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Article

Performance, Reaction Pathway, and Pretreatment of Au Catalyst Precursor in H2/O2 Atmosphere for the Epoxidation of Propylene

by
Zixuan Yang
†,
Huijuan Su
†,
Yanan Cheng
,
Xun Sun
,
Libo Sun
,
Lijun Zhao
and
Caixia Qi
*
Yantai Key Laboratory of Gold Catalysis and Engineering, Shandong Applied Research Center of Nanogold Technology (Au-SDARC), School of Chemistry & Chemical Engineering, Yantai University, Yantai 264005, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Catalysts 2022, 12(5), 540; https://doi.org/10.3390/catal12050540
Submission received: 21 April 2022 / Revised: 9 May 2022 / Accepted: 13 May 2022 / Published: 15 May 2022
(This article belongs to the Section Catalytic Materials)
Browse Figures
Versions Notes

Abstract

:
Gas-phase epoxidation of propylene in the copresence of H2 and O2 was performed over the catalyst of Au on as-synthesized TS-1 that contained a small amount of anatase TiO2. The catalytic performance was studied by washing or nonwashing the catalyst precursor to modulate the content of purity (K, Cl) and then calcining the samples in O2 or H2 prior to reaction. The results show that the catalytic performance of Au/TS-1 can be improved without washing (more K+ and Au maintained) and O2 pretreatment. It was found that the calcination in O2 was able to maintain more metallic Au and form more surface-active oxygen species and thus providing a better yield of propylene oxide with the assistance of potassium. Interestingly, more acrolein can be produced over the catalysts with respect to the in situ calcination in O2 than that in H2 when the feed only contained 10% O2 and 10% propylene in argon, while there was no formation of propylene oxide. On the other hand, the catalyst precursor calcined in H2 prefers the formation of successive oxygenates of PO.

Graphical Abstract

1. Introduction

Propylene oxide (PO) is an important bulk chemical that is widely used in the production of polyether polyols, propylene glycol, and many other products [1]. As shown in Figure 1, industrial production of PO includes a chlorohydrin process and an organic hydrogen peroxide process [2]. The chlorohydrin process suffers from a certain number of chlorinated organic pollutants and a large amount of CaCl2. The organic hydrogen peroxide process will also produce a large number of byproducts, such as tert-butanol and styrene. In recent years, the C3H6 epoxidation process (HPPO process) with H2O2 as the oxidant has been commercialized, but the high cost of H2O2 has hindered the widescale production of PO using this process [3]. Compared with the above processes, the direct epoxidation of C3H6 with H2 and O2 is considered to be a promising process because it is simple, green, and less costly.
In 1998, Haruta et al. first reported that TiO2-supported 2–5 nm Au nanoparticles were able to catalyze the epoxidation reaction of C3H6 with O2 and H2, providing a higher PO selectivity (>90%), and a relatively lower C3H6 conversion (~1.0%) [4]. Since then, many scholars have tried to improve the catalytic activity of Au catalysts by optimizing the catalyst support, preparation method, and pretreatment conditions. Nijhuis et al. studied the effects of TS-1, TiO2, and Al2O3 carriers on the epoxidation of C3H6 and found that only Au nanoparticles loaded on titanium oxide were conducive to the formation of PO [5]. Subsequently, the scientific community has primarily studied this reaction based on Ti, and Au nanoparticles were successively loaded on different types of Ti-based carriers, such as Ti/SiO2 [6], Ti-MCM-41/48 [7], Ti-TUD [8], TS-1 [9], and TS-2 [10]. Common preparation methods for the corresponding Au catalysts include deposition-precipitation (the DP method) [11], immersion [12], coprecipitation [13], and solid grinding [14]. Among them, the Au particle size of the catalyst prepared using the DP method is relatively uniform and currently recognized as a more effective method for Au catalyst preparation [15]. Before the propylene gas–phase epoxidation reaction, the catalyst precursor must be pretreated to obtain catalytically active gold nanoparticles or clusters. At present, many studies have been reported on the pretreatment of titanium oxide-supported gold catalysts. Qi et al. carefully studied the pretreatment conditions of Au catalyst and observed that after a Au/nonporous Ti-SiO2 catalyst was calcined in air at 300 °C, it exhibited better catalytic performance than the catalyst dried under vacuum at room temperature [16]. Sinha et al. found that the catalytic performance of Au/Ti-SiO2 could be further improved after calcination at 300 °C in air and continuous pretreatment in H2/Ar (250 °C) and O2/Ar (250 °C), but the reason was not fully explained [17].
For Au/TS-1 catalyst, vacuum drying at room temperature is the first choice of many researchers [18]. The method of activating the catalyst by raw gas has also been reported in some studies. Ren et al. studied the effect of different temperatures and different pretreatment conditions on Au/TS-1 catalyst performance [19]. At 300 °C, pretreatment with H2-C3H6 gas can provide the best propylene epoxidation activity. The hydrophobicity of the Au/TS-1 surface was enhanced through organic functional modification, and the adsorption of propylene and desorption of PO were strengthened. However, the effect of pretreatment in H2 and O2 on Au/TS-1 is still unclear.
Therefore, the aim of this study was to explore the effect of pretreatment in a flow of hydrogen or oxygen on the catalytic performance over Au/TS-1 catalyst. We prepared two Au/TS-1 catalysts with centrifugal washing (Au/TS-1-wash) and without washing (Au/TS-1) and then pretreated them with H2 or O2 gas before the propylene epoxidation reaction.

2. Results

2.1. Catalytic Results of Au-TS-1 Catalysts

A nominal gold loading of 0.5 wt% was applied to prepare Au catalysts: 0.5 wt%Au/TS-1 and 0.5 wt%Au/TS-1-wash, which were then calcined in H2 or O2 and then evaluated for direct propylene epoxidation in the copresence of H2 and O2. The steady-state reaction results after 30 min of reaction at certain temperatures are shown in Table 1. At 100 °C, there is no formation of acrolein and CO2, two undesirable products. One was formed via attacking the α-H of propylene and another from the complete oxidation of propylene. With the increase in reaction temperature, both of them were produced more, but the formation of CO2 was more pronounced. On the other hand, C3-oxygenates, which include PO and its successive products, i.e., propanal and acetone, are the main products, and their formation gradually decreased with increasing reaction temperature. Except for the case of Au/TS-1-O2, the selectivity to propanal was much higher than the target product of PO, followed by acetaldehyde and acetone. Moreover, no matter if it was Au/TS-1-wash or Au/TS-1, the catalysts pretreated with O2 exhibited higher PO selectivity than those pretreated with H2. Accordingly, for Au/TS-1-O2 at 100 °C, the optimal C3H6 conversion of 0.09% and PO selectivity of 54.7% were observed.
These results show that Au/TS-1 catalyst without washing accompanying an in situ pretreatment in oxygen prior to reaction favors the production of PO.
Four catalysts were tested in the reaction feed without hydrogen (10% propylene, 10% O2 and 80% Ar as balance) at 100 °C and 200 °C. The results shown in Figure 2a,b indicates three main features: (1) no PO formation; (2) two samples with O2 pretreatment (see orange areas) favor the formation of acrolein, the product via attacking a-H of propylene; (3) two samples with H2 pretreatment prefer the formation of the cycle-opening and decomposed products of PO. With the increase in temperature to 200 °C, more products appeared; almost half of the products are CO2. However, two nonwashed catalysts are inclined to produce more CO2 (right side of figure).
From these results, we concluded that the two samples pretreated with H2 are more inclined to form successive products of PO. After O2 pretreatment, the α-H in propylene is obviously attacked and forms acrolein.

2.2. Structural Analysis of TS-1 Support and Au/TS-1 Catalysts

Figure 3 shows the XRD pattern of TS-1 and the studied catalysts. Diffraction peaks are located at 2θ = 7.9, 8.8, 23.1, 23.9, and 24.4°, which confirms the MFI structure of TS-1. The two single diffraction peaks at 24.4 and 29.4° indicate the existence of the cell structure of the orthogonal crystal system, which is mainly caused by the isolated four-coordination titanium atoms entering the molecular sieve framework [20]. Almost no diffraction peak appeared at approximately 25.4°, a characteristic peak of anatase TiO2, further indicating that most of the titanium species in the support exists in the framework of the MFI structure [21]. However, the relative crystallinity, which was calculated by dividing the total intensity of the characteristic peaks of the samples by that of the standard TS-1, was low (only 87%) for the current sample [22], indicating that the purity of TS-1 was lower. In addition, the diffraction peaks of Au (2θ = 38.2 and 44.4°) were not detected in the pattern of all Au-TS-1 catalysts, indicating good dispersion of Au nanoparticles or lower loading of Au nanoparticles [23].
Raman resonance spectroscopy was used to quantitatively analyze the content of anatase TiO2 in the samples. As shown in Figure 4, the bands at 374, 815, and 960 cm−1 are characteristic of the MFI topology and the appearance of the strongest anatase TiO2 characteristic peak at 154 cm−1 confirms the existence of anatase TiO2 in as-synthesized TS-1 [24]. The relative intensity of the bands at 154 and 374 cm−1 (I154/374) can be used to calculate the relative content of anatase TiO2 [25]. The calculated value of I154/374 TS-1 is 0.26, indicating a small amount of anatase TiO2 in as-synthesized TS-1 support.
The coordination environment of titanium atoms in the catalyst was analyzed by UV-Vis spectroscopy. As shown in Figure 5a, an adsorption peak appeared at 206 nm for all samples, which can be attributed to the electron transfer between O2− and Ti4+ in TS-1. This is evidence of the entry of Ti into the framework of the TS-1 molecular sieve and the formation of TiO4 tetrahedral, accordingly confirming the presence of isolated Ti (IV) species in the MFI framework [26]. Unlike TS-1 support, Au/TS-1 catalyst has two weak absorption peaks in the spectrum. The shoulder peak at 270 nm could be caused by KOH applied during the loading of Au, which partially degraded the coordination environment of the skeleton titanium atom in TS-1 support and changed the coordination environment [19]. As shown in Figure 5a,b, the absorption peak at 526 nm can be attributed to the plasma resonance generated by Au particles in the catalyst, indicating that Au species on the catalyst are reduced [27].

2.3. Chemical State of Au and Concentration of Surface Elements in Au/TS-1 Catalysts

The chemical states of Au, Ti, and O in the Au/TS-1 catalysts calcined in different gaseous environments were determined by X-ray photoelectron spectroscopy (XPS), as shown in Figure 6a. For all the studied catalysts, two binding energy peaks at 83.8 and 84.4 eV were assigned to the Au0 and Au+ species, respectively [28]. The corresponding quantification analysis of Au species for Au/TS-1 calcined in different gases are shown in Table 2. Au/TS-1 pretreated with O2 had more metallic Au species (71.1%, 77.6%) than that of Au/TS-1 pretreated with H2 (55.1%, 68.4%). These results show that the calcination atmosphere can modulate the chemical states of Au on Au/TS-1. According to the literature, the chemical state of Au species is related to the product selectivity of propylene epoxidation. Metallic gold is favorable for the epoxidation of propylene with oxygen and hydrogen to form PO, while oxidized gold may promote the hydrogenation of propylene to produce propane byproduct [29]. However, there are also reports in the literature that an appropriate amount of gold oxide is conducive to the adsorption of oxygen in the reaction [30,31].
The Ti 2p XPS spectra of catalysts are shown in Figure 6b. All the spectra display one strong peak at around 460.2 eV and one weak peak at around 458.7 eV. Based on the work done by Grohmann et al. [32], the strong peak belongs to the Ti species which coordinated with the tetrahedron in the silicon structure, and the weak peak belongs to the octahedral coordinated nonframework Ti species (anatase TiO2). This is consistent with the Raman results shown in Figure 4. Two distinct binding energies were attributed to different oxygen coordination of titanium. The increase in the binding energy of titanium in the framework of Ti-silicate can be explained by an increase in the interatomic potential due to the decrease in the coordination number of titanium and the shortening of the Ti–O bond [32].
The O 1s XPS results in Figure 6c show that there are three types of oxygen species on the catalyst: (1) O atoms (OI) from a mixture of hydroxyl groups and adsorbed water on the surface of the catalyst at ca. 531.8 eV [33]; (2) surface-active O2− species (OII) at ca. 532.5 eV and (3) oxygen atoms (OIII) at ca.533.2 eV, which are associated with the silicon atoms of the crystal lattice [34,35]. The formation of octahedral coordination anatase TiO2 in synthesis may derive from tetracoordinated titanium located at the surface of the material by reaction with H2O in the atmosphere [36]. The low Ti 2p binding energy and the appearance of the first oxygen peak (OI) should therefore be caused by TiO2 species on the external surface of the silicate crystals.
Table 2 shows that compared with H2 pretreatment, the catalysts with O2 pretreatment exhibit more surface-active oxygen species OII and that without washing, which kept more K and chloride, strengthens the results. Therefore, 0.5%Au/TS-1-O2 catalyst provides the highest content of surface-active oxygen species. The resulting adsorption of O2 on the catalyst was also evidenced by XPS observation and DFT calculation in other references [34,35,37]. According to the well-known mechanism of direct epoxidation of propylene, it is generally believed that oxygen obtains electrons from reduced Au species, which then becomes surface-active oxygen (O2−) and forms -OOH [38,39], which is the rate-limiting step in the formation of PO (as shown in Figure 7).

2.4. Dispersion Situation of Gold Particles

Figure 8 shows the TEM images and size distribution of Au particles in the studied catalysts. Gold particles are well dispersed for all the catalysts; however, two washed catalysts (a & b) show a small mean diameter of Au particles than the other two nonwashed catalysts (c & d). It is reasonable that more Au, K, and Cl were maintained in two nonwashed catalysts, as shown in Table 3 than those in two washed catalysts. It is generally accepted that the existence of chloride ions leads to gold species migration and aggregation after heating, resulting in large gold particles. However, Table 3 shows that O2 pretreatment to some extent can remove the chloride ions from the catalysts, whether the catalysts were washed or not. In contrast, the potassium contents were not influenced by O2 pretreatment. During the O2-treated catalysts, the exchange between anion O and Cl− might occur during the reduction of Au catalyst precursor in O2 atmosphere. The two samples with O2 pretreatment (one washed and another nonwashed) showed a similar mean diameter of Au particles, indicating that oxygen plays a more important role in controlling the particle size of gold than chloride. It can be speculated that chloride ion mainly induces metallic Au particles to accumulate due to weak interaction with the support.

3. Discussion

The results show that Au/TS-1 calcined in O2 atmosphere displayed the best catalytic performance and more cationic Au and active-surface oxygen species were found in the sample. The studied catalysts were also tested in the feed without hydrogen, and it was found that the pretreating atmosphere can change the reaction pathway. Compared with the published work on the epoxidation of propylene in the presence of H2 and O2 over Au/TS-1 catalysts, the PO selectivity over the studied catalysts in this work was found to be smaller. This can be attributed to Au nanoparticles with sizes larger than 5 nm in the studied catalysts with the number of Au species and the formation of a small amount of anatase TiO2 in the synthesized TS-1 support (see Raman analysis, UV-Vis and XPS analysis in Figure 4 and Figure 5, and Table 2). It is well known that epoxidation of propylene is a very structure-sensitive reaction and high selectivity to PO highly depended on 2.0–5.0 nm gold nanoparticles nearby the tetrahedral Ti species in the mixture of H2 and O2 [40]. The size-dependent Au active structure and reaction mechanism [41], and a strong morphology-dependent interplay [42] between the Au-TiO2 interaction for the low-temperature propene epoxidation with H2 and O2 were studied by Chen et al. They found that a distinct increase of Au species in the Au/TiO2-{001} catalyst with large Au particle (7.0–2.0 nm) lowered the activity for PO formation, due to the easy adsorption of PO and increased deep oxidation.
Moreover, during the synthesis of hierarchical TS-1 or nano-sized TS-1, it is easy to form anatase TiO2 due to the mismatch in the formation rate of structural Ti and silicon [24]. TiO2 anatase leads to a low decomposition efficiency of H2O2, covering the active center of TS-1, which is not conducive to catalytic oxidation [43].
The Au/TS-1-O2 sample has a mean diameter of 6.09 of Au particles, the highest content of K (Table 2), and adsorbed capacity of oxygen species (Table 1). The Au/TS-1-wash-H2 catalyst with a mean diameter of 5.04 of Au particles, the lowest K content, and content of surface-active oxygen species showed the highest PO selectivity of 54.7% in this study. The positive role of K in propylene epoxidation reaction has been demonstrated in Huang’s work; basic salt or hydroxides of alkali (except Li) are speculated to interact with Au clusters to activate molecular O2 and then stabilize it [14].
Based on the catalytic results in Figure 2 (no H2 in feed) and Table 3 (H2 included in feed), we concluded that the two samples treated with H2 were more inclined to form isomerized products of PO. After O2 pretreatment, the α-H in propylene was obviously attacked to form acrolein. This also indirectly indicates the possible sequence of steps for propylene epoxidation with H2 and O2 on Au-supported titanosilicates (Figure 7) [38]; the formation of -OOH species in the absence of hydrogen atmosphere is not easy. We may say that the reaction pathway of catalysts could be changed after pretreatment with O2 or H2 and the presence of H2 can form an active -OOH species for propylene epoxidation, while only oxygen species can be formed in the absence of H2.
In addition to the crucial role of hydrogen in forming hydroxyl intermediate for PO production, we may confirm the promotion role of K+ and surface-activated O species in the oxidation of propylene. The production of PO is a synergy between the oxygen species and active center, as well as H2 activation via oxygen species to form a hydroxyl intermediate.

4. Materials and Methods

4.1. Catalyst Synthesis

All the chemical reagents were purchased from Sinopharm Chemical Reagent Co. Ltd., China, and used as received. According to the method developed by Khoman et al. [44], tween 20 (2 g) was dissolved in deionized water (24 mL), followed by the addition of tetrapropylammonium hydroxide (TPAOH, 25 wt%, 43.2 mL) while stirring. Tetraethyl silicate (TEOS, 38.6 mL) was slowly added to the solution under vigorous stirring for 1 h. Then, tetrabutyl titanate (TBOT, 0.48 mL) dissolved in isopropyl alcohol (IPA, 20 mL) was added dropwise (still under vigorous stirring) for another 2 h. The resulting solution was then crystallized (at 443 K, 18 h) under autogenous pressure. Finally, the solid recovered from centrifugation was washed with distilled water, dried in a vacuum oven (at 323 K, 12 h) and calcined (at 823 K, 5 h) in air. The BET surface area of synthesized TS-1 was 409.1 m2/g, which was determined by N2 adsorption-desorption measurements.
Au/TS-1 catalyst was prepared using an improved deposition-precipitation method. A certain amount of support was added to a suitable amount of solution of HAuCl4·xH2O (A.R., Sinopharm Group, Beijing, China) with a pH of 7.0 that was adjusted with 1.0 M KOH solution. The solution product was then treated with an ion exchange of aqueous ammonia solution. Next, the product was divided into two groups. One group was washed with deionized water and dried in vacuum overnight and designated as 0.5% Au/TS-1-wash. Then, this group was heated in a flowing hydrogen or oxygen stream at 300 °C for 1 h, and then denoted as 0.5%Au/TS-1-wash-H2 or 0.5%Au/TS-1-wash-O2. The other group of products was not washed but dried overnight in vacuum, and these catalysts were designated as 0.5%Au/TS-1. Then, the group of catalysts was calcined in a flowing hydrogen or oxygen stream at 300 °C for 1 h and then named as 0.5%Au/TS-1-H2 or 0.5%Au/TS-1-O2.

4.2. Catalyst Characterization

X-ray diffraction spectrum of the catalysts was obtained using a diffractometer with CuKα (λ = 0.154 nm) radiation(XRD-6100, Shimadzu Corporation, Berlin, Japan). The sample was scanned in the 2θ range of 5~80° at a scan rate of 2°/min. The dispersion situation of Au nanoparticles in the studied catalysts was observed using a JEM-200, which was operated at 200 kV (TEM, JEOL Corporation, Tokyo, Japan). The actual content of Au in the catalysts was determined by inductively coupled plasma-atomic emission spectrometry (ICP-AES, Optima 7300V, Perkin-Elmer, Waltham, MA, USA). The chemical state of Au in the catalyst was determined using an X-ray photoelectron spectrometer (XPS, ESCALAB250, Thermo Fisher, Waltham, MA, USA) equipped with a monochromatic Al Kα X-ray source (1486.6 eV). The binding energy (BE) of the C1s core level at 284.6 eV was used as the internal standard. The Ti species was identified using a UV–Vis–NIR spectrophotometer (LAMBDA750, PerkinElmer, Shanghai, China). The sample was dried with a BaSO4 UV lamp for 4 h and then pressed into tablets. Scanning analysis was performed in the range of 200-800 nm. The amount of anatase TiO2 was further measured by Raman spectroscopy (Senterra, Bruker company, Saarbrucken, German) in the range of 40_4000 cm−1.

4.3. Catalytic Tests

Au/TS-1 and Au/TS-1-wash catalyst precursors were pretreated in situ with H2 or O2 prior to the reaction where Ar was used as the balance gas. Typically, 0.15 g of Au/TS-1 was placed in a vertical quartz reactor (6 mm inner diameter, 8 mm outer diameter), and then heated from room temperature to the target temperature at a ramping rate of 2 °C∙min−1 under an appropriate atmosphere. If unspecified, the volume percent for H2 and O2 was 10% (Ar as the balance gas). At the target temperature for 2 h, the Au/TS-1 catalyst was purged with Ar and then cooled to room temperature under Ar flow. Then, the feed gas (10% H2, 10% O2, 10% C3H6, and 70% Ar) was introduced at a flow rate of 10 mL min−1, corresponding to a space velocity of 4000 mL g−1 cat h−1. The reactants and products are measured online using a gas chromatograph (Agilent 7820A, Palo Alto, Santa Clara, CA, USA) equipped with a thermal conductivity detector (TCD) (Porapak Q and 5A chromatographic columns) and flame ionization detector (FID) (FFAP capillary chromatographic columns). The propylene conversion, product selectivity, and carbon balance were calculated as follows:
Conversion = moles of (oxygenates + CO2/3)/moles of propylene in the feed;
Selectivity = n/3 (moles of product)/moles of (oxygenates + CO2/3);
Carbon balance = moles of (3 ∗ oxygenates + CO2/3)/moles of propylene in the feed;
where n is the number of carbon atoms in the product.

5. Conclusions

In short, centrifugally washed catalyst Au/TS-1-wash and unwashed catalyst Au/TS-1 were calcined in H2 or O2, and the performance of the propylene epoxidation reaction was evaluated in this study. By comparison, the unwashed Au/TS-1 catalyst showed better catalytic performance when calcined in oxygen. Although the purity of as-synthesized TS-1 support and larger Au particles did not provide high selectivity to PO as reported in the study, we found that the distribution of Au species and O species was affected by pretreating the Au catalyst precursor in H2 or O2 atmospheres. The catalyst with O2 pretreatment showed a higher content of surface-active oxygen species on the surface of the catalyst, leading to more acrolein formation when no hydrogen was added in the reactive feed and making more PO when hydrogen was in the feed with the assistance of potassium.

Author Contributions

Conceptualization, Z.Y. and C.Q.; methodology, Z.Y. and H.S.; software, Z.Y. and Y.C.; validation, Z.Y. and C.Q.; formal analysis, L.S. and X.S.; investigation, Z.Y.; resources, C.Q.; data curation, H.S. and L.Z.; writing—original draft preparation, Z.Y.; writing—review and editing, C.Q.; supervision, C.Q.; funding acquisition, C.Q. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 21773202, 21802117).

Data Availability Statement

The date is contained within the article.

Acknowledgments

This work is financially supported by the National Natural Science Foundation of China (Nos. 21773202 and 21802117). We also acknowledge the Key Research and Development Plan of Yantai (Grants. 2019LJRC140 and 2021XDHZ068).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 00540 g001 550
Figure 1. Reactions used in the commercial production of PO.
Figure 1. Reactions used in the commercial production of PO.
Catalysts 12 00540 g001
Catalysts 12 00540 g002 550
Figure 2. Catalytic results over Au/TS-1 with no-hydrogen feed gas. The reaction temperatures are: (a) 150 °C (b) 200 °C.
Figure 2. Catalytic results over Au/TS-1 with no-hydrogen feed gas. The reaction temperatures are: (a) 150 °C (b) 200 °C.
Catalysts 12 00540 g002
Catalysts 12 00540 g003 550
Figure 3. XRD pattern of TS-1 and the studied Au catalysts (inset: magnified region between 24° and 30°).
Figure 3. XRD pattern of TS-1 and the studied Au catalysts (inset: magnified region between 24° and 30°).
Catalysts 12 00540 g003
Catalysts 12 00540 g004 550
Figure 4. Raman spectra of TS-1 support and Au/TS-1 catalysts.
Figure 4. Raman spectra of TS-1 support and Au/TS-1 catalysts.
Catalysts 12 00540 g004
Catalysts 12 00540 g005 550
Figure 5. (a) UV-Vis spectra of TS-1 support and Au/TS-1 catalysts and (b) the enlarged region of the peak at 526 nm.
Figure 5. (a) UV-Vis spectra of TS-1 support and Au/TS-1 catalysts and (b) the enlarged region of the peak at 526 nm.
Catalysts 12 00540 g005
Catalysts 12 00540 g006 550
Figure 6. XPS spectra of the Au/TS-1 catalysts: (a) Au 4f, (b) Ti 2p and (c) O 1s.
Figure 6. XPS spectra of the Au/TS-1 catalysts: (a) Au 4f, (b) Ti 2p and (c) O 1s.
Catalysts 12 00540 g006
Catalysts 12 00540 g007 550
Figure 7. Possible sequence of steps for propylene epoxidation with H2 and O2 on Au-supported titanosilicates.
Figure 7. Possible sequence of steps for propylene epoxidation with H2 and O2 on Au-supported titanosilicates.
Catalysts 12 00540 g007
Catalysts 12 00540 g008 550
Figure 8. The TEM images and size distribution of Au/TS-1 catalysts: (a) Au/TS-1-wash-H2, (b) Au/TS-1-wash-O2, (c) Au/TS-1-H2 and (d) Au/TS-1-O2.
Figure 8. The TEM images and size distribution of Au/TS-1 catalysts: (a) Au/TS-1-wash-H2, (b) Au/TS-1-wash-O2, (c) Au/TS-1-H2 and (d) Au/TS-1-O2.
Catalysts 12 00540 g008
Table
Table 1. Epoxidation of propylene on Au/TS-1 and Au/TS-1-wash under H2 or O2 pretreatment atmospheres.
Table 1. Epoxidation of propylene on Au/TS-1 and Au/TS-1-wash under H2 or O2 pretreatment atmospheres.
SampleT.
(°C)		Conv.
(%)		Products Selectivity (%)Carbon
Balance
POPropanalAcetoneAcetaldehydeAcroleinCO2C3-
Oxygenates
0.5 wt% Au/TS-1-
wash-H2		100
150
200		0.08
0.89
3.74		26.1
31.5
14.6		56.7
36.7
45		0
8.6
10.3		17.2
6.6
8		0
3.9
3.3		0
12.7
18.8		82.8
76.8
69.9		0.98
1.00
1.04
0.5 wt% Au/TS-1-wash-O2100
150
200		0.01
1.13
3.87		40.1
32.1
20.5		59.9
35.5
24.4		0
6.7
18.1		0
6.5
6		0
2.8
1.8		0
16.5
29.1		100.0
74.3
63.0		1.00
1.00
1.03
0.5 wt% Au/TS-1-H2100
150
200		0.05
0.60
2.19		33.6
21.8
12.7		66.4
46.5
28.6		0.0
7.9
14.1		0.0
6.5
7.7		0.0
4.3
2.8		0.0
12.9
34.1		100.0
76.2
55.4		0.99
0.99
1.04
0.5 wt% Au/TS-1-O2100
150
200		0.09
1.01
3.87		54.7
27.9
10.7		34.9
35.4
33.1		0.0
8.4
19.5		10.5
7.7
7.2 		0.0
2.4
1.7 		0.0
18.2
27.8		100
71.7
63.3		0.96
0.98
1.02
Table
Table 2. Quantitative analysis of Au and O species on the catalyst based on XPS data.
Table 2. Quantitative analysis of Au and O species on the catalyst based on XPS data.
CatalystAu species (%)O species (%)
Au0Au+OIOIIOIII
0.5%Au/TS-1-wash-H268.431.66.0417.7776.19
0.5%Au/TS-1-wash-O277.622.48.7218.0373.25
0.5%Au/TS-1-H255.144.98.3417.6873.98
0.5%Au/TS-1-O271.128.99.3218.7471.94
Table
Table 3. The elemental content of Au, K, and Cl in the studied catalysts.
Table 3. The elemental content of Au, K, and Cl in the studied catalysts.
CatalystActual Loading (wt.%) aSurface Atom Content (%) b
AuKClK
Au/TS-1-wash-H20.250.1127.570.32
Au/TS-1-wash-O227.130.33
Au/TS-1-H20.260.1228.540.35
Au/TS-1-O228.310.39
a determined by ICP-AES, b determined by XPS measurement.
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Yang, Z.; Su, H.; Cheng, Y.; Sun, X.; Sun, L.; Zhao, L.; Qi, C. Performance, Reaction Pathway, and Pretreatment of Au Catalyst Precursor in H2/O2 Atmosphere for the Epoxidation of Propylene. Catalysts 2022, 12, 540. https://doi.org/10.3390/catal12050540

AMA Style

Yang Z, Su H, Cheng Y, Sun X, Sun L, Zhao L, Qi C. Performance, Reaction Pathway, and Pretreatment of Au Catalyst Precursor in H2/O2 Atmosphere for the Epoxidation of Propylene. Catalysts. 2022; 12(5):540. https://doi.org/10.3390/catal12050540

Chicago/Turabian Style

Yang, Zixuan, Huijuan Su, Yanan Cheng, Xun Sun, Libo Sun, Lijun Zhao, and Caixia Qi. 2022. "Performance, Reaction Pathway, and Pretreatment of Au Catalyst Precursor in H2/O2 Atmosphere for the Epoxidation of Propylene" Catalysts 12, no. 5: 540. https://doi.org/10.3390/catal12050540

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