Conversion of Isobutene and Formaldehyde to Diol using

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Conversion of Isobutene and Formaldehyde to Diol Using Praseodymium-doped CeO2 Catalyst Zhixin Zhang, Yehong Wang, Jianmin Lu, Chaofeng Zhang, Min Wang, Mingrun Li, Xuebin Liu, and Feng Wang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02134 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 4, 2016

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ACS Catalysis

Conversion of Isobutene and Formaldehyde to Diol Using atalyst Praseodymiumraseodymium-Doped CeO2 Catalyst

Zhixin Zhang,a,b Yehong Wang,a Jianmin Lu,a Chaofeng Zhang,a Min Wang,a Mingrun Li,a Xuebin Liu,c and Feng Wang*a a

State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, P. R. China. b State Key Laboratory of Fine Chemicals, Faculty of Chemical Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116024, P. R. China c Energy Innovation Laboratory, BP Office (Dalian Institute of Chemical Physics), Dalian 116023, P.R. China *Corresponding author. Tel: +86-411-84379762; Fax: +86-411-84379798; E-mail: [email protected]

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ABSTRACT: BSTRACT: Conversion of low-carbon olefins to higher ones via the formation of C–C bond is an increasingly concerned topic. We herein report an example of converting isobutene and formaldehyde (aqueous solution of 38 wt%) to 3-methyl-1,3-butanediol (MBD), a precursor for isoprene. The reaction occurs through a Prins condensation-hydrolysis reaction over a praseodymium (Pr)-doped CeO2 catalyst. The best 70% MBD yield is achieved over the Pr-doped CeO2 catalyst. Catalyst characterizations with high-angle annular dark field-transmission electron microscopy (HAADF-TEM), pyridine adsorption infrared (IR), Raman spectroscopy, and density functional theory (DFT) calculations show that the doped Pr is uniformly and highly dispersed in the CeO2 crystalline phase. Besides the Pr-doping creates more oxygen vacancy sites on CeO2 and thus enhances the Lewis acidity of the catalyst, which is responsible for the catalytic performance of the Pr-CeO2 catalyst. Keywords: CeO2 · metal-doped · Prins condensation · hydrolysis · Lewis acid · oxygen vacancy


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Introduction Alkanediols are important chemical intermediates for polymers1,2 and surfactants.3,4 Dehydration of alkanediols, such as 3-methyl-1,3-butanediol (MBD), produces isoprene for synthesizing rubber.5-7 Schneider et al. ever studied the selective hydroxylation of isoamyl alcohol using p-nitroperbenzoic acid as catalyst and obtained 5% yield of MBD as a minor product.8 MBD can also be synthesized by Prins condensation of isobutene and formaldehyde, but it was only obtained as byproduct with the yield of < 10% using an FeMCM-41 catalyst.9 Up to now, no study targeting at MBD as the major product has ever been successfully achieved from isobutene and formaldehyde. This require a catalyst which contains water-tolerant acidic sites and the acidity should be suitably strong enough to activate isobutene and formaldehyde and also avoid catalyze their polymerization. In aspiring for sustainable route for petroleum-based chemicals, it is interesting to extend the biomass product chain from C1–C2 alcohols to more functionalized alcohols, such as C4+ alcohols. For example, isobutene can be obtained from bioethanol10-13 via sugar platforms.14-18 Formaldehyde is industrially produced by oxidation of methanol.19,20 Therefore, the success of converting isobutene and formaldehyde to MBD can fill the gap between sustainable biomass and valuable chemicals (Scheme 1).

Gas Dehydration

Biomass

Coal

CH3OH

HCHO

This work

Prins Condensation

+ Chemical intermediates for polymers and surfactants

Oil

Scheme 1. 1. MBD production based on the biomass.

The Prins condensation of olefins and their derivatives with aldehydes can produce alkyl-m-dioxanes and alkanediols, and hydrolysis of dioxanes over acid catalysts can also give alkanediols.6,21,22 Thus, the Prins condensation forming a new C–C bond is a subject of study. Prins condensation reactions can be catalyzed by homogeneous catalysts, such as mineral acids (e.g. H2SO4) or Lewis acids (e.g. SnCl4).21 In recent


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years, heterogeneous acid catalysts (such as zeolites,23 clay,24 sulfates,25,26 phosphates,27-29 heteropolyacids30, and oxides31) have been pursued because of the reusability and operational simplicity. Recent studies by ours and others further show that the pristine CeO2 can efficiently catalyze various organic reactions.32-42 We find that the pristine CeO2 possesses weak and water-tolerant Lewis acidity (54 µmol/g). This nature makes it catalytically active in one-pot condensation-hydrolysis reaction even under hydrothermal reaction conditions, such as the synthesis of 1,3-butanediol from propene and formaldehyde.43 But the pristine CeO2 was inefficient in synthesizing MBD from isobutene and formaldehyde (merely 21% yield). Hegde, Nolan, Luo, and Fornasiero et al. report that the addition of metal dopants into the CeO2 lattice generates oxygen vacancies in CeO2.44-56 Recently, the group led by Metiu has reported a series of works on theoretical calculations of CeO2. Their results reveal that the metal-doping can increase the acidity or basicity of CeO2.57,58 But it is experimentally unknown how the metal-doping modifies the acid-base property of CeO2 and which metal is suitable for doping. Multivalent cation-doped CeO2 materials have been considered as outstanding catalysts for CO oxidation59 or soot oxidation.60-63 Among the various multivalent dopants, Pr is expected to be the most promising for dissolution into the host CeO2 to form a solid solution, due to its similar ionic radius to the Ce ions and the same fluorite structure of its oxide PrO2 as CeO2. Doping with multivalent Pr (Pr3+/4+) in CeO2-based materials is known to enhance the formation and migration of oxygen vacancies.64 And


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the oxygen vacancies can be generated and increased gradually by the addition of multivalent Pr ions up to 50 mol%.65 In the multivalent Pr-doped CeO2 material, Pr3+ ions play an important role to create oxygen vacancies and induce a local distortion, which will improve the degree of unsaturated coordination of Ce ions, and Pr4+ can contribute to diminishing reduction energy via the formation of an additional redox couple (Pr3+/4+).65 In this study we focus on this topic and employ the condensation-hydrolysis of isobutene and formaldehyde to MBD as a test reaction. We discovered that the Pr-doped CeO2 shows excellent activity and selectivity toward MBD synthesis. Combined catalyst characterizations suggest that the metal-doping created more oxygen vacancies on CeO2, and its concentration is proportional to the catalytically active acidic sites required for the isobutene-formaldehyde condensation reaction. Particularly the ionic radius of the metal dopants has distinctive influence on the density of observed oxygen vacancies and furthermore on catalytic activity. Catalytic reaction over the Pr-doped CeO2 offers 55% yield of MBD, which is 2.6 times higher than that of the undoped CeO2, and to the best of our knowledge, is the best result reported up to now.

Results and Discussion XRD and TEM Characterization of the Metaletal-doped CeO2


60

70

(331) (420)

(400)

(220)

(200)

Al-CeO2

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(311) (222)

Intensity (a.u.)

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(111)

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Zr-CeO2 Eu-CeO2 Pr-CeO2 La-CeO2 JCPDS 04-0593 CeO2 10

20

30

40

50

80

2θ (degree) Figure 1. 1. XRD patterns of the doped M-CeO2 (M = Al, Zr, Eu, Pr, or La)

The CeO2 and doped CeO2 catalysts were obtained with a precipitation and co-precipitation approach, respectively. The molar ratio of Ce to the doped metal was set to 4:1. For comparison, we selected some other doped metal, including Al, Zr, Eu, and La, based on their radii, valences, and properties. The ion radii of Al3+ (50 pm) and Zr4+ (80 pm) are smaller than that of Ce3+/4+, and they have different valences. The ion radius of Eu3+ (95 pm) is larger than that of Ce4+ (92 pm) and smaller than that of Ce3+ (103.4 pm). And the ion radius of La3+ (106.1 pm) is larger than that of Ce3+/4+. The substitution of Ce4+ by the low-valence dopant (such as Al3+, Eu3+, and La3+) generates oxygen vacancies via the charge neutrality criterion.58 Although the fact that substitution of Ce4+ by the isovalent Zr4+ is limited in terms of self-generation of oxygen vacancies for “charge compensation”, the oxygen vacancies can also be generated by large local relaxation in the crystalline lattice. And Zr-doped CeO2 is well known for various applications due to its high oxygen storage capacity.54,66 Doping of CeO2 with rare-earth elements (RE), such as Eu and La, has been widely applied in the field of catalysis, due to its unusual properties induced by oxygen vacancy defects.60,67 For brevity, metal-doped CeO2 catalysts were denoted as M-CeO2 (M = Al, Zr, Eu, Pr, or La), respectively. XRD shows all samples were indexed to be (111), (200), (220), (311), (222), (400), (331), and (420) crystal facets of the fluorite structure of CeO2 (JCPDS 04-0593) (Figure 1). No doped metal oxide phase was found, indicating the doped species were highly dispersed in CeO2. A slight peak shift of 2θ toward higher angles was observed on Al- and Zr-CeO2. The shift was probably due to the occupancy of Ce4+ (92 pm) sites by the smaller Al3+ (50 pm) and Zr4+ (80 pm) ions, causing lattice contraction.68 In contrast, the 2θ shift to smaller angles when CeO2 was doped with the


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larger metal ions Eu3+ (95 pm), Pr3+ (101.3 pm) and La3+ (106.1 pm) indicates lattice expansion.69

Figure 2. 2. HAADF-TEM EDS elemental mapping images of the Pr-CeO2 catalyst. The HAADF-TEM image (a), EDS elemental mapping images representing the Pr (b) and Ce (c).

Figure S1 shows the SEM, TEM and EDS elemental mapping results of the doped CeO2. Pr-CeO2 has a rod-like morphology as shown in TEM images (Figure S1f). HAADF-TEM EDS elemental mappings (Figure 2) show that Pr is uniformly and highly dispersed in CeO2. EDS analysis by randomly chosen eight zones indicated the Ce/Pr atomic ratios were 4.33 ± 0.04 (Figure S1g). The slight higher Ce/Pr ratio than the added value of 4 may indicate the little enriching of Ce and Pr infusion into CeO2 lattice. The HRTEM images shows the Pr-CeO2 is well crystallized and consists of crystallites of CeO2 or PrO2 (Figure S2b). The selected area electron diffraction (SAED) pattern also indicates the as-prepared Pr-CeO2 is polycrystalline and the diffraction rings were attributed to (111), (200), (311), (222), (400), and (331), respectively, which is in agreement with the XRD results (Figure S2c). Thus, no amorphous PrOx could be found in the Pr-CeO2. Furthermore, similar results were observed on Al, Zr, Eu, and La-doped CeO2 (Figure S1 and S5). Prins Condensationondensation-hydrolysis Reactivity of the M-CeO2 Catalysts Table 1: Prins condensation-hydrolysis of isobutene with formalin over CeO2 and M-CeO2 catalysts (M = Al, Zr, Eu, Pr, or La)

Entry 1 2

Catalyst CeO2 Al-CeO2

Formaldehyde Conv. (%) 27 35

Sel. (%) MBD + DMD > 99 > 99


GC yield (%) MBD

DMD 21 30

6 5

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3 4 5 6a 7

Zr-CeO2 Eu-CeO2 Pr-CeO2 Pr-CeO2 La-CeO2

41 51 67 94 53

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> 99 99 99 99 > 99

32 41 55 70 45

9 9 11 23 8

Reaction conditions: 50 mg catalyst, 1.5 mL H2O, 0.21 mL formalin (38 wt% HCHO), 3.0 g isobutene, 120 °C, 2 h. a At 135 °C, 3 h.

Catalytic results showed that the reaction of isobutene and formaldehyde gave 3-methyl-1,3-butanediol (MBD) as the major product, together with minor 4,4-dimethyl-m-dioxane (DMD) (Table 1). These two products may be obtained via a parallel mechanism reported by Dumitriu et al.70 The mechanism starts from the interaction of isobutene and HCHO to create a C5 hydroxyl-carbocation, which can react with water or a second HCHO molecule to form MBD or DMD, respectively. The reaction was conducted in water-phase. Thus, hydrolysis of DMD can also give MBD.43 Furthermore, the hydrolysis reaction is reversible.71 The overall selectivity of (MBD + DMD) was >99%. No isoprene was observed among products, which can be obtained by the dehydration of MBD in vapor-phase at high temperatures (usually >250 oC).70 Pristine CeO2 gave a relatively low formaldehyde (FA) conversion (27%) (Table 1, entry 1). All dopants increased the FA conversion in the order of Al-CeO2 < Zr-CeO2 < Eu-CeO2 < La-CeO2 < Pr-CeO2. The Pr-CeO2 was the best catalyst with a 67% conversion and 66% yield of (MBD + DMD) (Table1, entry 5). Further experiments at 135 °C using Pr-CeO2 catalyst offered 93% yield of (MBD + DMD) in 3 h (Table1, entry 6). After reaction, Pr-CeO2 was recovered by centrifugation, dried, and then used for a subsequent cycle. The catalyst retained its activity during its fourth run (Figure S6). These results indicated Pr-CeO2 was an effective and stable heterogeneous catalyst in this reaction. Correlation between Catalytic Activity Variation and Metal Dopants Prins condensation-hydrolysis is an acid-catalyzed reaction.21 The acidity of M-CeO2 catalysts is a critical factor for the reaction. Pyridine-adsorption IR (Figure S7) shows no Brønsted acidic sites were present on the doped CeO2. The presence of the strong band at 1440 cm–1, which was attributed to the coordinatively bound pyridine on Lewis acidic sites,72-74 indicated the M-CeO2 oxides contain Lewis acidic sites. Pyridine-IR showed the metal doping remarkably increased the Lewis acidity (Table 2). Particularly the Lewis acidity of Pr-CeO2 increased by 3.5 times compared with the pristine CeO2. In literatures, Sn-doping increased CeO2 acidity by 0.4–0.6;75 Zr-doping increased CeO2 acidity by 0.21–0.57;76 and Al-doping increased Ce-Zr mixed oxides acidity by 0.15–0.85.77 All these metals increase both Brønsted and Lewis acidity of CeO2. However, it is rarely known that the Pr-doping only increases Lewis acidity. Table 2: The concentration of Lewis acidic sites of CeO2 and M-CeO2 (M = Al, Zr, Eu, Pr, or La)


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Catalyst

CLa (µmol g–1)

1

CeO2

54

2

Al-CeO2

82

3

Zr-CeO2

74

4

Eu-CeO2

64

5

Pr-CeO2

188

6

La-CeO2

145

Entry

CL means the concentration of Lewis acidic sites and is calculated according to the Formula 1 (See Experimental Section). a

The oxygen vacancies of CeO2 were related to its Lewis acidity43 and were quantified by Raman spectroscopy.78 Two major Raman peaks at about 460 and 530–600 cm–1 are ascribed to the F2g symmetrical stretching vibration mode of metal-doped CeO2 in a fluorite structure and oxygen vacancies, respectively（Figure 3）.51,67,79,80 Different metal dopant results in different peak position. The assignments of Raman peaks are summarized in Table 3. The bands of F2g mode around 463 cm–1 in the pristine CeO2, Al-CeO2, and Zr-CeO2 samples are almost same. However, for the Eu-CeO2, La-CeO2, and Pr-CeO2, this F2g mode band is around 457 cm–1, which shifts to the lower frequency because of the larger ionic radius of metal ions (Eu3+ 95 pm, La3+ 106.1 pm, and Pr3+ 101.3 pm) than that of Ce4+ (92 pm). This observation agrees with the XRD results. The bands ascribed to the oxygen vacancies are located at 600 cm–1 for the pristine CeO2,81 Al-CeO2, and Zr-CeO2 samples. 82,83 And the oxygen vacancies induced band are situated at 537, 550, and 570 cm–1 for the Eu-CeO2,84 La-CeO2,67 and Pr-CeO2,65 respectively. Furthermore, two bands at about 640 and 694 cm–1 appearing on the spectra of Eu-CeO2 sample is closely correlated with crystalline Eu2O3, because of the europium (Eu) segregation on the Eu-CeO2 surface.85


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462

Al-CeO2 Zr-CeO2 Eu-CeO2 La-CeO2

Intensity (a.u.)

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570 537

600

Pr-CeO2 CeO2

640 694

550

300

400

500

600

700

800

900

1000

-1

Raman shift (cm ) Figure 3. Raman spectra of CeO2 and M-CeO2 (M = Al, Zr, Eu, La, or Pr)

Table 3: 3: Raman peaks assignment and the observed oxygen vacancy concentration of CeO2 and M-CeO2 (M = Al, Zr, Eu, La, or Pr). Entry

Catalyst

1 2 3 4 5 6

CeO2 Al-CeO2 Zr-CeO2 Eu-CeO2 La-CeO2 Pr-CeO2

F2g mode frequency –1

Oxygen vacancy –1

Cov

(cm )

frequency (cm )

A(VӦ)/A(F2g)a

463 462 466 459 453 457

600 600 600 537 550 570

0.009 0.015 0.021 0.166 0.215 2.601

a

A(VӦ)/A(F2g) means the peak area of F2g mode be divided by the peak area of oxygen vacancy, representing the observed oxygen vacancy concentration.

The observed oxygen vacancy concentration (Cov) was calculated based on the integrated peak area (A) of the bands of oxygen vacancy (VӦ) and F2g mode. The ratio of


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A(VӦ)/A(F2g) indicates the Cov (Table 3).50,86 The Al and Zr do not increase the Cov value obviously, while the Eu-, La-, and Pr-doping remarkably increase the Cov value. Among the five samples, Pr-CeO2 has the highest Cov value, 289 times than that of pristine CeO2. When Pr is doped into the CeO2, Pr3+ and Pr4+ ions are incorporated as majority and minority ions, respectively.65 And doping with Pr3+ generates the oxygen vacancies for “charge compensation”. And the oxygen vacancies could survive under the reaction environments, as proved by our previous work43, in which the Lewis acidic sites induced by oxygen vacancy over CeO2 surface is water-tolerate and can catalyze the Prins condensation-hydrolysis of propene with formalin. But the other dopants seem to be not good at introducing the oxygen vacancies. Under the same doping level and synthesis conditions, for the Al and Zr-CeO2, the dopant may be incorporated as interstitial ions rather than incorporated into the lattice of CeO2 due to their small radii compared to their host cations.87 Thus, the Cov values increase un-obviously. The concentration of oxygen vacancies over Eu-CeO2 sample increases but cannot compare with that of Pr-CeO2, due to the aggregation of Eu observed by Raman spectra. For the La-CeO2 sample, the Cov value also increase but less than the value of Pr-CeO2, the possible reason is the larger radius of La than that of Ce3+/4+.

Figure 4. Catalytic activity for Prins condensation-hydrolysis of isobutene and formalin over M-CeO2 and its relation to the ionic radius of the dopant and the concentration of Lewis acidic site (unit: mmol/g) and oxygen vacancy.


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A plot of the conversion of FA, the C L, and the Cov against the dopant ionic radius is also shown in Figure 4. The concentration of Lewis acidic sites (CL) and the conversion of FA against the doped metal ionic radius had a similar relationship, indicating that CL is vital for the condensation-hydrolysis of isobutene with formalin and the value can be modulated by different metal doping. The Cov has a similar trend as C L. The consistency apparently reveal that the observed oxygen vacancy of the doped CeO2 is related to its Lewis acidity. Based on our previous studies36,43 and others,32,33 we propose that the coordinatively unsaturated Ce cations next to the oxygen vacancies function as Lewis acidic sites for the Prins condensation-hydrolysis reaction. DFT Calculations alculations For the doped oxides, it comes with a question that what is the nature of catalytic active sites: the doped phase or the parent oxide phase? It is rather difficult to experimentally tackle the issue. Density functional theory (DFT) calculations may shed light on this issue. Formaldehyde adsorption and activation is a key step for the condensation.88,89 We constructed several adsorption models of HCHO on CeO2(111) and Pr-CeO2(111) with one oxygen vacancy, respectively and calculated the adsorption energy of HCHO. In some studies,90,91 solvation could improve the stability of transition states and affect the calculated adsorption energy of substrates. In the present study, when we compared the calculated adsorption energy of HCHO on CeO2 and Pr-CeO2, the effect should exhibit the similar trend for these two catalysts and the net difference value between these two adsorption models should be constant. As shown in Figure 5, HCHO is adsorbed on the (111) crystal face of CeO2 with one oxygen vacancy, which is optimized on the Lewis acid-base pair of Ce–O (Figure 5a) next to the oxygen vacancy with an adsorption energy of -1.39 eV. When the Pr is doped into the lattice of CeO2, the calculated adsorption energy of HCHO on Pr-CeO2 on the same adsorption site is -1.60 eV, which is 0.21 eV lower than that on the CeO2 surface and is found to be the lowest state. In comparison, the adsorption energy of HCHO on Pr–O site of Pr-CeO2 is -1.33 eV, which is close to that on the CeO2 surface. Next, the bond length of carbonyl group in HCHO adsorbed on Pr-CeO2 is also slightly larger (1.391 Å) than the value of the HCHO (1.370 Å) adsorbed on CeO2 due to the different interaction between the carbonyl group in HCHO and the Lewis acid-base pair (Ce–O). These two values are larger than that of the isolated HCHO (1.215 Å), also indicating the CeO2-based material can be a good catalyst of the activation of HCHO. These DFT calculation results demonstrate an obvious promoting effect of the highly dispersed Pr dopant on CeO2 surface for the adsorption and activation of HCHO. It is known that the presence of oxygen vacancies can promote the interaction of aldehydes with oxide surfaces via the C=O group.92 We have also considered the adsorption model of HCHO at oxygen vacancy sites of CeO2 and Pr-doped CeO2 catalyst and calculated the adsorption energies of HCHO. The adsorption energies are -1.13 eV and -0.85 eV, respectively, which are weaker than that of HCHO on the Ce–


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O sites next to the oxygen vacancies. Thus, the adsorption model of HCHO on the Ce– O sites is optimized. Therefore, the increased quantity of oxygen vacancies enhances the interaction (activation) of HCHO with oxide surface via the effect on the Ce–O sites next to the oxygen vacancies, and thus the reactivity with isobutene. Prins condensation of olefin with HCHO is a bimolecular reaction. In heterogeneous catalysis, two main reaction mechanisms proposed are the Langmuir-Hinshelwood (LH) mechanism and the Rideal-Eley (RE) mechanism.93-95 For the vast majority of surface catalytic reactions, the LH mechanism is preferred.93 In this study, the adsorption of isobutene was also considered. As shown in Figure 6, isobutene is adsorbed on the oxygen vacancy over CeO2 and Pr-CeO2 with an adsorption energy with -0.72 and -0.28 eV, respectively. The adsorption of isobutene is 0.67 eV weaker than that of HCHO on CeO2, and 1.32 eV weaker on the Pr-CeO2 surface. This reveals that the Prins condensation may proceed via the RE mechanism in the catalytic system. Similar calculation results were obtained by Fu et al., who found the reaction between HCHO and propylene over MgY zeolite proceeded via the R-E mechanism.89 Thus, the adsorption and activation of HCHO is the vital step for the Prins reaction.

Figure 5. Summary of adsorption models (top views) of HCHO on CeO2(111) and Pr-CeO2(111). Adsorption energies (eV) and bond length (Å) are labeled in the figure. Atom colors: H = white; C = black; O = red; Ce = pale yellow; Pr = green. (a) HCHO adsorbed on O-Ce site over CeO2. (b) HCHO adsorbed on O-Ce site over Pr-CeO2. (c) HCHO adsorbed on O-Pr site over Pr-CeO2.


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Figure 6. Summary of adsorption models of isobutene on CeO2(111) (top view) and Pr-CeO2(111) (top and sided views). Adsorption energies (eV) are labeled in the figure. Atom colors: H = white; C = black; O = red; Ce = pale yellow; Pr = green. (a) Isobutene adsorbed on oxygen vacancy over CeO2. (b) HCHO adsorbed on oxygen vacancy over Pr-CeO2.

Conclusions In summary, we have demonstrated that Pr-doped CeO2 acts as a reusable and efficient heterogeneous catalyst for the 3-methyl-1,3-butanediol synthesis from isobutene and formalin via the Prins condensation-hydrolysis reaction. This route is both scientifically and industrially meaningful for synthesizing higher and complex alcohols from sustainable biomass. HAADF-TEM, pyridine adsorption IR, and Raman spectroscopy indicate that the superior catalytic activity can be ascribed to the enhanced Lewis acidity of catalyst induced by the more oxygen vacancy sites via the Pr-doping. DFT calculations show that the adsorption and activation of HCHO is the vital step for the Prins reaction and the Pr dopant on the CeO2 surface promotes the adsorption and activation of HCHO. Experimental Section Chemicals and Reagents All chemicals were of analytical grade, purchased from Aladdin Chemicals, and used without further purification. Preparation of CeO2 and the Metaletal-doped CeO2 Catalysts The CeO2 sample was prepared by a conventional precipitation method.43 Briefly, 5.0 g of Ce(NO3)3·6H2O was dissolved in 100 mL of Millipore-purified water (18 mΩ·cm), and the solution was adjusted to pH = 11.0 by the addition of NH4OH (3.4 M) under magnetic stirring at room temperature. The resulting gel mixture was washed with pure water, dried in an oven at 120 °C for 12 h, and calcined at 550 °C in air (50 mL min–


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1

) for 4 h. The metal-doped CeO2 catalysts (Al, Zr, Eu, La, or Pr-doped CeO2) were prepared by a co-precipitation method. Ce(NO3)3·6H2O and the corresponding metal nitrates (Al(NO3)3·9H2O, ZrO(NO3)2·6H2O, Eu(NO3)3·6H2O, La(NO3)3·6H2O, or Pr(NO3)3·6H2O) were used as precursors. Ce(NO3)3·6H2O and doped-metal nitrate were dissolved by deionized H2O. The ammonia aqueous was added drop-wise to the above solutions under stirring conditions. Consequently, the mixture was stirred for 2 h. Then the mixture was filtered and dried at 120 °C overnight, and then calcinated at 550 °C in air (50 mL min–1) for 4 h to obtain the final materials. The molar ratio of Ce to added metal was about 4:1. For brevity, metal-doped CeO2 catalysts were denoted as M-CeO2 (M = Al, Zr, Eu, La, or Pr), respectively. Catalyst Phase Analysis by Powder XX-ray Diffraction Powder X-ray diffraction patterns are conducted on a PANalytical X-Pert PRO diffractometer, using Cu-Kα radiation at 40 kV and 20 mA. Continuous scans are collected in the 2θ range of 5 ~ 80o. Prins Condensationondensation-hydrolysis Reaction Formaldehyde (38% formalin, 0.21 mL, 3.0 mmol of formaldehyde), catalyst (50 mg), water (1.5 mL), and a magnetic stir bar were loaded into a 15 mL Teflon-lined autoclave reactor. The reactor was then sealed and placed in a preheated red copper mantle at the desired temperature. Isobutene (99.99%) was supplied from a cylinder and charged into the reactor and its amount was based on the difference of weight. Products were analyzed by gas chromatography−mass spectrometry (GC−MS) using an Agilent 7890A/5975C instrument equipped with an HP-5MS column. The formaldehyde solution was analyzed by a GC instrument equipped with a packed column and a TCD detector. An external standard method was used to quantify the formaldehyde conversion. Acidity Measurement by Pyridineyridine-IR Spectra Quantification of acidity by the pyridine-adsorption IR method was conducted on a Bruker 70 IR spectrometer. The sample was pressed into a self-supporting disk (13 mm diameter, 21.4‒38.3 mg) and placed into a homemade IR cell attached to a closed glass-circulation system. Prior to pyridine-adsorption, the sample disk was pretreated by heating at 350 °C for 30 min under Ar and for 30 min in vacuum (pressure < 10‒2 mbar) and then cooled to 30 °C. After a spectrum was collected, the sample disk was exposed to pyridine vapor. IR spectra of the chemisorbed pyridine were recorded after evacuation at 150 °C for 30 min to eliminate physically adsorbed pyridine. Spectra were collected after cooling to 30 °C. The calculation of the number of Lewis acidic sites was based on the following formula:96 = 1.42 × IA × ∕ (Formula 1) Where C is the concentration [mmol (g of catalyst)‒1], IA is the integrated absorbance of the L band (cm‒1), R is the radius of the catalyst disk (cm), and W is the mass of the disk (mg). Raman Characterization


ACS Catalysis

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Raman spectra were recorded on a micro-Raman spectrometer (Renishaw) equipped with a CCD detector using a He/Ne laser with a wavelength of 532 nm. DFT Calculation Settings The periodic plane wave based density functional theory program VASP (Vienna ab initio simulation package)97,98 has been employed to perform all the DFT calculations. The electron-ion interactions are described by the projector-augmented wave method (PAW), which is a frozen core all-electron method using the exact shape of the valence wave functions instead of pseudo-wave functions.99 The exchange correlation energy has been calculated within the generalized gradient approximation by the Perdew-Burke-Ernzerhof formulation (GGA-PBE).100,101 The kinetic energy cutoff of plane-wave basis sets was fixed to 500 eV in all calculations. 2s22p4 electrons of O, 5s25p64f5d6s2 of Ce and 5p64f36s2 of Pr are explicitly taken into consideration. We used DFT+U corrections102 to describe the CeO2 with the value of U-J = 4.5 eV103 for Ce 4f orbitals and U-J = 6.0 eV104 for Pr 4f orbitals. This level of theory computes the lattice constant of a = 5.495 Å for bulk CeO2, in reasonable agreement with the experimental value (a = 5.411 Å).105 This lattice constant has been used to construct a periodic CeO2 (111) p (3 × 3) slab with 9 atomic layers (which are equal to 3 stoichiometric layers) separated by a vacuum layer of 15 Å to eliminate interactions between the slab and its periodic images. In total, there are 27 Ce atoms and 54 O atoms in the slab. The bottom 4 atomic layers are fixed to their optimized bulk configuration during all computations, and the top 5 atomic layers and surface intermediates are fully relaxed. All atomic coordinates of the adsorbates and the atoms in the relaxed layers are optimized to a force of < 0.03 eV/Å on each atom. All self-consistent field calculations are converged to 1 × 10−5 kJ/mol. Brillouin zone integration is performed using a 3 × 4 × 1 Monkhorst−Pack grid and a Gaussian smearing of 0.05 eV. ASSOCIATED ASSOCIATED CONTENT Supporting Information Available: Description of TEM-EDX and HAADF-TEM characterization, Catalyst recycling tests, and pyridine adsorption IR spectra of M-CeO2. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements This work was supported by the National Natural Science Foundation of China (21403216, 21233008 and 21303189) and by Strategic Priority Research Program of Chinese Academy of Sciences (XDB17020300). Computing resources from National Supercomputing Center in Shenzhen, China and National Supercomputing Center in Tianjin, China are gratefully acknowledged. References


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TOC Graphic


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Conversion of Isobutene and Formaldehyde to Diol using

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