Journal of Chemistry
Volume 2018 (2018), Article ID 5608243, 7 pages
https://doi.org/10.1155/2018/5608243

Research Article

An Effective Pt-Cu/SiO₂ Catalyst for the Selective Hydrogenation of Cinnamaldehyde

Dong Wang¹ and Yujun Zhu²

¹School of Chemical and Environmental Engineering, Hanshan Normal University, Chaozhou 521041, China
²Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, School of Chemistry and Materials, Heilongjiang University, Harbin 150080, China

Correspondence should be addressed to Yujun Zhu; yujunzhu@hlju.edu.cn

Received 14 December 2017; Accepted 18 March 2018; Published 24 April 2018

Academic Editor: Ioannis D. Kostas

Copyright © 2018 Dong Wang and Yujun Zhu. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The bimetal catalyst Pt-Cu/SiO₂ was prepared by the impregnation method. Its catalytic performance was investigated by the selective hydrogenation of cinnamaldehyde. Pt-Cu/SiO₂ exhibited much higher selectivity (64.1%) to cinnamyl alcohol than Pt/SiO₂ (3.7%), while they showed similar conversion of cinnamaldehyde. This enhancement was attributed to the increase in the amount of the Pt⁰ species on the Pt-Cu/SiO₂ surface, which is derived from the interaction between Pt and Cu revealed by XRD and XPS.

1. Introduction

Selective catalytic hydrogenation of α,β-unsaturated aldehydes is an important step in industrial production of fine chemicals, such as pharmaceuticals, flavours, and fragrances [1–5]. Cinnamaldehyde (CAL) is representative α,β-unsaturated aldehydes, which can be hydrogenated first via C=C or C=O double bonds to hydrocinnamaldehyde (HALD) or cinnamyl alcohol (COL), followed by total hydrogenation of these products to hydrocinnamyl alcohol (HALC) (Scheme 1) [1]. In general, the reduction of the C=C group is favored thermodynamically [1, 2]. Thus, it is still a challenging task to enhance the selectivity for the hydrogenation of C=O group to form COL [2, 6, 7].

Scheme 1: Hydrogenation products of cinnamaldehyde.

Nowadays, Pt-based catalyst facilitates the production of COL via the hydrogenation of the C=O bond [8–12]. The unique advantage of such catalysts is that activity and selectivity can be tuned by controlling the size, shape, and surface species of Pt particles [1, 13–15]. In fact, many methods have been developed to enhance the performance of Pt-based catalyst, such as investigating novel support and promoter [15–19]. In our previous works, it was found that graphene-supported Pt catalysts showed high selectivity to COL [13]; and the activity and selectivity of Pt/carbon nanosheets (CNS) catalysts increased with the rise of the graphitization degree of CNS [20]. Moreover, the selectivity of Pt catalyst does not depend on particle size, at least within the 1.5–2.5 nm particle size range over Pt/SiO₂ [21]. We also found that Mo₂N could enhance the efficiency of Pt on ternary catalyst Pt-Mo₂N/SBA-15 due to the synergistic effect of Pt and Mo₂N [22]. Furthermore, some other studies illustrated that the introduction of a second metal promoter was also a valid way to improve the catalytic properties of Pt nanoparticles. It has been reported that metal promoters (Ga, Fe, Ni, and Ru) play an important role in CAL hydrogenation [7, 23, 24].

In light of the above, a thought stimulates us to investigate whether the nonnoble metal of Cu could promote the catalytic performance of Pt for the hydrogenation of cinnamaldehyde. Thus, the Pt-Cu bimetal catalyst on SiO₂ aerogel (Pt-Cu/SiO₂) was prepared by a simple impregnation method, and its performance on selective hydrogenation of cinnamaldehyde was investigated. To our delight, the Pt-Cu/SiO₂ catalyst showed much higher selectivity than Pt/SiO₂. It was found that the addition of Cu enhanced the dispersion of Pt on SiO₂. Moreover, there were more Pt⁰ species on bimetal catalyst, which benefitted the enhancement of the selectivity to COL.

2. Material and Method

2.1. Catalyst Preparation

SiO₂ aerogel was purchased from Aladdin. Bimetal catalyst was synthesized by impregnation and hydrogen reduction method. Firstly, 0.4 g Cu(NO₃)₂3H₂O was dissolved in 20 mL water at room temperature, and then 2 mol/L ammonia water was dropped to the mixture until there was no precipitation. Secondly, 1.00 g SiO₂ aerogel was added to the above solution. After stirring for 4 h, the blue powder was collected by filtration and washing and dried at 80°C. Thirdly, 0.300 g blue powder was impregnated with 1.5 mL H₂PtCl₆ (10 mmol/L) ethanol solution. The resulting suspension was stirred at room temperature to remove the solvent and dried at 80°C for 12 h. Finally, the sample was placed inside a furnace with a heating rate of 5°C min⁻¹ to 300°C under N₂ atmosphere which was maintained at 300°C for 1 h under pure H₂ atmosphere. Thus, the obtained sample was denoted as Pt-Cu/SiO₂. As references, Pt supported on SiO₂ aerogel sample was also prepared under similar condition, which was denoted as Pt/SiO₂.

2.2. Catalyst Characterization

The X-ray diffraction (XRD) patterns were performed by using a Bruker D8 Advance X-ray diffractometer with Cu Kα ( = 1.5418 Å) radiation (40 kV, 40 mA). The scan range of the wide-angle XRD was 10–80°. Based on XRD results, the average particles sizes can be calculated from Pt(111) by Scherrer’s equation. The average particles sizes were recalculated to dispersion values assuming spherical shapes and using the formula described by Scholten et al.:

is dispersion (/), is the atomic weight (195.1 g/mol for Pt), is the platinum surface site density (12.5 Pt atoms/nm²), is particle size (nm), is the metal density (21.45 g/cm³ for Pt), and is the Avogadro constant. Thus, = 1.13/d (nm) for Pt [25, 26].

Properties of surface area were derived from N₂ adsorption-desorption isotherms at 77 K with a Micromeritics Tristar II surface area and a porosimetry analyzer. Before the measurement, samples were degassed under vacuum at 150°C for 5 h. The surface area was determined from desorption isotherms by the BET method.

The morphology and structure of the samples were analyzed by using a JEM-2100 transmission electron microscopy (TEM) with an acceleration voltage of 200 kV.

X-ray photoelectron spectroscopy (XPS) was conducted on a Kratos-AXIS ULTRA DLD with an AlKα radiation source. Binding energies (BE) refer to the C (1s) binding energy of carbon taken to be 284.7 eV.

The content of Pt and Cu on the Pt-Cu/SiO₂ catalyst was determined by inductively coupled plasma-optical emission spectrometry (ICP-OES), which was performed by using a PerkinElmer Optima 7000DV analyzer. Each sample was dissolved in a diluted HF and chloroazotic acid solution before measurement.

2.3. Selective Hydrogenation of Cinnamaldehyde

The catalyst (50 mg) was mixed with 1.00 g CAL and 30 mL isopropanol. The mixture was subsequently transferred to a 100 mL autoclave. After flushing three times with N₂ at 5 bar and three times with H₂ at 5 bar, the reaction was allowed to proceed at 80°C under 10 bar of H₂ for 2 h.

The products were analyzed by GC (Agilent 7820 A) with flame ionization detection (FID) and a HP-5 capillary column (30 m × 0.32 mm × 0.25 mm). Quantitative analysis of the products was performed via calibration curves using the benzyl alcohol as an external standard. The products were further identified by GC-MS (Agilent 6890/5973N).

3. Results and Discussion

3.1. Characterization of Catalysts

Figure 1 shows the XRD patterns of SiO₂, Pt/SiO₂, and Pt-Cu/SiO₂ samples. All of them displayed a diffuse peak of amorphous SiO₂ around 22.5°. For Pt/SiO₂, a series of characteristic diffraction lines of platinum at 39.5, 46.4, and 67.4° were observed, which was identified as the (111), (200), and (220) reflections of Pt with face-centered cubic structure [22]. According to Scherrer’s equation, the grain size of Pt nanoparticles on the Pt/SiO₂ sample was about 21.54 nm (Table 1). Compared with the Pt diffraction peak of the Pt/SiO₂ sample, the Pt (111) diffraction peak of the Pt-Cu/SiO₂ sample becomes broadened and shifts to 41.1°, and the reflection of Pt (200) and (220) apparently disappears. Meanwhile, the diffraction peaks assigned to the Cu species are not observed, which may be ascribed to the little amount of Cu (0.33 wt%, Table 1) measured by ICP.

Table 1: Physicochemical properties of Pt-Cu/SiO₂ catalysts.

Sample

  (m²/g)
Pt loading (wt%)
Cu loading (wt%)
XRD
(nm)
TEM
(nm)
D(Pt)
(%)

SiO₂
550
-
-
-
-

Pt/SiO₂
364
1.21
-
21.54
-
5.24

Pt-Cu/SiO₂
410
1.14
0.33
11.05
10.25
10.22

Derived from nitrogen adsorption-desorption isotherm. The contents of Pt and Cu are tested by ICP-OES. Average particle sizes calculated from XRD patterns of Pt based on Scherrer’s equation. Average particle sizes calculated from at least 50 individual crystallites in TEM images. Estimated from the mean particle size of Pt calculated from XRD patterns.

Figure 1: Wide-angle XRD patterns of (a) SiO₂, (b) Pt/SiO₂, and (c) Pt-Cu/SiO₂.

Comparing the XRD patterns of the Pt-Cu/SiO₂ (Figure 1(c)) with those of the Pt/SiO₂ (Figure 1(b)), on one hand, the broadening peak of Pt (111) and the disappearance of the Pt (200) and Pt (220) peaks for the Pt-Cu/SiO₂ imply that the addition of Cu is able to decrease the size of Pt particles and improve the dispersion of Pt. In view of the calculation using Scherrer’s equation, the Pt particle size for the Pt-Cu/SiO₂ sample (11.05 nm) was much smaller than that of the Pt/SiO₂ (21.54 nm), which also implies that the addition of Cu benefits gaining relatively small Pt particles. On the other hand, the diffraction peak of the Pt (111) shifted to higher angle with the addition of Cu. It is likely that the Cu atom is incorporated into the crystal lattice of Pt and then occupies the position of Pt atoms. The atomic radius of Cu (0.128 nm) is a little smaller than Pt (0.139 nm); the substitution of Cu for Pt results in the crystal cell of Pt shrinking and the parameters of crystal cell decreasing. The difference of XRD patterns illustrates that there are interactions between Pt and Cu in the Pt-Cu/SiO₂ sample.

The textural properties of the samples were investigated by N₂ adsorption-desorption isotherms. The BET specific surface area is summarized in Table 1. It was worthwhile to note that the surface area significantly decreased after metal modification. The BET surface area of the Pt/SiO₂ decreased to 364 m² g⁻¹ from 550 m² g⁻¹ for SiO₂. For the Pt-Cu/SiO₂, the corresponding surface area decreased to 410 m² g⁻¹. This indicated that Pt and Cu were loaded on the SiO₂ support and occupied its surface. According to the results revealed by XRD patterns, the particle size of Pt-Cu/SiO₂ was smaller than that of Pt/SiO₂; therefore the surface area of the Pt-Cu/SiO₂ was slightly larger than that of the Pt/SiO₂.

Figure 2 shows the TEM images of Pt/SiO₂ and Pt-Cu/SiO₂. Unfortunately, we failed to take the HRTEM images of the samples, maybe due to the wrapping of SiO₂ aerogel. From Figures 2(a)–2(c), it can be seen that the dispersion of Pt nanoparticles on Pt/SiO₂ catalysts was poor. The Pt nanoparticles size was not uniform, and the aggregation is serious (Figure 2(b)). Thus, it is difficult to gain its average particle size determined by statistical analysis. With regard to the Pt-Cu/SiO₂ sample in Figures 2(d)-2(e), the Pt nanoparticles were highly dispersed on SiO₂ supporter with uniform size ( = 10.25 ± 0.30 nm), which was in line with the XRD results. Although the average particle size of the Pt/SiO₂ was not available, it is obvious that the Pt-Cu/SiO₂ sample exhibits much smaller particle size and far better dispersion. The observations disclose that the addition of Cu helps to disperse Pt and effectively avoids agglomeration of Pt.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: ((a), (b), and (c)) TEM images for the Pt/SiO₂ sample; ((d), (e)) TEM images and size distribution of particles for the Pt-Cu/SiO₂ sample. (f) is the size histogram of the particle distribution and the average size of the particles determined by statistical analysis of the TEM images; at least 50 individual crystallites were analyzed.

XPS was conducted to investigate the surface properties and the chemical states of Pt. Figure 3 shows the Pt 4f XPS of Pt/SiO₂ and Pt-Cu/SiO₂ together with corresponding deconvolution by fitting Gaussian peaks after Shirley-background subtraction. As shown in Figure 3, the Pt XPS of Pt/SiO₂ and Pt-Cu/SiO₂ presented a doublet corresponding to Pt 4f7/2 and Pt 4f5/2, and the corresponding binding energy (BE) is listed in Table 2. As for the Pt/SiO₂ sample, the Pt 4f7/2 BE at 71.14 eV was ascribed to platinum in the metallic state (Pt⁰); the Pt 4f7/2 peak around 73.34 eV could be assigned to the Pt²⁺ species and the Pt 4f7/2 peak at 75.52 eV was attributed to the Pt⁴⁺ species. But there were only two kinds of Pt species in the Pt-Cu/SiO₂ sample. As shown in Figure 3(b), the peak located at 70.93 eV was attributed to Pt⁰, while the peak around 72.99 eV corresponded to the Pt²⁺ species. Interestingly, the BE of Pt⁰ and Pt²⁺ species for the Pt-Cu/SiO₂ sample exhibited a negative shift compared with that for Pt/SiO₂ sample. This shift should be attributed to the electrons transfer from Cu to Pt by their intimate contact. In other studies [10, 11, 22, 23, 28], this electron transfer phenomenon between precious and promoter was also found, and the direction of this transfer changed with the nature of precious metal and promoter.

Table 2: Binding energies of the Pt 4f levels and the content of Pt species for the different catalysts.

Sample
Pt species
Pt species BE (eV)
Content of Pt species (%)

Pt/SiO₂
Pt⁰
71.14
74.49
28

Pt²⁺
73.34
76.69
53

Pt⁴⁺
75.52
78.87
19

Pt-Cu/SiO₂
Pt⁰
70.93
74.28
60

Pt²⁺
72.99
76.34
40

Figure 3: Pt 4f XPS spectra of (a) Pt/SiO₂ and (b) Pt-Cu/SiO₂.

Furthermore, what is noticeable is the difference in the quantitation of the surface Pt species. As listed in Table 2, Pt⁰, Pt²⁺, and Pt⁴⁺ species simultaneously existed in the Pt/SiO₂ sample and the corresponding proportion was 28%, 53%, and 19%, respectively. Obviously, for the Pt/SiO₂, metal Pt exists mostly in the form of the Pt²⁺ species.

As far as the Pt-Cu/SiO₂ was concerned, there were only Pt⁰ and Pt²⁺ species, and the ratios were 60% and 40%, respectively. Compared with the Pt/SiO₂, the Pt-Cu/SiO₂ contains more Pt⁰ content and less Pt⁴⁺ amount. This suggests that the strengthening of metallic characteristics of Pt in Pt-Cu/SiO₂ may be attributed to the presence of the transition metal Cu.

3.2. Catalytic Performance

The selective hydrogenation of CAL was applied to evaluate the catalytic performance of the Pt/SiO₂ and Pt-Cu/SiO₂, and the results are shown in Table 3. It can be seen that SiO₂ and Cu/SiO₂ showed little activity to the selective hydrogenation of CAL. And Pt/SiO₂ showed 38.3% conversion of CAL, and the selectivity to COL, HALD, HALC, and other products was 3.7%, 1.3%, 35.4%, and 59.6%, respectively. 10.2% CAL could be converted over Cu/SiO₂, and the selectivity to COL, HALD, and HALC was 1.4%, 5.9%, and 9.3%, respectively. However, the Pt-Cu/SiO₂ catalyst performed 37.9% CAL conversion and 64.1% selectivity to COL, while the selectivity to HALC, HALD, and other side products was 5.4%, 11.3%, and 19.2%, respectively. And the TOF of Pt to form COL increased to 3076 h⁻¹ from 329 h⁻¹ for the Pt/SiO₂. Therefore, it is clear that the Pt-Cu/SiO₂ catalyst displays much higher selectivity to COL than Pt/SiO₂, while they showed similar conversion of CAL. This result is different from a past report supporting Pt and Cu on carbon nanotube by microwave-assisted polyol reduction [23]. The different results imply that the interaction between Pt and promoter may be different due to the distinctive preparation strategies. In this work, the enhanced selectivity to COL caused by promoter Cu could be attributed to the increased fraction of the metallic Pt⁰ species on the surface of the Pt-Cu/SiO₂ catalyst. According to our previous studies, the amount of Pt⁰ on the catalyst has a much larger influence on the selectivity of COL, while the Pt²⁺ and Pt⁴⁺ species are responsible for the high-molecular-weight secondary products [13, 20–22]. For Pt/SiO₂, there are more Pt²⁺ and Pt⁴⁺ species and less Pt⁰ species, so it exhibits less selectivity to COL and higher selectivity to those condensation products. On the other hand, the Pt-Cu/SiO₂ catalyst possesses more Pt⁰ species and less species on its surface; thus it displays much higher selectivity to COL than Pt/SiO₂.

Table 3: Catalytic activities of Pt-Cu/SiO catalysts for the hydrogenation of CAL.

Sample
Con. (%)
TOF (h⁻¹)
Sel. (mol%)

COL
HALD
HALC
Others

SiO₂
5.8
-
0
0.9
1.5
97.6

Cu/SiO₂
10.2
-
1.4
5.9
6.3
86.4

Pt/SiO₂
38.3
329
3.7
1.3
35.4
59.6

Pt-Cu/SiO₂
37.9
3076
64.1
5.4
11.3
19.2

Reaction conditions: 50 mg catalyst, 1.00 g CAL, 30 mL isopropanol, 10 bar H₂, 80°C, 2 h. Turnover frequency (TOF) = [moles of cinnamyl alcohol formed]/[(moles of Pt loading) × (reaction time) × D(Pt)] [27]. Including possibly 1-(3-propoxyprop-1-enyl)benzene, cinnamyl formate, cinnamic acid, benzyl cinnamate, 4,4-diphenylcyclohexane-1,5-dienyl acetate, and other condensation products that could be identified by GC-MS because of their large molecular weights.

4. Conclusions

The Pt-Cu bimetal catalyst was prepared on SiO₂ aerogel. The Pt-Cu/SiO₂ showed uniform Pt NPs benefitted from the introduction of Cu. The intensive interaction between Pt and Cu was observed in the Pt-Cu/SiO₂ catalyst, which results in the difference of Pt species on its surface and the electron transfer between Pt and Cu. The Pt-Cu/SiO₂ catalyst exhibited a remarkably enhanced selectivity to cinnamyl alcohol for the liquid-phase hydrogenation of cinnamaldehyde. This promotion can be ascribed to the fact that Cu increases the amount of the Pt⁰ species in the surface of the Pt-Cu/SiO₂ catalyst.

Conflicts of Interest

The authors declare no conflicts of interest.

Acknowledgments

This work was supported by Natural Sciences Fund of Heilongjiang Province (B2015009) and the Ph.D. Fund of Hanshan Normal University (no.QD20170503).

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