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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Visible–Light–Promoted Selective Hydrogenation of Crotonaldehyde by Au Supported ZnAl–Layered Double Hydroxides: Catalytic Property, Kinetics and Mechanism Investigation Lei Fang, Wei Luo, Yue Meng, Xiaobo Zhou, Guoxiang Pan, Zheming Ni, and Sheng-jie Xia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b05463 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

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The Journal of Physical Chemistry

Visible–Light–Promoted Selective Hydrogenation of Crotonaldehyde by Au Supported ZnAl Layered Double Hydroxides: Catalytic Property, Kinetics and Mechanism Investigation

Lei Fanga, Wei Luoa, Yue Mengb, Xiaobo Zhouc, Guoxiang Panb, Zheming Nia*, Shengjie Xiaa* a

Department of Chemistry, College of Chemical Engineering, Zhejiang University of

Technology, Hangzhou 310032, P R China b

Department of Materials Engineering, Huzhou University, Huzhou 313000, P R

China c

Entegris, Inc., 129 Concord Road, Billerica, MA 01821, USA

* Corresponding author. E–mail: [email protected] (S.J. Xia); [email protected] (Z.M. Ni) Telephone number: 086–0571–88320373

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The Journal of Physical Chemistry 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: α,β–unsaturated alcohol are a group of important chemicals and have frequently been synthesized by catalytic hydrogenation method due to its low cost and environmental friendly nature. However, the synthesis of high reactivity and selectivity catalyst that favors C=O hydrogenation is very challenging. Many references have been reported the results of selective hydrogenation of crotonaldehyde to crotonyl alcohol, either have high conversion low selectivity, or high selectivity low conversion. Herein, Au supported ZnAl layered double hydroxides (Au/ZnAl–LDHs) was synthesized and used on the hydrogenation reaction of crotonaldehyde (CDE) to crotonyl alcohol (COL) under the moderate reaction condition supplied with light irradiation. The experimental results indicated that the reactivity of CDE hydrogenation and selectivity of COL both have been greatly improved under visible light irradiation, from 42% to 99% for conversion of CDE, from 59.5% to 96% for COL selectivity, and from 101 h–1 to 272 h–1 for TOF, compared with no irradiation. Based on the proposed kinetics equation, it can be concluded that irradiation can not only remarkably increase the reaction rate of selective hydrogenation of CDE to COL, but also obviously decrease the activation energy of the reaction system. The enhancement of photocatalytic property of Au/ZnAl–LDHs is exactly due to the supporting of Au, which can be proved by the dependence of the catalytic performance on the wavelength range of light as well as UV–vis result. In addition, the possible CDE catalytic reaction mechanism was concluded based on the calculation of surface adsorption and hydrogenation reaction path using DFT method, which is well explain and support the experimental results.


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1. INTRODUCTION α,β–unsaturated alcohol is group of important chemicals with the general formula of R–CH=CHCH2OH. CH3CH=CHCH2OH–also known as crotonyl alcohol–is the simplest member of this group of compounds.1,2 Crotonyl alcohol, also called butenol, is a colorless liquid with special odor which has been widely to synthesize butyl aldehyde, butanol, alcohol denaturant, herbicide, and plasticizer.3,4 Currently, α,β–unsaturated alcohol is synthesized by α,β–unsaturated aldehyde reduction, including both the traditional direct reduction method and catalytic hydrogenation method.5,6 The traditional direct method uses expensive reductants such as lithium aluminum hydride, sodium borohydride, aluminum isopropoxide etc. in the process, and it has the disadvantages such as high cost, harsh reaction condition, contaminates generation, and challenge in product separation.7–9 The new catalytic hydrogenation method has been drawing attention due to its low cost and environmental friendly nature.10,11 The possible hydrogenation products of crotonaldehyde (CDE) include crotonyl alcohol (COL), butyraldehyde (BDE), ENOL and butanol (BOL), which means there is competition of the hydrogenation on different double bonds. The bonding energy of C=C is around 615.0 kJ/mol, and for C=O it is about 715.0 kJ/mol, which make the hydrogenation reaction favors the C=C bond, at the same time the conjugation effect will enhance this trend.12–14 Therefore, the synthesis of high reactivity and selectivity catalyst that favors C=O hydrogenation is very challenging.15 Many references have been reported the results of selective hydrogenation of crotonaldehyde to crotonyl alcohol, either have high conversion low selectivity,4,16 or have high selectivity low conversion.14,17,18 Noble metals, such as Au, Pd, Co, Pt and Rh, have been used as one of the major groups of catalyst in α,β–unsaturated aldehyde



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hydrogenation, they have high reactivity but the selectivity towards C=O bond is usually low.19–23 In recent years, bimetallic catalyst has been developed to improve the selectivity. They are synthesized by modifying a base noble metal with another element. The modification metal element used in this process includes Fe, Sn, Ir, In, Cu ad Ag.24–26 Although the modification could improve the performance of the single metal catalysts, the catalyst itself is still usually expensive and not ideal for large scale production.27 Thus, the choice of appropriate supports is one of the key to achieve high catalytic performance of noble metals. The support can not only stabilize the highly dispersed metal nanoparticles, but substantially promote the reaction through metal–support cooperation.28–31 Besides, based on the strong visible light absorption of Au nanoparticles (NPs), Au NPs and Au based materials can be used as highly efficient photocatalysts for many reactions which are usually difficult to react or with low efficiency.32,33 Therefore, we conceive that Au nanoparticles supported on a suitable support material used as a photocatalyst may largely promote selective hydrogenation of crotonaldehyde to crotonyl alcohol. Layered double hydroxides (LDHs) with the huge advantages of low cost, easy to synthesize, excellent chemically stable, high surface area and good catalytic activity itself, which has been widely used as supports in different catalytic reactions,34–38 maybe is a good choice. Thus,

in

this

paper,

Au

supported

ZnAl

layered

double

hydroxides

(Au/ZnAl–LDHs) was used on the crotonaldehyde (CDE) hydrogenation reaction supplied with light irradiation. The data showed that the reactivity and selectivity both have been improved a lot under light irradiation. According to the kinetics study, we aimed to calculate and compare the reaction rate and activation energy of CDE hydrogenation in dark and under irradiation. We proved that the enhancement of photocatalytic property of Au/ZnAl–LDHs is due to the supporting of Au for the


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reason from the dependence of the catalytic performance on the wavelength range of light as well as UV–vis result. We also concluded the possible CDE catalytic reaction mechanism catalyzed by Au/ZnAl–LDHs based on the surface adsorption calculation and hydrogenation reaction path using DFT method. 2. EXPERIMENTAL SECTION 2.1. Materials and Characterizations The detailed instruments and materials list are given in supporting material. 2.2. Synthesis of Au/ZnAl–LDHs 2.2.1 Preparation of ZnAl–LDHs Solution A contains 60 mL ultrapure water, NaOH (6.40 g, 0.16 mol) and Na2CO3 (1.06 g, 0.01 mol); solution B containing 60 mL ultrapure water, Zn(NO3)2·6H2O (17.82 g, 0.06 mol) and Al(NO3)3·9H2O (7.50 g, 0.02 mol); solution A and B were added with drop by drop into a 250 mL three–neck flask containing 60 ml ultrapure water at room temperature. The pH of reaction system was maintained at range of 9.5–10 with stirring rate of 300 rpm, the total process would last for 20 min. After that, the resultant slurry was aged at 85 °C for 24 h, and then, the product was centrifuged (5000 rpm for 5 min) and washed with ultrapure water until the pH=7. Finally, the sample was dried in vacuo at 65 oC for 18 h. After grounded, the product of ZnAl–LDHs could be obtained (content by weight: Zn: 45.88%, Al: 6.34%; the molar ratio of Zn:Al=3.03). 2.2.2 Preparation of Au/ZnAl–LDHs ZnAl–LDHs (2.0 g) was added into a 250 mL flask containing a designated quantity of 5 mmol/L HAuCl4 aqueous solution. After the mixture was stirred with a rate of 300 rpm for 10 min, aqueous NH3 (10 %, 2 mL) was added, and the resulting mixture was stirred at room temperature for 18 h. The resulting slurry was



centrifugated (5000 rpm for 5 min), washed with ultrapure water (50 ml×3), and dried at room temperature under vacuum, giving the supported hydroxide precursor. Then, the precursor (1.0 g) was dispersed in ultrapure water (100 mL) and treated with NaBH4 (50 mg) at room temperature for 2 h. After centrifugation, washing and drying, Au/ZnAl–LDHs catalyst can be prepared (Au:2.02%; Zn: 44.93%; Al: 6.22%). 2.3. Visible–light–promoted selective hydrogenation In a typical selective hydrogenation procedure, the transparent glass vessel (250 ml) was loaded with 50 mL 1,4–dioxane/water (volume ratio is 1:1) mixture contained 0.1 mol/L crotonaldehyde, sonicated for 15 min and then 50 mg of pretreated Au/ZnAl–LDHs catalyst were added. After that, the vessel was attached to the reactor and then evacuated and filled with hydrogen three times to exclude atmosphere off. After the vessel was full of hydrogen under 0.2 MPa, the temperature was heated and maintained at 60 oC, then, the stirring device started with a constant rate of 600 rpm and the reaction started and timed. The reaction time was 4 h, and the reaction products were identified by GC and NMR (1H and 13C) analyses. In addition, a special–designed circinate–shape 300 W Xenon lamp (400 nm＜λ＜ 800 nm) with light intensity of 1.0 W/cm2 was used for the experiments of visible light irradiation selective hydrogenation (temperature of reaction system is maintained at 60 oC). All other conditions and processes are the same with no irradiation selective hydrogenation. After catalytic hydrogenation experiment, the reaction solution containing Au/ZnAl–LDHs sample was filtered to get catalyst solid. The filtering product was washed with 25 ml mixed solution of water and alcohol (1:1 in volume) twice and then dried at 85 oC overnight for testing and recycle experiments. 2.4. Theoretical calculation


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2.4.1 Calculation methods and parameters of Au/ZnAl–LDHs Hydrotalcite bulk laminate with Zn/Al=3 was built in a stacking 2H mode. Carbonate was selected as the interlayer anion to balance the positive charge on the surface of the laminate. After optimization, an Au3 cluster-loaded hydrotalcite laminate model was constructed on the surface model of hydrotalcite laminates, as shown in Figure 1a. Full-optimization and property analysis of the model was performed with the program package of CASTEP in the Materials Studio 7.0 of Accelrys Inc, using Ceperly-Alder- Perdew-Zunger (CA-PZ) Local Density Approximation (LDA) exchange-correlation functional. The atomic electronics were represented by ultrasoft pseudopotential and a 340 eV cut-off energy was used. The Monkhorst-Pack mesh k-points was 4×4×1 for calculations, and the convergence criteria was 2×10−6 eV/atom for SCF. All the results were based on spin polarization calculations with density mixing scheme and the other parameters were set to the program's default values. 2.4.2 Molecular models CDE has four different geometric isomers: E–(s)–trans、E–(s)–cis、Z–(s)–trans and Z–(s)–cis, due to the position of methyl group, C=C and C=O, the results are showed in Figure 1b. It also shows the relative energy (ER) for these isomers (ER of E–(s)–trans–CDE is set as 0), from which we can say the stability are in the order of: E–(s)–trans>E–(s)–cis >Z–(s)–trans>Z–(s)–cis, this is consistent with the reference paper. Thus, we choose E–(s)–trans–CDE to study the CDE adsorption and selective hydrogenation due to the fact that it is the most stable isomer of all four. 3. RESULTS AND DISCUSSION 3.1. Structural characteristics of ZnAl–LDHs and Au/ZnAl–LDHs



Figure 2a is the XRD patterns of ZnAl–LDHs and Au/ZnAl–LDHs. The characteristic peaks of 003, 006, 009/012, 015, 018, 110, 113, and so on, can be observed in all two curves, which indicate the successful synthesis of typical LDH material.39–41 Particularly, there are obvious peaks at about 37.6 o (111), 44.0 o (200) and 61.5

o

(220) of 2θ for Au/ZnAl–LDHs, which represents the existence of Au

species in Au/ZnAl–LDHs sample.42,43 The UV−vis curves for two samples are showed in Figure 2b. From which we can see that, there is very little absorption in visible range for ZnAl–LDHs. But, Au/ZnAl–LDHs sample has a strong and broad absorption peak at the visible light region, especially for the range of 500–650 nm. Such enhancement of both intensity and width of absorption may indicate that the photo performance in visible light region of ZnAl–LDHs would be improved after Au supported, which may be resulted from the localized surface plasmon resonance (LSPR) effect of Au.33 TEM image of ZnAl–LDHs is given in Figure 2c. From which we can see that ZnAl–LDHs with layered structure display obvious compact lamellar crystal, the crystallite size of these nanoparticles is at the range of 50–150 nm. TEM image of Au/ZnAl–LDHs (Figure 2d) reveals that there are a lot of Au dark dots dispersed in LDH sheets, and the average Au nanoparticle size is about 8.4 nm. We also tested the HRTEM image and Electron diffraction pattern (SAED) of Au/ZnAl–LDHs, which are shown in Figure 2e and 2f. HRTEM image of Au/ZnAl–LDHs shows the single crystalline nature of the Au nanoparticles with characteristic d–spacings of 0.244 nm for (111) plane of Au. SAED pattern of the polycrystalline diffraction rings, (111),


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(200), (220) and (311) planes of Au nanoparticles are indexed, which is quite consistent with XRD result. In addition, STEM images with elemental mapping and EDS intensity line profiles for Au/ZnAl–/LDHs (Figure 3) further indicate that Au nanoparticles are supported on the surface of ZnAl–LDHs. 3.2. Selective hydrogenation and reusability GC–FID chromatograph of the hydrogenation of crotonaldehyde catalyzed by ZnAl–LDHs and Au/ZnAl–LDHs are added into supporting. As shown in Figure S1, there are obvious peaks representing butyraldehyde (BDE), crotonaldehyde (CDE), butanol (BOL) and crotonyl alcohol (COL) in the case of ZnAl–LDHs; the products of Au/ZnAl–LDHs only contain crotonaldehyde (CDE) and crotonyl alcohol (COL)as well as very little crotonaldehyde (CDE). Table 1 shows the activity and selectivity of crotonaldehyde (1a) hydrogenation reaction towards different final products such as crotonyl alcohol (2a), butyraldehyde (4a) and butanol (5a). Conversion of crotonaldehyde (CDE) and selectivity of crotonyl alcohol (COL) curves as a function of time catalyzed by ZnAl–LDHs and Au/ZnAl–LDH are also given in Figure S2. The conversion rate of 1a is 31% and 39%, the selectivity of 2a is 38.7%, 43.6%, respectively, before and after light was introduced when ZnAl–LDHs is used for the reaction. It is obvious that the enhancement of both conversion and selectivity for ZnAl–LDHs after irradiation is just a little. While in the case of Au/ZnAl–LDHs, 1a conversion increases from 42% in dark to 99% with light, 2a selectivity also increases a lot from the 54.8% without light to 96.0% under visible light. The TOF value of Au/ZnAl–LDHs before and after irradiation is 101 h–1 and 272 h–1, respectively. Obviously, the photocatalyzed reaction dramatically enhanced the activity and selectivity of Au supported ZnAl layered double hydroxides towards the production of crotonyl alcohol. As a contrast, we also investigated the selective hydrogenation of



crotonaldehyde to crotonyl alcohol catalyzed by p25 TiO2 and Au/TiO2 under visible light irradiation. The results show that the comprehensive performance of p25 TiO2 and Au/TiO2 (includes 1a conversion, 2a selectivity and TOF value) are much lower than that of Au/ZnAl–LDHs. FTIR spectra of gaseous crotonaldehyde adspecies on ZnAl–LDHs and Au/ZnAl–LDHs were tested and added into Figure 4a. The first two bands at 1688 and 1668 cm−1 can be assigned to ν(C=C) of crotonaldehyde adsorbed on the catalyst, and the last band at 1641 cm−1 can be assigned to ν(C=C) of crotonaldehyde. These results are quite assistant with other reference, which indicates that crotonaldehyde was adsorbed on both two catalysts.44 After the catalytic hydrogenation reaction, we recycled the Au/ZnAl–LDHs by filtration, rinsing and baking process. The recycled Au/ZnAl–LDHs was re–used to repeat the same reaction under visible irradiation, the result of which is shown in Figure S3. We can easily make the conclusion that 1a conversion and 2a selectivity remained at high rates of 97% and 94% even after 5 recycles, respectively. Besides CDE, several other α,β–unsaturated aldehydes were also studied under the optimized condition, and the results are summarized in Table 2. All the α,β–unsaturated aldehydes tested showed photoinduced selective hydrogenation over Au/ZnAl–LDHs with high selectivity toward the production of corresponding unsaturated alcohols. Aromatic unsaturated aldehydes (entries 5−10) basically exhibited higher activity than the aliphatic unsaturated aldehydes (entries 2−4, except enter 1 for CDE), likely due to the difference in the conjugation effects from the unsaturated carbon bonds.45 The summary of reaction conditions and catalytic activities of selective hydrogenation of CDE to COL over various supported catalysts are listed in Table S1. Compared with the results listed in Table S1, the overall


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performance of CDE hydrogenation to COL catalyzed by Au/ZnAl–LDHs is better the most based on the view of moderate experimental condition and high catalytic activity. 3.3. Kinetics equation and activation energy According to classical kinetic model, the kinetic equation of selective hydrogenation of crotonaldehyde (CDE) to crotonyl alcohol using Au/ZnAl–LDHs under irradiation can be described as follows:

r=−

dPH 2

dt

=−

dcCDE m PHn2 (1) = kcCDE dt

The conversion of CDE under the condition of different temperatures (from 30 oC to 60 oC), pressures (0.10 MPa to 0.25 MPa) and different reaction times, which is given in Table S2–S5 in supporting material. In order to calculate the reaction order of CDE, we maintain the pressure of hydrogen during the reaction, thus, equation (1) can be transformed as equation (2): −

dcCDE m = k ′cCDE (2), k ′ =kPH 2 dt

Figure S4 shows the relationship between 1n cCDE and reaction time under different temperature and pressure. From which we can see that all the plots can be fitted as well straight lines, and this is the typical characteristic of first order reaction. That is to say, the reaction order of CDE is 1, or m=1. And then, at temperature of 60 oC, we keep the concentration of CDE and make the pressure of hydrogen is the only variable, thus, equation (1) can be transformed as equation (3):

−

dPH 2

dt

m = k ′′PHn2 (3), k ′′ = kcCDE



k’’ and n can be calculated by using initial rate (r0) method: the expression of

dcCDE/dt can be obtained from first–order derivative of nonlinear fitting curve of cCDE VS t; set t=0, r0 value can be calculated. All parameters of nonlinear fitting equation are given in Table S6.

r0 = k ′′PHn2 (4) The plots and fitting curves of ln r0 VS ln PH 2 based on equation (4) are given in Figure S5, which indicate there is well linear relationship between ln r0 and ln PH 2 . The slope of fitting line is 0.4983, which means n≈0.5, or the reaction order of hydrogen in CDE hydrogenation is 0.5. Thus, in this experiment, the kinetic equation of selective hydrogenation of crotonaldehyde (CDE) to COL can be described as equation (5):

r = kcCDE PH0.52 (5) In addition, under the condition of PH2=0.2 MPa and reaction temperature is at the range of 30 oC–60 oC, the reaction rate constant k value is calculated and given in Table S7. According to Arrhenius equation, the activation energy of selective hydrogenation of CDE to COL catalyzed by Au/ZnAl–LDHs in dark and under irradiation is calculated and given in both Figure 4b. It indicates that irradiation can not only remarkably increase the reaction rate of selective hydrogenation of CDE to COL, but also obviously decrease the activation energy of the reaction system.

3.4. Contribution of different wavelength ranges of visible light onto photocatalytic efficiency In order to prove the enhancement of photocatalytic property of Au/ZnAl–LDHs is due to the supporting of Au, the dependence of the catalytic performance on the wavelength range of light was investigated. A series of optical lowpass filters were


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used to block light below specific cut–off wavelengths. During the experimental procedures, we block the certain range of light and let the other wavelength of light pass through. The visible light range is divided as 400–450 nm, 450–500 nm, 500–600 nm and 600–800 nm. For example, the result of 800–450 nm means that light from 400–450 nm is blocked. So, in that case, the contribution of 400–450 nm equals 99% (400–800 nm) subtract 89.4% (800–450 nm) and that is 9.6%. The result is illustrated in Figure 5. From Figure 5a we can see that the CDE conversion of Au/ZnAl−LDHs within each wavelength range was about 9.6% of the total conversion for 400−450 nm, 13.7% for 450−500 nm, 49.6% for 500−600 nm and 26.1% for 600−800 nm. Figure 5b shows the contribution ratio of different wavelength ranges of visible light onto the total CDE conversion. It indicates that the contribution is mainly come from the wavelength of 500−600 nm, which contributes about 50.1% (49.6%/99%) of the total CDE conversion rate, where Au NPs would strongly absorb the light due to the LSPR effect. Evidently, the light absorbed by Au NPs is the major driving force of the reaction. The conduction electrons of Au NPs gain the energy of the incident light through the LSPR effect and yields energetic electrons at the surface of nanoparticles.46 The resonance of conduction electrons with the electromagnetic field of incident light results in a significant enhancement of the local electromagnetic fields near the surface of Au NPs. The light–excited electrons can activate the adsorbed reactant molecules on the surface of Au NPs. The enhanced local field may also promote the reaction.47 Under the visible light irradiation, electrons are activated and transferred from valence band (VB) to conduction band (CB), the supported Au on the surface of ZnAl–LDHs can be used as accepter for electrons, which would influence the transposition of electrons and holes in material. This migration would result in the



separation of electrons and holes, which is benefit for not only effective inhibition of the recombination of electrons and holes, but also obvious extension of the existence of electrons. The transposition and enrichment of electrons onto the materials’ surface would inhibit the hydrogenation of C=C, which is favor the C=O activation. Figure 6 is the schematic diagram of transposition of electrons and holes in the material of Au/ZnAl–LDHs.

3.5. Theoretical calculation In this section, DFT method was used to further explain the selectivity of the CDE hydrogenation process (for simplification the calculation workload and also due to the limitation of light simulation, DFT work just focus on the hydrogenation without irradiation). The crotonyl alcohol model was built to study its adsorption behavior at the surface of the Au/ZnAl–LDHs (more details are given in the supporting material, which also contained Table S8: Adsorption energies of CDE on Au/ZnAl–LDHs surface and Figure S6: Structure of CDE and its most stable configuration on Au/ZnAl–LDHs surface). We also calculated the most possible reaction path way based on the difference between partial and complete hydrogenation, the calculation offers the theoretical explanation for the reaction mechanism. 3.5.1. Partial Hydrogenation Mechanism Based on our results and references,48 we concluded the CDE hydrogenation as Figure 7a shows that the hydrogenation of CDE could produce the following compounds: COL, BDE and ENOL. Based on different hydrogenation sites and products, the partial hydrogenation mechanism has three different possibilities, which are 1,2(2,1)–addition (mechanism A), 3,4(4,3)–addition (mechanism B) and 1,4(4,1)–addition (mechanism C). We optimized the structure of the reactants and products of the major elementary reactions, and then did transition state (TS) search


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using the stable configurations after optimization. Table 3 shows the activation energy (Ea) and energy change of all the elementary reactions (∆E). The product of mechanism A is COL. Based on the order of hydrogenation; it could be further defined as A1 and A2 mechanisms. In the first step of A1 mechanism, the hydrogenation generates 43.8 kJ/mol energy with Ea=35.1 kJ/mol, the second step creates 47.5 kJ/mol with Ea=150.7 kJ/mol. In A2 type of reaction, the 1st step generates 10.2 kJ/mol with Ea=70.3 kJ/mol and the second step outputs 81.0 kJ/mol with Ea=28.4 kJ/mol. Figure 8a is the energy change histogram, from which we can see that A2 requires lower activation energy compared to A1 reaction, which makes the reaction favoring A2 more than A1. The 1st step hydrogenation of A2 needs high activation energy compared to the second step, therefore make the 1st step of elementary reaction the speed control step. Also, it is easy to tell this reaction is exothermic reaction and lower temperature would help the reaction. BDE is the product from mechanism B. This mechanism would be also further illustrated by B1 and B2 mechanisms. In B1 mechanism, the first hydrogenation step gives out 32.5 kJ/mol with Ea=240.2 kJ/mol; the second step release 124.7 kJ/mol after overcoming the Ea=301.7 kJ/mol. In B2 mechanism, the reaction only produces 0.4 kJ/mol under Ea=288.1 kJ/mol during the first step and followed by creating 156.9 kJ/mol with the Ea=220.3 kJ/mol. Figure 8b summarizes the mechanism B energy change. We can see that every mechanism has to overcome the activation energy higher than 220.3 kJ/mol, which makes the reaction very difficult. Reaction B also favors a lower temperature to shift the reaction equation to the right side. Mechanism C is the model to produce ENOL. It can be further illustrated as C1 and C2 mechanisms. During C1 mechanism, the reaction heat from the first step is 43.8 kJ/mol with an activation energy of Ea=35.1 kJ/mol. The second step needs to excel



and activation energy of 312.2 kJ/mol before generating 89.6 kJ/mol heat. In C2, the first step generates a low energy of 0.4 kJ/mol with a high Ea=288.1 kJ/mol; the second step has a much lower Ea of 42.6 kJ/mol and reaction heat of which is 133.0 kJ/mol. Figure 8c is the energy change of mechanism C, from which we can see that the hydrogenation of C4 on CDE requires an activation energy higher than 288.1 kJ/mol, this makes mechanism C very difficult from happening. Similar with A and B, this reaction also prefers lower temperature. Based on above mentioned analysis, for partial hydrogenation reaction mechanism, the reaction is exothermic, thus lower temperature will help the reaction moving forward. The possibilities for these 3 mechanisms are in the order of A>B>C. In other words, the most possible product from partial hydrogenation is COL, followed by ENOL and BDE should be the least favored product. Between A1 and A2, we can say that most likely the reaction will follow A2 and generate COL due to its lower activation energy gap. 3.5.2. Complete Hydrogenation Mechanism Based on 3.3.1, CDE will create mostly COL when following the partial hydrogenation mechanism, the further hydrogenated to form BOL to complete the whole hydrogenation reaction. We further studied the CDE complete hydrogenation mechanism in order to see if COL could exit stably. The complete mechanism is further detailed into 2,1,4,3–addition (D1) and 2,1,3,4–addition (D2) sub–mechanisms, as shown in Figure 7b. In addition, Table 4 is the summary of the activation energy (Ea) and energy change during the elementary reactions (∆E) under the complete reaction mechanisms. D2 requires an activation energy 17.7 kJ/mol and 18.5 kJ/mol lower than D1 for the COL hydrogenation, according to the data in Figure 8d. Therefore, D2 is favorable


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during the complete hydrogenation. The total reaction heat of this reaction is 196.5 kJ/mol. For D2 reaction, the first two steps require fairly low activation energy of 70.3 kJ/mol and 28.4 kJ/mol, but the followed 2 steps need the activation energy of 282.6 kJ/mol and 244.8 kJ/mol. The high activations make the third and fourth step very difficult, therefore we can say that Au/ZnAl–LDHs catalyst could avoid further COL hydrogenation. 3.5.3. Summary of CDE Selective Hydrogenation Mechanism As we discussed above, the reaction would mostly follow A2 during partial hydrogenation, the C2 of CDE will be hydrogenated first to form MS1 before going through the next TS2 step to finally become COL. When following the complete hydrogenation mechanism, the C3 will be hydrogenated firstly before becoming the intermediate MS5, which will go through TS4 after further hydrogenation before becoming the final product BDE. Comparing these 2 mechanisms, D2 needs higher activation energy which makes A2 mechanism favorable. The most possible reaction of CDE hydrogenation will follow A2 and the final product is COL. Au/ZnAl–LDHs catalyst shows the excellent selectivity towards C=O and also could avoid further hydrogenation of COL, this brings high COL yields, which is consistent with our previous experimental result.

4. CONCLUSIONS The visible light irradiation dramatically enhanced the activity and selectivity of Au/ZnAl–LDHs towards the production of crotonyl alcohol. The reactivity of crotonaldehyde hydrogenation and selectivity of crotonyl alcohol both have been greatly improved under visible light irradiation, from 42% to 99% for conversion of CDE and from 59.5% to 96% for COL selectivity, compared with no irradiation, when Au/ZnAl–LDHs was used as photocatalyst. Kinetics studies indicate that visible light



irradiation can not only remarkably increase the reaction rate of selective hydrogenation of CDE to COL, but also obviously decrease the activation energy of the reaction system. The wavelength of 500−600 nm contributes over 50% of the total CDE conversion rate, where Au NPs would strongly absorb the light due to the LSPR effect, which could prove the enhancement of photocatalytic property of Au/ZnAl–LDHs is due to the supporting of Au. Based on the DFT results, the most possible reaction of crotonaldehyde hydrogenation will follow A2 and the final product is crotonyl alcohol. Au/ZnAl–LDHs catalyst shows the excellent selectivity towards C=O and could also avoid further hydrogenation of crotonyl alcohol, this brings high crotonyl alcohol yields, which is consistent with the experimental result.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acs.jpcc.xxxxxx. Experimental details about the materials’ synthesis as well as additional characterization, kinetic fitting and DFT calculation.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S.J. Xia); [email protected] (Z.M. Ni)

ORCID Shengjie Xia: 0000-0002-3601-0927

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation of China (21503188) and National College Students Innovation and Entrepreneurship Training Program for


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the year 2017.

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Table 1 Selective hydrogenation of crotonaldehyde catalyzed by different samples.

Condition Conv. of 1a Yield of 2a Select. of 2a TOF (%) (%) (%) (h–1) dark 31 12 38.7 – ZnAl–LDHs light 39 17 43.6 – dark 42 25 59.5 101 Au/ZnAl–LDHs light 99 95 96.0 272 dark 11 3 27.3 – p25 TiO2 light 19 6 31.6 – dark 38 15 39.5 81.5 Au/TiO2 light 74 56 75.7 199 Reaction conditions: 50 mL 1,4–dioxane/water (volume ratio is 1:1) mixture contained 0.1 mol/L crotonaldehyde, 50 mg catalyst, 60 oC, 4 h, 0.25 MPa. TOF was calculated after 20 min reaction when conversion is low, by the following equation: TOF (h−1) = (converted CDE (mmol)) / ((total Au amount (mmol)) (reaction time (h))). Sample


Page 26 of 38

Page 27 of 38 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2 Visible–light–promoted selective hydrogenation of α,β-unsaturated aldehydes over Au/ZnAl–LDHs catalyst. Entry

Substrate

Product

Conv.(%)

Select.(%)

TOF(h-1)

1

99

96

272

2

82

80

149

3

78

87

103

4

81

78

135

5

100

100

436

6

98

98

237

7

88

95

205

8

86

93

196

9

95

98

232

10

93

95

218

Reaction conditions: 50 mL 1,4–dioxane/water (volume ratio is 1:1) mixture contained 0.1 mol/L α,β-unsaturated aldehydes, 50 mg catalyst, 60 oC, 0.2 MPa; 300 W Xenon lamp (400 nm＜λ＜800 nm) with light intensity of 1.0 W/cm2. The reaction time was 4 h, except for entry 5 (3 h), entries 3 (6 h). TOF was calculated after 20 min reaction when conversion is low, by the following equation: TOF (h−1) = (converted aldehydes (mmol)) / ((total Au amount (mmol)) (reaction time (h))).



Page 28 of 38

Table 3 Activation energy (Ea) and reaction energy (∆E) of main elementary reactions for the partial hydrogenation of CDE. Reaction

Ea/(kJ·mol−1) ∆E/(kJ·mol−1)

Mechanism

CDE*+H*→MS2*+*

35.1

–43.8

A1

MS2*+H*→COL*+*

150.7

–47.5

Mechanism

CDE*+H*→MS1*+*

70.3

–10.2

A2

MS1*+H*→COL*+*

28.4

–81.0

Mechanism

CDE*+H*→MS4*+*

240.2

–32.5

B1

MS4*+H*→BDE*+*

301.7

–124.7

Mechanism

CDE*+H*→MS3*+*

288.1

–0.4

B2

MS3*+H*→BDE*+*

220.3

–156.9

Mechanism

CDE*+H*→MS2*+*

35.1

–43.8

C1

MS2*+H*→ENOL*+*

312.2

–89.6

Mechanism

CDE*+H*→MS3*+*

288.1

–0.4

C2

MS3*+H*→ENOL*+*

42.6

–133.0

CDE+*→CDE*

–

–93.7

COL *→COL +*

–

51.6

ENOL *→ENOL +*

–

56.2

BDE *→BDE +*

–

60.9

* represents a single surface site occupied by the corresponding adsorbent


Page 29 of 38 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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 4 Activation energy (Ea) and reaction energy (∆E) of main elementary reactions for the full hydrogenation of CDE. Reaction Ea/(kJ·mol−1) ∆E/(kJ·mol−1) CDE+*→CDE*

–

–93.7

CDE*+H*→MS1*+*

70.3

–10.2

Mechanism

MS1*+H*→COL*+*

28.4

–81.0

D1

COL*+H*→MS6*+*

300.3

23.9

MS6*+H*→BDE*+*

263.1

–217.2

CDE*+H*→MS1*+*

70.3

–10.2

Mechanism

MS1*+H*→COL*+*

28.4

–81.0

D2

COL*+H*→MS5*+*

282.6

23.7

MS5*+H*→BDE*+*

244.8

–217.0

BDE*→BDE+*

–

48.4



Page 30 of 38

(a) H O C Zn

Al Au

(b)

Figure 1 Structural model of Au/ZnAl–LDHs (a) and configuration and relative energy of four isomers of CDE (b).


Page 31 of 38

009/012

Au/TiO2

110 113

1.4

*220

018 *200

*111

(b)

1.6

TiO2 015

006

(a)

ZnAl-LDHs Au/ZnAl-LDHs TiO2

1.8

Absorbance

003

2.0

ZnAl-LDHs Au/ZnAl-LDHs Au/TiO2

*Au

Intensity (a.u.)

1.2 1.0 0.8

0.4 0.2

116 220

*200

204

004

105 211

101

*220

0.6

004/*111

200

0.0 -0.2

10

20

30

40

50

60

70

200

300

400

Two Theta (degree)

500

600

700

800

Wavelength (nm)

(c)

(d)

40

Frequency (%)

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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 20 10 0

0

4

8

12

16

20

Particle Size (nm)

(e)

(f)

0.244 nm Au (111)

3 2

1

4

Figure 2 Characterization of different of catalysts: (a) XRD curves; (b) UV–vis curves; (c) TEM image of ZnAl–LDHs; (d) TEM image of Au/ZnAl–LDHs; (e) HRTEM image and (f) Electron diffraction pattern (SAED) of Au/ZnAl–LDHs. Note: inset pictures for (d) is the size distribution curves of the sample.



Figure 3 STEM images with elemental mapping and EDS intensity line profiles for Au/ZnAl–LDHs.


Page 32 of 38

Page 33 of 38

(a)

Absorbance

ZnAl-LDHs Au/ZnAl-LDHs

1750

1700

v (C=C) 1641

1668

1689

v (C=O)

1800

1650

1600

1550

-1

Wavenumber (cm ) 1.2

(b)

ln k= -3749 1/T + 12.34 2 R =0.9874 Ea=31.17 kJ/mol

0.9 0.6

Dark Light

0.3 0.0

ln k

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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ea=50.31 kJ/mol -0.3 -0.6

ln k= -6051 1/T + 18.70 2 R =0.9712

-0.9 -1.2 -1.5 0.0030

0.0031

0.0032

0.0033

-1

1/T (K )

Figure 4 FTIR spectra of gaseous crotonaldehyde adspecies on ZnAl–LDHs and Au/ZnAl–LDHs (a); fitting of activation energy of selective hydrogenation of crotonaldehyde to crotonyl alcohol using Au/ZnAl–LDHs under the condition of dark and irradiation (b).



140

49.6%

50

Conversion of CDE (%)

Conversion of CDE (%)

120

100

80

30

(a)

800-600 nm 800-500 nm 800-450 nm 800-400 nm

40

26.1%

20

99.0%

13.7% 10

9.6%

89.4%

0 400-450 nm 450-500 nm 500-600 nm 600-800 nm

75.1%

60

40

26.1%

20

0 0

30

60

90

120

150

180

210

240

270

Time (min) 60

1.2

(b) 500-600

50

50.1%

0.8

0.6

40

600-800

20

0.4

450-500 400-450 13.8% 9.7%

0.2

0.0 200

30

300

400

500

26.4% 10

600

700

Ratio of CDE Conversion (%)

1.0

Absorbance

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 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 38

0 800

Wavelentgh (nm)

Figure 5 The curves of crotonaldehyde conversion (a) and the ratio of whole CDE conversion (b) for different wavelength ranges of visible light using Au/ZnAl–LDHs as catalyst.


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Figure 6 The schematic diagram of transposition of electrons in the material of Au/ZnAl–LDHs.



(a)

(b)

Figure 7 Different reaction mechanisms for the partial hydrogenation (a) and the full hydrogenation (b) of CDE.


Page 36 of 38

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Figure 8 Schematic diagram for potential relative energy of reaction mechanisms: (a) to (d) represent mechanism A to D, respectively.



TOC Graphic


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