The Aldolization Nature of Mn4+-Nonstoichiometric Oxygen Pair Sites

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The Aldolization Nature of Mn -Nonstoichiometric Oxygen Pair Sites of Perovskite-type LaMnO in the Conversion of Ethanol 3

Ren-Kai Chen, Ting-Fang Yu, Meng-Xun Wu, Tai-Wei Tzeng, Po-Wen Chung, and Yu-Chuan Lin ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02269 • Publication Date (Web): 21 Jul 2018 Downloaded from http://pubs.acs.org on July 22, 2018

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ACS Sustainable Chemistry & Engineering

The Aldolization Nature of Mn4+-Nonstoichiometric Oxygen Pair Sites of Perovskite-type LaMnO3 in the Conversion of Ethanol Ren-Kai Chen1, Ting-Fang Yu1, Meng-Xun Wu2, Tai-Wei Tzeng2, Po-Wen Chung 2,*, and YuChuan Lin1,* 1

Department of Chemical Engineering, National Cheng Kung University, No. 1 University Road,

Tainan 70101, Taiwan 2

Institute of Chemistry, Academia Sinica, 128 Academia Road, Section 2, Nankang, Taipei 115,

Taiwan *E-mails: [email protected] (P.W.C.); [email protected] (Y.C.L.)

ACS Paragon Plus Environment

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ABSTRACT

This study reports that the C2-C2 aldolization in ethanol conversion to C4 products, particularly butadiene, can be catalyzed by silica-supported LaMnO3 catalysts. The concentration and strength of Mn4+ was discovered to be related to the particle size of supported LaMnO3: the smaller the particle size is, the higher the concentration and acidity of Mn4+ are. The presence of high concentration and acidity of Mn4+ of small LaMnO3 particles concurrently increases the amount of weak basic nonstoichiometric oxygen, with which the surface concentration of Lewis acid-base adducts can be elevated. The Mn4+/nonstoichiometric oxygen pair is intrinsically active in C2-C2 aldolization, and the concentration of the paired site is positively correlated to the selectivity of C4 products. By co-reacting ethanol with its evolved intermediates, i.e. acetaldehyde and crotonaldehyde, the aldol condensation of acetaldehyde molecules was discovered to be rate-limiting. Accordingly, a plausible mechanism of aldolization of acetaldehyde molecules into C4 products mediated by the Mn4+/nonstoichiometric oxygen adduct of LaMnO3 was established.

KEYWORDS: Aldol condensation, butadiene, ethanol, nonstoichiometric oxygen, perovskite


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Introduction

Perovskite is a crystalline oxide with an ABO3 formula. Perovskite catalysts are extensively used in oxidation of hydrocarbons and NOx reduction.1-2 The reasons why perovskite catalysts are widely applied in the aforementioned reactions are because perovskite is rich in mobile oxygen species and it possesses oxygen vacancies.3 The mobile oxygen species and the oxygen vacancies facilitate the redox cycle through oxygen transfer between reactants and the surface of a catalyst. LaMnO3, sometimes denoted as LaMnO3+δ, is one of the perovskite catalysts rich in oxygen.3 The oxygen nonstoichiometry (or overstoichiometry, δ) allows LaMnO3 to possess electron acceptors (Lewis acid) of Mn4+/3+ and electron donors (Lewis base) of O2-,4 i.e. a surface acid-base adduct. The acidity is related to the effective positive charge of surface cations, while the basicity is originated from the effective negative charge and the coordination of surface anions.5 Even though LaMnO3 has its own rhombohedral structure, the outermost surface, at which reactions occur, has an inhomogeneous nature compared to its crystallographic structure.6 Moreover, mixed ratios of Mn4+-to-Mn3+ were frequently found because of varying extents of anion deficiency or cation vacancies, and through the mixed ratios of Mn4+-to-Mn3+, the strength of Lewis acid–base adducts can be tailored.7 Hence, LaMnO3 perovskite is deemed to be a potential catalyst in catalyzing acid-base reactions.5 Unexpectedly, there are a handful of researches using perovskite catalysts, not to mention LaMnO3, in acid-base catalysis reactions, such as ethanol conversion to butadiene (BD). Weckhuysen et al.,8 Sels et al.,9 and Dumeignil et al.10 provided detailed surveys of tested catalysts that exclude perovskites in BD synthesis from ethanol. Trikalitis and Pomonis11 and,


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more recently, Foo et al.12 employed 2-propanol conversion over LaSrVO3, and over Sr(Ti, Zr)O3 and Ba(Ti, Zr)O3, respectively. Both groups of researchers agreed the synergy of acid-base properties existed in different perovskite catalysts, while the latter group reported that the inhomogeneity of surface composition (enrichment of SrO or BaO) is vital in mediating the mechanism of 2-propanol conversion. Klimkiewicz and coworkers studied the ketonization of 1butanol (BuOH) to 4-heptanone by the mixtures of LaMnO3 and Al2O313 and of LaMnO3 and carbon black.14 The conversion of BuOH and the distribution of products were found to be dependent on the acid-base chemistry of tested catalysts. Tesquet et al.15 investigated ethanol conversion over LaFeO3-based perovskites in a differential reactor (less than 20% conversion of ethanol). They discovered that the surface rich in La2O3 holds high concentration of base sites, resulting in an increase of BuOH selectivity through the Guerbet process.16 Conversely, additional La in Zn-Zr-Si mixed oxide catalysts promotes ethanol transformation into BD instead of into BuOH due to its basicity.17 The difference between these reaction results indicates the significant influence cast by the composition of active sites on the catalyst surface upon the result of the conversion of ethanol into C4 products. This study intends to investigate the aldolization nature of the Lewis acid (Mn4+)-base (nonstoichiometric oxygen, O2-) pair site of perovskite-type LaMnO3 and uses ethanol conversion to C4 products (i.e. BD and BuOH) as a probe reaction. By varying the surface area of silica support and the amount of loaded perovskite, the chemistry of a Lewis acid-base adduct can be tailored: compared with the bigger particle size immobilized LaMnO3, the smaller particle size of immobilized LaMnO3 possessed more Lewis acid sites and stronger Lewis acidity. This trait is related to a higher Mn4+/Mn3+ ratio in the solid solution of perovskite and, coherently, a higher amount of nonstoichiometric oxygen is formed to neutralize the positive charge of


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tetravalent Mn cations. The Mn4+ and neighboring nonstoichiometric oxygen are likely to enhance an aldolization activity for the generation of C4 products in ethanol conversion. A plausible mechanism of C4 species production through the route of C2-C2 condensation mediated by the Lewis acid (Mn4+) and base (nonstoichiometric oxygen) adducts is therefore proposed.

Experimental Section Catalyst Synthesis The wet impregnation method was employed to synthesize silica-supported LaMnO3 catalysts

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. Designated amounts of lanthanum nitrate, (Alfa-Aesar, 98%), manganese nitrate

(Alfa-Aesar, 98%), and 1.53 g citric acid (J. T. Baker, 98%), corresponding to a molar ratio of 1:1:2, were well mixed in a 30 mL ethanol solution (50% in water). The pH value of the solution was adjusted to be at approximately 7 by using an ammonia solution (25% in water). Approximately 1 g of silica, SiO2-L (Cabot-Sil L-90 with the surface area of ca. 90 m2/g) or SiO2-H (Cabot-Sil EH-5 with the surface area of ca. 300 m2/g) was added into the neutralized solution, and the solution was stirred for 24 h. The yielded paste was dried in a vacuum system (10-3 pa) at 60 oC for 12 h, and the produced powder was calcined at 700 oC for 4 h in an air stream (50 mL/min) to form silica-supported LaMnO3 catalysts. Unsupported LaMnO3, denoted as LaMn, was also prepared using the same procedure without adding silica. The amount of loaded perovskite on SiO2 was estimated using the specific heat of the precursor of LaMnO3 during crystallization in an air stream (50 mL/min) above 600 oC, recorded by a simultaneous differential scanning calorimetry and thermogravimetry (SDT, TA Instruments Q600) using the


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specific heat of LaMn (3402 J/kg/oC) as the reference (see Figure S1 in Supporting Information). Three samples, corresponding to 45 wt%, 18 wt%, and 18 wt% of loadings of LaMnO3 on SiO2H, SiO2-H, and SiO2-L, were denoted as LaMn0.45/SiO2-H, LaMn0.18/SiO2-H, and LaMn0.18/SiO2L, respectively.

Catalyst Characterization The Brunauer-Emmett-Teller (BET) surface area was obtained with an automated N2 physisorption analyzer (Micromeritics ASAP 2020). Prior to the measurement, the sample (approximately 0.2 g) was dehydrated at 300 oC in vacuum for 3 h. The elemental composition of each catalyst was measured by inductively coupled plasma-atomic emission spectrometry (ICPAES, Kontron S-35). X-ray powder diffraction (XRD) patterns were recorded in the scanning angle (2ϴ) range from 10o to 80o on a Rigaku D/Max-IIB diffractometer using Cu Kα radiation operated at 40 kV and 30 mA. Field emission transmission electron microscopy (TEM) images were acquired using a JEOL JEM 2100F microscope at 200 kV. The X-ray photoelectron spectroscopy (XPS) measurement was conducted on an AXIS Ultra DLD Kratos spectrometer equipped with a monochromatized aluminum source with a wavelength at 1486.6 eV. The C 1S binding energy of adventitious carbon at 285.0 eV was used to correct the energy shift. The core level spectra were curve fitted using symmetrical Gauss–Lorentzian curve fitting after Shirleytype subtraction of the background. Temperature-programmed reduction (TPR) of H2, temperature-programmed desorption (TPD) of NH3, CO2, and O2 were all carried out on an automated chemisorption analyzer (Micromeritics AutoChem II) using 0.15 g of a sample with the particle size ringing from 40 mesh to 80 mesh per trial. H2-TPR was recorded using a thermal


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conductivity detector (TCD), while NH3-, CO2-, and O2-TPD were monitored with a quadrupole mass gas analysis system (ThermoStar GSD 320 T, Pfeiffer Vacuum). Detailed pretreatments and operating conditions of temperature-programmed analyses can be found in our recent studies.19-20 The Fourier transform infrared spectroscopy (FTIR) of pyridine and CO2 adsorption was conducted using a Thermo Scientific Nicolet iS50 spectrometer and an in situ quartz cell (transmission mode, Dalian Xuanyu Technology). The sample was dehydrated in an air stream (50 mL/min) at 300 oC for 1 h. For pyridine IR measurements, the dehydrated sample was cooled to 150 oC and exposed to pyridine vapor. Excess pyridine was removed at 150 oC in a N2 stream (50 mL/min) for 1 h, and the IR spectra were recorded at 150 oC. For CO2 IR measurements, the dehydrated sample was flushed with a CO2 stream for 1 h (40 mL/min) at 30 oC, purged in a nitrogen stream (40 mL/min) for 1 h, and heated up to desired temperature to record the spectra.

Activity Evaluation A continuous fixed-bed reactor (10 mm i.d.) was used for activity evaluation. Approximately 0.2 g of a catalyst was sandwiched by quartz wool in the center of the catalyst bed. The reactant was fed into the system (0.1 mL/h) by a KDS 100 syringe pump, vaporized at 140 oC at the inlet, and mixed with a 20 mL/min N2 stream. N2 also served as the internal standard and was used to estimate the relative gas chromatography (GC) response factor of detected species. The weight hourly space velocity (WHSV) of ethanol was set at 0.38 h-1. Before each test, the catalyst was pretreated in air at 450 oC for 30 min. The off-gas was analyzed using an on-line GC (SRI 8610C) with a Porapak Q column annexed with a Molecular Sieve 5A column. The operating condition was designed to obtain the conversion of a reactant


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less than 15% for each trial to obtain intrinsic catalytic performances. To evaluate the acid-base nature of tested catalysts, isopropanol (IPA) conversion under differential conditions (at any conversion less than 15%) was performed in the fixed-bed system described above at 200 oC with a WHSV of 0.19 h-1. The conversion was calculated as moles of a reactant that reacted divided by moles of a reactant that is injected. The selectivity of carbon-containing products was defined as 100 × (moles of a reactant converted to product i)/(the sum of moles of carboncontaining products). Three to five trials were recorded after arriving at the steady state (usually about 1 h after the initiation of the inline GC system). The results of these multiple trials were averaged as reported data.

Results and Discussion Physicochemical Characterizations Table 1 presents the surface area estimated with N2 physisorption and the elemental composition of each catalyst obtained through ICP-AES. The BET equation estimated that the surface area of LaMn was merely 14.3 m2/g, while silica-supported LaMnO3 catalysts had higher surface areas than LaMn; their surface areas are listed in ascending order as follows: LaMn0.18/SiO2-L (70.9 m2/g) < LaMn0.45/SiO2-H (94.6 m2/g) < LaMn0.18/SiO2-H (192.3 m2/g). The surface areas of silica-supported LaMnO3 catalysts were lower than their blank supports (300 m2/g for SiO2-H and 90 m2/g for SiO2-L), implying the possible pore blockage after impregnation. The relative ratio of La to Mn is in a good agreement of 1:1, and the weight loading estimated with ICP-AES is close to its nominal value of each catalyst. The measurement of each La-to-Mn ratio being approximate to 1 and the closeness between nominal and practical


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loadings indicated that LaMnO3 perovskite can be successfully deposited on different silica supports. Table 1. Specific surface area and bulk composition of tested catalysts Catalyst

SBET (m2/g)

La

Mn

Si

Oa

La/Mn

wt% of LaMnb

LaMn

14.3

18.3

16.9

0

64.9

1.1

100

LaMn0.45/SiO2-H

94.6

3.9

3.9

34.1

58.2

1.0

46.0

LaMn0.18/SiO2-L

70.9

1.2

1.2

26.7

70.9

1.0

18.2

LaMn0.18/SiO2-H

192.3

1.4

1.3

26.4

70.9

1.1

19.6

a

Calculated by 100 - (La + Mn + Si)

b

Estimated by the results of ICP-AES

Figure 1 shows the XRD patterns of tested catalysts. All the catalysts exhibited characteristic peaks of crystalline perovskite of LaMnO3 (JCPDS NO. 01-075-0440). Neither the responses of La2O3 nor those of MnO2 were detected. Responses of amorphous silica were observed in the 2Θ range from 20o to 40o for silica supported catalysts. The intensities of index peaks of crystalline perovskite increased gradually with increasing LaMnO3 loadings. Using the full width at half the maximum (FWHM) of the (1 1 0) plane, the estimated crystalline sizes obtained from the Scherrer equation are listed in ascending order as follows: LaMn0.18/SiO2-H (5.0 nm) < LaMn0.18/SiO2-L (5.7 nm) < LaMn0.45/SiO2-H (10.2 nm) < LaMn (11.1 nm).


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Figure 1. The XRD patterns of tested catalysts Figure 2 shows the TEM images of silica-supported LaMnO3 catalysts. The d-spacings of the (1 0 1) plane (0.3840 Å) and the (1 1 0) plane (0.2732 Å) of perovskite-type LaMnO3 can be clearly observed on LaMn0.45/SiO2-H (see Figure 2(d)), indicating that the perovskite phase can be successfully immobilized on silica supports.


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Figure 2. TEM images of (a) LaMn0.18/SiO2-H, (b) LaMn0.18/SiO2-L, and (c) LaMn0.45/SiO2-H; (d) the d-spacings of the (1 0 1) and the (1 1 0) planes of LaMnO3 of LaMn0.45/SiO2-H Figure 3 displays the XPS patterns of Mn 2p signals. The Mn 2p3/2 peak in the range from 630 eV to 645 eV and the Mn 2p1/2 peak in the range from 650 eV to 660 eV can be deconvoluted into four peaks. The peaks centered at 642.9 eV and 654.5 eV are assigned to Mn4+ cations; 641.5 eV and 653.1 eV, Mn3+ cations.21-22 Accordingly, the relative ratio of Mn4+ to Mn3+ can be estimated, and the results of the estimation are listed in ascending order as follows:


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LaMn (0.95) < LaMn0.45/SiO2-H (0.97) < LaMn0.18/SiO2-L (1.30) < LaMn0.18/SiO2-H (1.47), on the basis of the deconvoluted peak areas of Mn 2p3/2 and Mn 2p1/2 responses. The increasing order underlines that LaMn0.18/SiO2-H had the highest Mn4+ concentration in the LaMnO3 phase while LaMn had the lowest one. This difference also implied that the crystalline size of LaMnO3 can affect the composition of Mn4+ and M3+ in perovskite solution. Malavasi23 and Minot et al.2425

have reported that the surface defects of perovskite-type LaMnO3 can be crucial in tuning the

Mn4+/Mn3+ ratio due to the oxygen nonstoichiometry and cation vacancies. We believe that similar behavior could be stated for LaMnO3 with varying particle diameters since the size and shape of LaMnO3 affect the surface structure and composition greatly,5 and this influence is possibly caused by the differences of the surface area of silica supports.

Figure 3. The XPS spectra of the Mn 2p level of tested catalysts


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Figure 4 shows the H2-TPR profiles. The two reduction bands with 600 oC as the demarcation could be observed. The low-temperature response can be correlated with a little amount of nonstoichiometric oxygen (also called α-O2,1 which is desorbed below 250 oC) and the reduction of Mn4+ to Mn3+, and the high-temperature response is related to the reduction of Mn3+ to Mn2+.26-28 Table 2 presents the quantitative analysis of hydrogen consumption. Theoretically, if all Mn cations in LaMnO3 are trivalent, a full reduction of Mn3+ to Mn2+ should consume 2.07 mmol of H2 for each gram of perovskite; if all Mn cations are tetravalent, 4.13 mmol of H2 should be consumed for each gram of perovskite.29 The amount of H2 consumption per gram of loaded LaMnO3 of each sample was in the range from 2.07 mmol/g to 4.13 mmol/g, and each amount is arranged in ascending order as follows: LaMn (2.23 mmol/g LaMnO3) < LaMn0.45/SiO2-H (2.32 mmol/g LaMnO3) < LaMn0.18/SiO2-L (2.46 mmol/g LaMnO3) < LaMn0.18/SiO2-H (2.69 mmol/g LaMnO3). This ascending order indicates that synthesized LaMnO3 is a mixture of Mn3+ and Mn4+ solid solution, and LaMn0.18/SiO2-H had the highest Mn4+/Mn3+ ratio while LaMn had the lowest one. Moreover, the nonstoichiometric oxygen (δ) of each catalyst can be estimated with the amount of hydrogen consumption, and the estimation results are listed in ascending order as follows: LaMn (δ = 0.04) < LaMn0.45/SiO2-H (δ = 0.06) < LaMn0.18/SiO2-L (δ = 0.08) < LaMn0.18/SiO2-H (δ = 0.15).


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Figure 4. H2-TPR profiles of tested catalysts

Table 2. Quantitative analysis of hydrogen consumption of tested catalysts < 600 oC

> 600 oC

Total

(mmol/g)

(mmol/g)

(mmol/g)

LaMn

0.93

1.30

2.23

2.23

LaMn0.45/SiO2-H

0.56

0.49

1.05

2.32

LaMn0.18/SiO2-L

0.32

0.12

0.44

2.46

LaMn0.18/SiO2-H

0.36

0.13

0.48

2.69

Catalyst

Normalization of Total (mmol/g LaMnO3)

The maximum rates of reduction of low- and high-temperature responses (TRL and TRH) are listed in ascending order as follows: LaMn0.18/SiO2-H (425 oC and 787 oC) < LaMn0.18/SiO2L (434 oC and 846 oC) < LaMn0.45/SiO2-H (463 oC and 852 oC) < LaMn (465 oC and 898 oC).


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The ascending order of TRL and TRH implies that the smaller the LaMnO3 particles are, the more reducible the catalyst can be.29 Moreover, the relative ratios of H2 consumed at low and high temperatures are listed in ascending order as follows: LaMn (0.72) < LaMn0.45/SiO2-H (1.15) < LaMn0.18/SiO2-L (2.55) < LaMn0.18/SiO2-H (2.87). Since the low-temperature reduction response is attributed to the reduction of tetravalent Mn cations to the trivalent state, the above-mentioned ascending order suggests that the concentration of Mn4+ increased as the LaMnO3 particle size decreased. This tendency is in line with the Mn4+/Mn3+ ratio estimated with the Mn 2p XPS spectra.

Acid-base Nature Figure S2 displays the NH3-TPD profiles of tested catalysts, and the signal of m/e = 17 was used to monitor the desorption of NH3. Silica supports showed no response of desorbed NH3. The amount of desorbed NH3 was used to estimate the overall acid site concentration, which is listed in Table 3. It should be noted that LaMn had a relatively weak MS response, and therefore the integrated area of TCD signals below 500 oC was used to quantify the acid site of LaMn. A more detailed analysis was conducted through pyridine IR measurements. Figure 5 shows the spectra of pyridine IR measurements in the frequency range from 1650 cm-1 to 1400 cm-1 on tested catalysts. LaMn and its reference, MnO2, displayed no response. Two bands at approximately 1444 cm-1 and 1600 cm-1, corresponding to the ν19b and ν8a modes of CCN stretching vibrations30 of pyridine molecules chemisorbed on Lewis acid sites31-32 could be identified. The response of pyridine adsorbed on Brønsted acid at approximately 1545 cm-1 was absent. IR responses of pyridine adsorption were observed over LaMn0.45/SiO2-H (1443.9 cm-1


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and 1591.9 cm-1), LaMn0.18/SiO2-L (1443.0 cm-1 and 1595.4 cm-1) and LaMn0.18/SiO2-H (1444.0 cm-1 and 1599.9 cm-1). The blue shift, particularly that of the ν8a band, proves that LaMn0.18/SiO2-H had the strongest Lewis acidity among tested catalysts.32 The outcome of pyridine IR measurements highlighted that LaMn0.18/SiO2-H had not only the highest Mn4+/Mn3+ ratio, but also the strongest Lewis acidity among tested catalysts. Table 3. Quantitative analysis of the acid and base sites of tested catalysts (Sbase/Sacid)a

(Sbase/Sacid)b

186.1

7.0

30.3

48.9

125.1

2.6

24.1

LaMn0.18/SiO2-L

62.9

46.7

0.7

8.4

LaMn0.18/SiO2-H

74.6

29.5

0.4

4.2

Normalized adsorbed NH3

Normalized adsorbed CO2

(µmol/g LaMnO3)

(µmol/g LaMnO3)

LaMn

26.7

LaMn0.45/SiO2-H

Catalyst

a

Estimated by (normalized adsorbed NH3)/(normalized adsorbed CO2)

b

Estimated by (selectivity of acetone)/(selectivity of propylene) from IPA conversion under the differential analysis condition.


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Figure 5. FTIR spectra of pyridine adsorbed onto tested catalysts

Figure S3 exhibits MS fragments of CO2 (m/e = 44) that change with temperature. CO2 is an acidic adsorbent and is preferentially adsorbed on base sites.25, 33 A broad response in the range of 100 oC to 200 oC, which is attributed to the carbonate species weakly bonded to hydroxyls,34-35 was observed on each catalyst. Listed in Table 3, the amount of desorbed CO2 per gram of supported LaMnO3 was used to estimate concentration of base sites. Figure 6 exhibits the IR spectra of adsorbed CO2 at 30 oC and 250 oC. At 30 oC, three responses, corresponding to a symmetric O-C-O stretching of an asymmetric O-C-O stretching of unidentate carbonate (1400 cm-1 and 1510 cm-1) and an asymmetric O-C-O stretching of bidentate carbonate (1625 cm-1),


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were observed.36-37 However, these adsorbed CO2 species almost disappeared at 250 oC, suggesting a weak surface basicity of each catalyst.

Figure 6. FTIR spectra of CO2 adsorbed onto tested catalysts at 30 oC (solid curve) and 250 oC (dash curve)

The concentration of adsorbed NH3 and CO2 can be used to estimate the relative composition of acid and base sites. Table 3 includes the ratio of Sbase/Sacid calculated through the adsorbed amounts of CO2 and NH3. The Sbase/Sacid ratio decreased monotonically with the decreasing LaMnO3 particle size, suggesting that a catalyst loaded with small LaMnO3 clusters has a higher concentration of Lewis acid sites. To reveal the relative ratio of acid to base sites, catalytic conversion of isopropanol (IPA) was performed under the conditions of differential analysis (the IPA conversion was kept below


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15%) to avoid secondary reactions. IPA can be dehydrated to form propylene and water on an acid site, it can also be dehydrogenated to form acetone and hydrogen on a base site. Accordingly, the ratio of selectivities of acetone and propylene can be used to estimate the baseto-acid ratio of each catalyst.38-39 Table 3 also presents the (Sbase/Sacid) value estimated according to the results of IPA conversion. The Sbase/Sacid ratios are listed in descending order as follows: LaMn (30.3) > LaMn0.45/SiO2-H (24.1) > LaMn0.18/SiO2-L (8.4) > LaMn0.18/SiO2-H (4.2). The descending order of the Sbase/Sacid ratios calculated according to the results of IPA conversion is in line with the above-mentioned order of the Sbase/Sacid ratios observed in NH3- and CO2-TPD analyses. This confirmation further supports our claim that supported LaMnO3 with a small particle size contains higher concentration of Lewis acid than those catalysts with a large LaMnO3 particle size.

Activity Evaluation Table 4 presents the catalytic results of differential analysis of tested catalysts. Selectivities of ethylene and diethyl ether (DEE), generated by intra- and inter-molecular dehydration due to nonselective thermal influence,40 were less than 3% for all trials. The low conversion of ethanol and low selectivities of dehydrated products underlined that the reported activities were intrinsic. Moreover, each catalyst displayed 16-hour stability with a carbon mass balance for each data point higher than 97% (see Figures S4 to S7), indicating that the initial catalytic results are representative. The activity of each catalyst is listed in ascending order as follows: LaMn (0.18 mmol/h/g LaMn) < LaMn0.45/SiO2-H (0.48 mmol/h/g LaMn) < LaMn0.18/SiO2-L (1.12 mmol/h/g LaMn) < LaMn0.18/SiO2-H (1.16 mmol/h/g LaMn). This order


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implies that the smaller the size of immobilized LaMnO3 particles, the higher the activity of the catalyst can be achieved. Acetaldehyde (AC) was the major product, accounting for more than 80% of tested catalysts excluding LaMn0.18/SiO2-H (77.9%). The lower selectivity of AC of LaMn0.18/SiO2-H was compensated by higher selectivities of BD (13.0%) and BuOH (3.9%). The sums of selectivities of BD and BuOH are listed in descending order as follows: LaMn0.18/SiO2H (16.9%) > LaMn0.18/SiO2-L (11.8%) > LaMn0.45/SiO2-H (2.6%) > LaMn (1.0%). The mixed powder of LaMnO3 and silica in a 1:4 weight ratio was also tested, and a trivial selectivity of BuOH (1.1%) was achieved. These catalytic performances highlight the unique activity of bimolecular

condensation

that

characterizes

silica-supported

LaMnO3,

particularly

LaMn0.18/SiO2-H. Table 4. Catalytic performances of LaMnO3-based catalysts in ethanol conversiona Catalyst

XEtOH

Activity

(%)

(mmol/h/g LaMn)

C2H4

DEE

AC

BD

BuOH

Othersb

LaMn

10.3

0.18

1.8

2.7

91.1

0

1.0

5.0

LaMn0.45/SiO2-H

12.5

0.48

0.1

0

92.6

0

2.6

4.6

LaMn0.18/SiO2-L

11.8

1.12

0.2

0.2

81.0

8.2

3.6

6.8

LaMn0.18/SiO2- H

12.1

1.16

0.4

0.6

77.9

13.0

3.9

4.2

LaMn0.2 + SiO2-Hc

8.9

-

0.1

0.3

94.9

0

1.1

3.6

Selectivity (%)

a

The carbon mass balance exceeds 97% for each trial

b

Other products may include 1-butene, 1-butenal, and 2-butenol

c

The mixture of LaMn and SiO2-H in a 1:4 weight ratio

Co-feeding Basic (pyridine) and Acidic (CO2) Species


20

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To clarify the influence of the acid and base sites of LaMnO3 in ethanol conversion, ethanol was co-fed with 3% of pyridine (see Table 5) and 10% of CO2 (see Table S1), respectively, to selectively deactivate the acid and base sites. Compared to the results with the use of pure ethanol as the feed (see Table 4), the use of pyridine as an additive made the conversion of ethanol and the activity of each catalyst drop substantially, especially for LaMn0.18/SiO2-H and LaMn0.18/SiO2-L (more than 50% of fractional decreases). The significant drop of ethanol conversion and activity underlines that the active phases of tested catalysts were mainly Lewis acid sites. More than 90% acetaldehyde was detected when each catalyst was used. The significant drop of selectivity of C4 products accompanying a substantially increase of acetaldehyde emphasized that the Lewis acid site for C4 formation is mostly deactivated. The formation of acetaldehyde can be attributed to the enriched electron density of the nearest oxide ions of the pyridine-adsorbed Lewis acid sites.41 Table 5. Catalytic performances of LaMnO3-based catalysts in ethanol conversion with the cofeeding of 3% pyridinea Catalyst

a

XEtOH

Activity

(%)

(mmol/h/g LaMn)

C2H4

DEE

AC

BD

BuOH

Othersb

LaMn

9.8

0.17

0.1

1.8

92.3

0

1.1

3.9

LaMn0.45/SiO2-H

10.9

0.42

0.2

0.9

93.6

0

2.6

2.7

LaMn0.18/SiO2-L

5.3

0.51

0.4

0.5

95.3

0

2.1

1.7

LaMn0.18/SiO2- H

3.4

0.32

0.5

0.1

96.5

0

2.4

0.5

Selectivity (%)


b



21


Page 22 of 39

10% CO2 was added in the ethanol stream to evaluate the influence of surface base sites since strong bases can be poisoned by CO2 and thus form a surface layer of stable carbonate.3 However, the product distributions were nearly identical to those of the trials using pure ethanol as the reactant, indicating a weak basic surface possessed by LaMnO3-supported catalysts.

Determining the Rate-limiting Step in the Formation of C4 Products The highest sum of selectivities of C4 products of LaMn0.18/SiO2-H indicates that the activity of bimolecular condensation can be manipulated with the domain size of LaMnO3 clusters on silica supports. Several mechanisms of ethanol conversion were proposed, such as the Lebedev (one-step) mechanism,42 the Kagan (two-step) mechanism,43 and the Cavani mechanism,44 and the rate-limiting steps of these mechanisms varied depending on the surface acid-base properties.41, 45 Scheme 1 shows the generally acknowledged reaction network of C4 product formation.9 This reaction network was also validated through the testing of LaMn0.18/SiO2-H at 250 oC with different WHSVs, and the results were reported in Table S2 in the Supporting Information. Hence, using AC or crotonaldehyde as a co-reactant should be able to clarify the decisive step of C4 product formation over silica-supported LaMnO3 catalysts.


22

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Scheme 1. Reaction network for the production of C4 species from ethanol9 Tables 6 and 7 present the differential analysis data of using mixtures of 10% AC in ethanol and 10% crotonaldehyde in ethanol as the reactants. The catalytic performances of using an AC-ethanol mixture as the reactant were nearly identical as those of using pure ethanol as the feed (see Table 4). The undisturbed product distribution was confirmed by the use of 5% and 20% AC in ethanol as the reactants and was tested under the same reaction condition with the use of LaMn0.18/SiO2-H (see Table S3). In contrast, the significantly high BD selectivity and the little AC selectivity can be observed through the use of 10% crotonaldehyde in ethanol as the feed, as compared to the results observed through the use of ethanol as the reactant, particularly for those of LaMn0.18/SiO2-H (increasing from 13.0% to 79.0%) and LaMn0.18/SiO2-L (increasing from 8.2% to 76.1%). Conversion of crotonaldehyde (XCRON) of each catalyst was within the range from 1.6% to 77.2%, indicating that crotonaldehyde was consumed. Therefore, crotonaldehyde acted as the main precursor of BD. The role of crotonaldehyde was further explored through the use of 5% and 20% crotonaldehyde in ethanol as the feeds and was tested under the same reaction condition with the use of LaMn0.18/SiO2-H (see Table S4). Crotonaldehyde was fully converted with the use of 5% crontonaldehyde in ethanol as the


23


Page 24 of 39

reactant. The decreased AC selectivity from 77.9% to 48.5% and the increased BD selectivity from 13.0% to 36.9% were discovered compared to the conversion of using pure ethanol as the feed (see Table 4). The product distributions of the transformation using 10% and 20% crontonaldehyde in ethanol as the reactants were similar. Moreover, BuOH was trivial when using the crotonaldehyde-ethanol mixture as the feed. According to Scheme 1, crotonaldehyde is the key intermediate to the MPV route for BD formation; therefore, enriched crotonaldehyde in the stream could facilitate hydrogen atom transfer from ethanol to crotonaldehyde and suppress the route of hydrogenation of crotonaldehyde with surface hydrogen atoms generated from ethanol dehydrogenation.46-47 In other words, the rate-controlling step is not MPV reduction, since excess crotonaldehyde could propel BD formation; instead, the step of aldol condensation of acetaldehyde molecules, i.e. the formation of crotonaldehyde, is. Table 6. Catalytic performances of LaMnO3-based catalysts using 10% acetaldehyde in ethanol as the reactanta Catalyst

a

XEtOH

Activity

(%)

(mmol/h/g LaMn)

C2H4

DEE

AC

BD

BuOH

Othersb

LaMn

10.7

0.18

0.9

1.4

93.7

0

1.3

2.7

LaMn0.45/SiO2-H

13.2

0.50

0.1

0.2

94.6

0

2.9

2.2

LaMn0.18/SiO2-L

12.6

1.20

0.2

0.2

81.2

7.9

4.2

6.3

LaMn0.18/SiO2- H

14.8

1.41

0.7

0.4

78.0

12.9

4.7

3.8

Selectivity (%)


b



24

Page 25 of 39 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 7. Catalytic performances of LaMnO3-based catalysts using 10% crotonaldehyde in ethanol as the reactanta Catalyst

a

XEtOH

Activity

XCRON

(%)

(mmol/h/g LaMn)

(%)

C2H4

DEE

AC

BD

BuOH

Othersb

LaMn

10.2

0.18

1.6

0.2

0.7

94.9

0

0.2

4.2

LaMn0.45/SiO2-H

9.9

0.38

18.7

1.3

0.3

73.7

19.4

0

5.4

LaMn0.18/SiO2-L

10.5

1.00

69.3

6.2

2.7

5.3

76.1

0

10.0

LaMn0.18/SiO2- H

13.2

1.26

77.2

8.9

0.1

0.1

79.0

0

10.3

Selectivity (%)


b

Other products may include 1-butene, 1-butenal, and 2-butenol Using Lewis acid-directed C2-C2 condensation to synthesize C4 products, to our best

knowledge, is mostly focused on supported catalysts of ZnO, ZrO2, and the combination of ZnO and ZrO2.48-51 The use of ZnO and ZrO2 possibly benefited from the dehydrogenation nature of ZnO52 and the MPV reduction activity of ZrO2.10 Interestingly, using ZnO-ZrO2/SiO248 and ZrO2/SiO250 catalysts in converting AC-ethanol mixtures can generate a substantial increase of BD (more than 50%, using the BD selectivity produced by pure ethanol conversion as the base). The difference in the catalytic results is in stark contrast to our findings, which showed no beneficial effect when using AC as the co-reactant. Such a deviant in catalytic results of using AC as a co-reactant implies that the activity of C4 products generated by using LaMnO3 may not be attributed to Lewis acid sites (Mn4+) solely. Presumably, surface hydroxyls of silica and/or the neighboring nonstoichiometric oxygen of Mn4+ may play a role to facilitate the aldolization of acetaldehyde to form C4 products.


25


Page 26 of 39

According to Jones et al.,48 acidic hydroxyls of silica and the mixture of ZnO and ZrO2 supported on silica have a synergistic effect, resulting in a substantial increase in the yields of C4 products, particularly BD, in ethanol conversion. To support this claim, they neutralized surface hydroxyls through a post-synthesis alkali-leached treatment and discovered that the BD yields can be suppressed greatly with alkali-treated catalysts. However, this is not the case in our study: the NaOH-leached catalysts produced through the same post-treatment48 displayed the catalytic results of ethanol conversion, and their activity was nearly identical to the performance of their unaltered counterparts (see Table S5). The similarity proves that the acidic hydroxyls of silica of LaMnO3-supported catalysts did not participate in the C2-C2 aldolization. Another factor that should be considered is the nonstoichiometric oxygen, which coexists with neighboring Mn4+ cations to achieve an electron balance of LaMnO3. By removing the nonstoichiometric oxygen, its paired Mn4+ should be reduced to Mn3+ to achieve charge neutralization, by which the aldolization nature of a catalyst surface should be deactivated due to the suppression of Lewis acidity and concentration. Table 8 presents the catalytic results of each tested catalyst by subjecting a thermal treatment in an N2 stream at 650 oC for 1 h to partially remove the nonstoichiometric oxygen from LaMnO3 phase. The criteria of the removal of nonstoimetric oxygen were determined according to the O2-TPD results shown in Figure S8. Note that at 650 oC, the desorption of α-O2 can occur, implying the possible extraction of nonstoichiometric oxygen from perovskite structure.1, 53 Moreover, experiments of XRD and H2TPR of nonstoichiometric oxygen-deficient samples were performed, shown in Figures S9 and S10. The nonstoichiometric oxygen-deficient samples displayed the XRD patterns that were nearly identical to those of their pristine counterparts. The similar XRD patterns suggest the thermal treatment has little impact on the crystalline structure of supported LaMnO3. In addition,


26

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all of the estimated δ values of nonstoichiometric oxygen-deficient catalysts approximated 0.01, which is much lower than those δ values of their respective as-synthesized counterparts. That is, nonstoichiometric oxygen can be mostly extracted from the surface after the tested catalyst is subjected to the thermal treatment. Compared to the outcomes in Table 1, the conversion of ethanol, activity, and C4 product selectivity dropped remarkably, showing that AC takes up approximately 90% of the products for each catalyst. The loss of C2-C2 coupling activity of the nonstoichiometric oxygen-deficient catalysts underlines that the LaMnO3-based Lewis acid (Mn4+)-base (nonstoichiometric oxygen) adduct is indeed the active phase in condensation of acetaldehydes to C4 products.

Table 8. Catalytic performances of LaMnO3-based catalysts subjected to a post-synthesis treatment for the removal of nonstoichiometric oxygen in ethanol conversiona Catalyst

XEtOH

Activity

(%)

(mmol/h/g LaMn)

C2H4

DEE

AC

BD

BuOH

Othersb

LaMn

6.4

0.11

1.1

0.9

88.5

0

2.1

7.4

LaMn0.45/SiO2-H

5.9

0.23

1.5

0.6

89.1

0

1.5

7.3

LaMn0.18/SiO2-L

4.8

0.46

1.7

1.3

90.1

0

2.6

4.3

LaMn0.18/SiO2- H

4.1

0.39

1.3

1.7

89.5

0

3.3

4.2

Selectivity (%)

a

The carbon mass balance exceeds 97% for each trial. The nonstoichiometric oxygen removal treatment was conducted through the following procedure: the sample was linearly heated to 650 o C with a 10 oC/min heating rate and was kept at 650 oC for 1 h in a N2 stream (50 mL/min). b


Reaction Mechanism


27


Page 28 of 39

Given the experimental evidence above, we proposed that aldol condensation of acetaldehyde derived from ethanol over aforementioned catalysts might take place in the acidbase cooperative interaction as shown in Scheme 2. First, the carbonyl group of acetaldehyde would be activated by the Lewis acid site, and the enol form of acetaldehyde formed through keto-enol tautomerism might be chemisorbed onto the adjacent metal center bridged by the basic nonstoichiometric oxygen. Then, the proposed enol would serve as a weak nucleophile to attack an activated carbonyl group of adsorbed acetaldehyde, leading to the formation of C-C bond propagation. Subsequently, the C4 product selectivity to BD would be observed after a series of dehydration and dehydrogenation of preceding intermediates. Furthermore, this significant loss of C4 chemical selectivity, which was proved to be caused by the catalysts possessing a relatively low amount of nonstoichiometric oxygen after the thermal treatment at a high temperature, evidently reflected the importance of nonstoichiometric oxygen on the surface for C-C bond formation of aldol condensation derived from ethanol.


28

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Scheme 2. A plausible mechanism of aldol condensation of acetaldehyde over silica supported LaMnO3 catalysts for BD formation It is worth mentioning that Sushkevich and Ivanova54 recently employed the steady-state isotopic transient kinetic analysis (SSITKA) method to elucidate that an adsorbed enol on a Lewis acid site can interact with a gaseous acetaldehyde molecule to promote C2-C2 coupling with the use of Ag/ZrO2/SiO2 catalysts. In other words, aldol condensation is more likely to undergo the Eley-Rideal mechanism (i.e., an enol interacts with a gaseous acetaldehyde molecule), but is less likely to proceed through the Langmuir-Hinshelwood mechanism (i.e., interaction between two adsorbed acetaldehyde molecules). The former mechanism is complimentary to the latter; therefore, we should not rule out the possibility of enol activation by gaseous acetaldehyde in our system. Conclusions We presented that the Lewis acid-base adduct, i.e. the adduct generated from Mn4+ and adjacent nonstoichiometric oxygen, is aldolization-active in the conversion of ethanol to C4 products. The concentration and strength of Lewis acids depend on the particle size of LaMnO3 on silica. The quantity of and strength of Lewis acid sites are positively correlated to the concentration of Mn4+ in LaMnO3 solid solution: the higher the Mn4+/Mn3+ ratio is, the higher the number and the stronger strength of the Lewis sites existing on the surface. The phenomenon inherently leads to the forming of a higher amount of nonstoichiometric oxygen to achieve a charge balance, resulting in a higher concentration of Mn4+ and nonstoichiometric oxygen pair sites. The rate-limiting step of the formation of C4 products through LaMnO3 was discovered to be the aldol condensation of acetaldehyde molecules, and the Mn4+/nonstoichiometric oxygen


29


Page 30 of 39

adduct is deemed to be the active phase. Accordingly, a plausible mechanism of aldol condensation of acetaldehyde molecules mediated by the Lewis acid (Mn4+)-base (nonstoichiometric oxygen) adduct of LaMnO3 is established. This study, thus, opens a new window for perovskite-type LaMnO3 catalysts in the practice of acid-base reaction systems. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet on the following site: http://pubs.acs.org. This supporting information contains: Calculation for the selectivity of acetaldehyde in the co-feeding test of 10% acetaldehyde in ethanol and calculation for the conversion of crotonaldehyde in the co-feeding test of 10% crotonaldehyde in ethanol, durability tests, and catalytic results of LaMnO3-based catalysts in ethanol conversion by cofeeding 10% CO2, DTG profiles, TPD profiles of NH3, CO2, and O2, XRD patterns and H2-TPR profiles of fresh and thermal-treated catalysts.

ACKNOWLEDGMENT This study was supported by Taiwan's Deep Decarbonization Pathways toward a Sustainable Society (Project 106-0210-02-11-05), the Ministry of Science and Technology (Projects 106-2221-E-006-188-MY3 and 106-2218-E-155-005), and the Ministry of Economic Affairs (Project H354DP2120).

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SYNOPSIS The tetravalent Mn and nonstoichiometric oxygen of LaMnO3 form a Lewis acid-base adduct, which is intrinsically active in C2-C2 aldolization.


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The Aldolization Nature of Mn4+-Nonstoichiometric Oxygen Pair Sites

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