m2 (i.e. 147 ␮mol/g) respectively, however with difference in strength. CO2 on TiO2 NWs is loosely bonded as shown by the very low desorption temperatures (<400 ◦ C); whereas on the NP support, mainly medium strong basic sites (desorption between 400 ◦ C and 600 ◦ C) are observed as seen in Fig. 3 [35]. The results are similar to those reported earlier to TiO2 and titania-nanotubes [36,37]. 3.1.3. Results from temperature programmed reduction and desorption of hydrogen The catalysts have been pre-reduced during catalyst preparation and prior to their characterization in hydrogen TPR (Fig. 4a). According to the literature, monometallic Pt/TiO2 forms at 160 ◦ C and 100 ◦ C, when using hexachloroplatinic acid [38] and PtCl4 as a metal precursor, respectively [39]. In our current work, the metal precursor is Pt(acac)2 and only a minor peak is present close to 100–160 ◦ C, since the bulk of Pt is already in the reduced state, as confirmed by XRD. The TPR results of Pt/TiO2 showed by Kim et al. [39] revealed that the reduction of Pt surface occurs at 100 ◦ C, whereas other peaks responsible for H2 consumption at 180 ◦ C and 300 ◦ C originate from the reducible support. It was also stated that with increasing calcination temperature, the peak at 100 ◦ C decreases. This is the case also in the current work, since no peak

Fig. 2. XRD patterns of different catalyst samples.  Anatase, 䊉 ␤-phase of TiO2 , * Pt metal and ⊗ Pty Snx .

Fig. 3. Temperature programmed CO2 desorption of TiO2 nanowire (red) and nanoparticle (black). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

was obtained at 200 ◦ C. The peak for the highest hydrogen consumption appears at ∼390 ◦ C for monometallic 2 wt.% Pt/TiO2 NW, whereas the corresponding peak for 1 wt.% Sn-2 wt.% Pt/TiO2 is obtained at ∼405 ◦ C indicating that the presence of Sn slightly increases the reduction temperature. This is rather plausible since Sn has considerably lower redox potential than that of the relatively easy-to-reduce precious metals’ such as Pt. The bimetallic catalysts have also a H2 consumption peak at ∼190 ◦ C indicating also increase in reduction temperature of Pt [40,41]. Temperature programmed reduction of Pt supported on alumina showed that Pt alone on alumina was reduced at 395 ◦ C, whereas Pt-Sn/Al2 O3 was reduced already at 384 ◦ C and the second peak at 440 ◦ C [42]. Thus these results are not totally in line with those published by Håkonsen et al. [42], it should be kept in mind that direct comparison is not necessary correct due to the different support. Furthermore, much lower reduction temperatures have been achieved for Pt-Sn/TiO2 , in which case the two main hydrogen consumption peaks were achieved below 200 ◦ C [38]. It should be noted that in that case the support had low specific surface area and the metal precursors applied were hexachloroplatinic acid and tin chloride dehydrate. Furthermore, the temperature ramping rate was only 5 ◦ C/min, whereas it was 10 ◦ C/min in the current case and in Håkonsen et al. [42], which could additionally increase the reduction temperature. The amount of hydrogen taken up by the bimetallic 1 wt.% Sn–2 wt.% Pt-/TiO2 NW is only 36% of the one for Pt/TiO2 NW, which is easily understood by taking into account the poor hydrogen adsorption capacity of Sn. Desorption of hydrogen from monometallic 2 wt.% TiO2 NW and bimetallic 1 wt.% Sn–2 wt.% TiO2 NW was also studied by using H2 TPD (Fig. 4b). As reported by several groups earlier for Pt-TiO2 system, desorption of hydrogen takes place in three steps. The first, a low temperature (LT) peak is observed at 115 ◦ C caused by desorption from Pt crystals, secondly a medium temperature (MT) at 285 ◦ C caused by the desorption from interfaces of Pt and support and finally a high temperature (HT) at 350 ◦ C which is caused by the hydrogen chemisorbed on the supporting TiO2 [43–45]. TPD patterns obtained for our samples were similar to those described earlier in the literature, desorption starting at around 75 ◦ C for bimetallic and at 115 ◦ C for monometallic catalysts followed by the other two desorption peaks at 200 ◦ C and at 330–370 ◦ C. For bimetallic catalyst an additional desorption peak in very high temperatures (555 ◦ C) was found, which is also visible in the TPD profile of Pt/TiO2 in Kim et al. [43]. The main peak, caused by the chemisorbed hydrogen on the support, seems to be at lower temperature (330 ◦ C) for the bimetallic sample than that for the monometallic one (370 ◦ C). Decrease in HT and LT peaks for bimetallic catalyst can be explained by the addition of the rather electropositive Sn to the sample. Although, the results of Panagiotopoulou and Kondarides [45] indicate that the addition of promoters such as alkali metals do not affect the position of LT, in our case, the formation of alloy naturally influences the desorption of hydrogen from the platinum surface. The hydrogen desorption was ∼2 times higher for the bimetallic catalyst than for the monometallic one. Higher hydrogen adsorption capacity of platinum is typical for bimetallic samples [46,47]. No desorption below 70 ◦ C (i.e. at the temperatures where hydrogenation of citral was studied) was detected indicating that all hydrogen is strongly bonded to catalyst.

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Fig. 4. a) Temperature programmed reduction and b) hydrogen desorption of 2 wt.% Pt/TiO2 NW (black line) and 1 wt.% Sn-2 wt.% Pt/TiO2 (red line) catalysts. The hydrogen consumption is shown as a negative and desorption as positive peak in the pictures. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Table 4 Catalyst metals supported on TiO2 nanoparticles (NPs) and nanowires (NWs), solvents and the corresponding initial reaction rates, turnover frequencies, conversions and selectivity values. Entry

Catalyst

Solvent

Initial rate (min−1 gcat −1 )

TOF (s−1 )

1 2 3 4

2 wt.% Pt/NP 2 wt.% Pt/NW 2 wt.% Pt/NW 1 wt.% Sn-2 wt.% Pt/NW 2 wt.% Sn-2 wt.% Pt/NW

Toluene Toluene 2-Pentanol 2-Pentanol

0.093 0.505 0.365 0.059

n.m.a n.m.a 0.43 0.59

95 100c 100d 95

2-Pentanol

0.005

0.08

13

5

X (%)at 180 min

SCAL (%) at X = 50%b

SN + G (%) at X = 50%b

SCOL (%) at X = 50%b

SDNAL (%) at X = 50%

SDMOL (%) at X = 50%b

29 (15) 88 (70) 82 (71) 16 (9)

18 (27) <1 (<1) 4 (<1) 66 (63)

40 (30) 11 (11) 12 (22) 18 (27)

0 (3) 0 (11) 0 0

7 (14) 0 (9) 2 (6) 0

19e

80e

0e

0e

0e

Note, in entry 1, 10% of products are unidentified. a Not measured in toluene as a solvent. b Values in parenthesis refer to 90% conversion. c After 30 min. d After 60 min. e 10% conversion.

3.2. Catalytic results 3.2.1. Effect of support Comparison of the performance of monometallic Pt/TiO2 nanoparticles and nanowires as supports in citral hydrogenation was performed in toluene as a solvent (Table 4, entries 1 and 2). The monometallic Pt/TiO2 NW catalyst was very active since complete conversion was achieved already in 30 min whereas NP supported one had conversion of 95% after 3 h. Also the product distributions were very different when using Pt/TiO2 NP or Pt/TiO2 NW (Tables 4 and 5, Figs. 5 and 6). In the case of the TiO2 nanoparticle supported catalyst, practically each product (CAL, N + G, COL and DMOL) starts to form simultaneously from the beginning of the reaction without favoring any route, i.e. no selectivity is observed. On the other hand, the catalyst based on the TiO2 nanowire support very rapidly produced CAL first, which then converted to DNAL and COL. Once DNAL and COL were present in the reaction mixture, those react further to DMOL. The differences in Pt/TiO2 NP and Pt/TiO2 NW activities and selectivities must be caused by the support material itself because the catalyst metals have very similar particle size distributions. Also the specific surface areas of the two different support materials were close to each other (NP 45–55 m2 /g and NW ∼ 52 m2 /g)

Table 5 Comparison of the ratios of initial rates for different products for Pt/TiO2 NP and Pt/TiO2 NW in toluene. Ratio of initial rates

Value

r0 ,CAL,NW /r0,CAL ,NP r0 ,COL,NW /r0,COL ,NP r0 ,DMOL,NW /r0,DMOL , NP

11.5 38 19.6

[30] thus this parameter is not expected to influence the catalytic behavior either. The main reason for the very different product distributions is most likely the difference in the strength of alkaline sites on the nanowires and on the nanoparticles. The weak basic sites seem to direct the reaction towards the hydrogenation of ethylenic double bond, which is known as the least energy demanding route, whereas the medium strong basic sites have only little effect on the selectivity. The effect of alkaline nature has been reported earlier with the increased selectivity towards the reduction of unsaturated ethylenic bonds in e.g. citral on Pt/MgAl [48], cinnamaldehyde on alkaline titanate nanotubes [49], 3-methyl-2-butenal over potassium promoted Ru/SiO2 [50], ethyl benzoylacetate on Pd/C [51] and benzene over alkaline promoted Ru/ZrO2 [52]. The alkalinity of the nanowires is due to the synthesis method used to prepare the TiO2 nanowire support, in which sodium hydrogen titanate nanowires (with a Na+ content of 4–5 wt.%) were annealed in air to transform those to self-similar anatase nanowires (measured Na+ content of ∼3.3 wt.%). 3.2.2. Effect of solvent Two different solvents, toluene and 2-pentanol were compared in citral hydrogenation on Pt/TiO2 NW catalyst. The main difference between the solvents is in their polarity and consequently in their dielectric constant (13.71 for 2-pentanol and 2.38 for toluene) [53]. Since more polar solvents favor to the formation of alcohols, in 2-pentanol we expect to observe better selectivity towards N + G products than towards CAL. Although, the conversion rate in toluene is somewhat faster than in 2-pentanol, surprisingly the product distributions were found rather similar for the two solvents applied (Table 4, entries 2 and 3). The comparison of the ratios of

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Fig. 5. a) Citral conversion with Pt/TiO2 NP and Pt/TiO2 NW and b) selectivities to CAL and COL in citral hydrogenation in toluene at 70 ◦ C.

Fig. 6. Product distribution in citral hydrogenation over a) Pt/TiO2 NP and b) Pt/TiO2 NW in toluene.

Fig. 7. a) Citral conversion in toluene and in 2-pentanol over Pt/TiO2 NW, b) selectivities to CAL and COL in toluene and in 2-pentanol.

the initial rates for formation of different products in toluene and in 2-pentanol showed that the initial rates were for each compound always higher in toluene than in 2-pentanol (Table 6). Furthermore, this ratio increased for secondary and tertiary products (COL, DMOL). The product distribution and selectivities in toluene and in 2-pentanol are relatively similar (Fig. 7). The reason for the lowered rate in 2-pentanol is probably the adsorption of 2-pentoxy species on the metal surface. On the other hand, when comparing literature data of the initial rates of the reaction carried out in different solvents, both increasing [54] and decreasing [55] rates for increasing solvent polarities.

unsaturated alcohols. The initial reaction rates on the Sn promoted catalyst are ∼6 times lower than on the monometallic ones (Table 4, entries 4 and 5). The TOF values on the bimetallic catalysts are also reduced somewhat due to the less efficient adsorption of

3.2.3. Comparison between the catalytic performances of monoand bimetallic Pt/TiO2 NW catalysts Addition of tin to the monometallic catalyst was made to decrease the reaction rate and increase the selectivity towards

Table 6 Comparison of the ratios of initial rates for different products for Pt/TiO2 NW in toluene and in 2-pentanol. Ratio of initial rates

Value

r0 ,COL,tol. /r0,COL ,2-pent. r0 ,DMOL,tol. /r0,DMOL ,2-pent.

2.4 9.0

Fig. 8. Citral conversion after 3 h as a function of the apparent average metal particle size measured by CO chemisorption.

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Fig. 9. Product distribution as a function of time in citral hydrogenation over a) 2 wt.% Pt/TiO2 NW and b) 1 wt.% Sn-2 wt.% Pt/TiO2 NW in 2-pentanol.

Fig. 10. a) Conversion of citral in 2-pentanol over 2 wt.% Pt/TiO2 NW, 1 wt.% Sn-2 wt.% Pt/TiO2 NW and 2 wt.% Sn-2 wt.% Pt/TiO2 NW b) selectivities of 2 wt.% Pt/TiO2 NW, 1 wt.% Sn-2 wt.% Pt/TiO2 NW.

hydrogen on Sn than on Pt. The lowered active surface area (or the increase of apparent particle size as measured with CO chemisorption) after the addition of Sn to Pt is reasonable, considering that SnO is reduced at relatively low temperatures in H2 gas to Sn [56], which in turn wets Pt surface rather well and may form various intermetallics [57]. For the 1 wt.% Sn-2 wt.% Pt/TiO2 catalyst the TOF value was larger than for monometallic one which indicates that although some of the active sites of Pt have been blocked by the tin there are still, considering the low dispersion, several active sites left. TOF values obtained for monometallic catalyst are not as high as values achieved by other groups [12,17] but the bimetallic catalyst seems to out shine the results in literature [12]. The conversion of citral after 3 h decreased with increasing apparent metal particle size determined by CO chemisorption (Fig. 8) showing also that chemisorption results correlate rather well with the reaction data, whereas the particle size determined by TEM showed very small differences, although the catalytic activity of the different catalysts varied a lot. It should be emphasized however, that TEM is a practical way to analyze the actual physical size of the catalyst nanoparticles, whereas CO desorption is providing us with information related to the chemical surface area and indirectly with an apparent particle size. The product distributions were very different for mono- and bimetallic Pt/TiO2 NW catalysts (Table 4, Fig. 9) and when comparing the ratios for the initial rates for formation of CAL to NG (Table 7). This indicates that monometallic catalyst preferentially produces CAL and promotes hydrogenation of ethylenic double bond, whereas the bimetallic ones are much more selective towards

Table 7 Comparison of the ratio of the initial formation rate of CAL to N + G and COL to DNAL when using mono-and bimetallic TiO2 NW catalyst. Ratio of initial rates

Value for Pt/TiO2 NW

Value for Sn-Pt/TiO2 NW

r0 ,CAL/ r0,N+G r0 ,COL /r0,DNAL

21 3.5

0.6 Large

hydrogenation of a carbonyl bond. This difference in selectivities between mono and bimetallic catalysts has been reported earlier also with other supporting materials [13,14,58]. The measured selectivity values towards unsaturated alcohols are in good agreement with the results of other groups measured earlier on Pt/TiO2 and Pt-Sn/TiO2 [11,12]. Although, the reaction conditions, apart from pressure, are the same in our experiments as used in Refs. [11,12], the selectivities we obtained towards N + G are somewhat lower, which is caused by the alkalinity of our NW support that directs the reaction towards CAL and COL (as discussed earlier in Section 3.2.1). 3.2.4. Effect of tin loading The effect of tin loading on the reaction was studied in 2pentanol and was found crucial for the catalytic activity (Fig. 10). When comparing the performances of 1 wt.% Sn-2 wt.% Pt/TiO2 NW and 2 wt.% Sn-2 wt.% Pt/TiO2 NW, the activity of the latter catalyst was only ∼10% of the one with the lower Sn content (Table 4, entries 4 and 5). This result correlates well with the low metal dispersion determined by CO chemisorption (Fig. 8). The selectivity towards unsaturated alcohols is higher with higher tin loading in the catalyst. This is not surprising because similar trends were observed earlier also on other support materials such as MgAl2 O4 , Al2 O3 [14], activated carbon felt and powder [13]. Also, hydrogenation of carbonyl bond in crotonaldehyde is reported to be more selective when Sn/Pt ratio is higher [9,32]. 4. Conclusions Chemoselective hydrogenation of citral was achieved by using ultra-small Pt and Pt-Sn catalyst nanoparticles supported on titanium oxide nanoparticles and nanowires. Because of the alkaline nature of TiO2 nanowires with weak basic sites, the catalyst based on the nanowires showed higher activity and better selectivity towards citronellal than the catalyst based on the neutral TiO2 nanoparticle support having mainly medium strong basic sites. The

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effect of solvent polarity on the reaction rate was found insignificant since less than 40% increase in the rate was observed when 2-pentanol was applied instead of toluene. To the best of our knowledge, no reports are published yet where toluene or 2-pentanol had been used as a solvent in citral hydrogenation, which is surprising. According to the results obtained in this work, both solvents perform rather well similar to that of the typically applied solvent 2-propanol. Addition of tin to the platinum catalyst increased the selectivity towards unsaturated alcohols and slowed the reaction sufficiently to avoid the formation of tertiary products even at nearly complete conversion of citral. With the increase of the tin loading in the catalyst, the selectivity towards nerol and geraniol is improved; however the catalytic activity is reduced due to the decreased number of active Pt sites on the catalyst. The results demonstrate that TiO2 nanowires developed recently are also suitable for liquid phase hydrogenation and – as a consequence of their basic character – may even outperform the conventional TiO2 nanoparticle based catalyst supports. Acknowledgments Authors would like to thank Prof. Konya (Univ. Szeged) for providing us with titanate nanowires and Jouko Virkkala (Mass and heat transfer process laboratory, Univ. Oulu) for performing BET measurement. A.-R. Rautio is grateful for the post-graduate position and for the personal grants received from the Graduate School in Electronics, Telecommunications and Automation and Tauno Tönning foundation. The work was supported by Tekes (projects Urakamu2 and Imphona), by the Academy of Finland (project OPTIFU) and by the European Union FP7 programme (Napep). References [1] P. Mäki-Arvela, J. Hajek, T. Salmi, D.Yu. Murzin, Appl. Catal. A Gen. 292 (2005) 1–49. [2] U.K. Singh, M.A. Vannice, J. Catal. 199 (2001) 73–84. [3] P. Mäki-Arvela, N. Kumar, D. Kubicka, A. Nasir, T. Heikkilä, V.-P. Lehto, R. Sjöholm, T. Salmi, D.Yu. Murzin, J. Mol. Catal. A Chem. 240 (2005) 72–81. [4] S. Galvagno, C. Milone, A. Donato, G. Neri, R. Pietropaolo, Catal. Lett. 18 (1993) 349–355. [5] S. Mukherjee, M.A. Vannice, J. Catal. 243 (2006) 108–130. [6] P. Mäki-Arvela, L.-P. Tiainen, M. Lindblad, K. Demirkan, N. Kumar, R. Sjöholm, T. Ollonqvist, J. Väyrynen, T. Salmi, D.Yu. Murzin, Appl. Catal. A Gen. 241 (2003) 271–288. [7] J. Aumo, J. Lilja, P. Mäki-Arvela, T. Salmi, M. Sundell, H. Vainio, D.Yu. Murzin, Catal. Lett. 84 (2002) 219–224. [8] L.-P. Tiainen, P. Mäki-Arvela, A. Kalantar Neyestanaki, T. Salmi, D.Yu. Murzin, React. Kinet. Catal. Lett. 78 (2) (2003) 251–257. [9] J. Ruiz-Martínez, A. Sepúlveda-Escribano, J.A. Anderson, F. Rodríguez-Reinoso, Catal. Today 123 (2007) 235–244. [10] N.M. Bertero, A.F. Trasarti, B. Moraweck, A. Borgna, A.J. Marchi, Appl. Catal. A Gen. 358 (2009) 32–41. [11] L.N. Protasova, E.V. Rebrov, H.E. Skelton, A.E.H. Wheatley, J.C. Schouten, Appl. Catal. A Gen. 399 (2011) 12–21. [12] Z.R. Ismagilov, E.V. Yakutova, L.N. Protasova, I.Z. Ismagilov, M.A. Kerzhentsev, E.V. Rebrov, J.C. Schouten, Catal. Today 147S (2009) S81–S86. [13] M.J. Vilella, S.R. de Miguel, C. Salinas-Martinez deLecea, A. Linares-Solano, O.A. Scelza, Appl. Catal. A Gen. 281 (2005) 247–258. [14] P.D. Zgolicz, V.I. Rodriguez, I.M.J. Vilella, S.R. de Miguel, O.A. Scelza, Appl. Catal. A Gen. 392 (2011) 208–217. [15] S.A. Ananthan, V. Narayanan, Nanoscience, Inter. Conf. Eng. Tech. (ICONSET) (2011) 23–29. [16] T. Ekou, L. Ekou, A. Vicente, G. Lafaye, S. Pronier, C. Especel, P. Marecot, J. Mol. Catal. A Chem. 337 (2011) 82–88. [17] U.K. Singh, M.A. Vannice, J. Mol. Catal. A Chem. 163 (2000) 233–250. [18] A.K. Datye, D.S. Kalakkad, M.H. Yao, D.J. Smith, J. Catal. 155 (1995) 148–153.

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