Strong Lewis Base Ga4B2O9: Ga-O Connectivity Enhanced Basicity

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Functional Inorganic Materials and Devices

Strong Lewis Base Ga4B2O9: Ga-O Connectivity Enhanced Basicity and Its Applications in the Strecker Reaction and Catalytic Conversion of n-propanol Shixiang Hu, Weilu Wang, Mufei Yue, Guangjia Wang, Wenliang Gao, Rihong Cong, and Tao Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04144 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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ACS Applied Materials & Interfaces

Strong Lewis Base Ga4B2O9: Ga-O Connectivity Enhanced Basicity and Its Applications in the Strecker Reaction and Catalytic Conversion of npropanol Shixiang Hu, Weilu Wang, Mufei Yue, Guangjia Wang, Wenliang Gao,* Rihong Cong, and Tao Yang* College of Chemistry and Chemical Engineering, Chongqing, 401331, China

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 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

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ABSTRACT

Heterogeneous solid base catalysis is valuable and promising in chemical industry, however is insufficiently developed compared to solid acid catalysis, due to the lack of satisfied solid base catalysts. To gain the strong basicity, the previous strategy was to basify oxides with alkaline metals to create surficial vacancies or defects, which suffers from the instability under catalytic conditions. Mono-component basic oxides like MgO are literally stable but deficient in electronwithdrawing ability. Here we prove a special connectivity of atoms could enhance the Lewis basicity of oxygen in mono-component solids exemplified by Ga4B2O9. The structure-induced basicity is from the µ3-O linked exclusively to 5-coorindated Ga3+. Ga4B2O9 behaved as a durable catalyst with a high yield of 81% in the base-catalyzed synthesis of α-aminonitriles by Strecker reaction. In addition, several mono-component solid bases were evaluated in the Strecker reaction, and Ga4B2O9 has the largest amount of strong base centers (23.1µmol/g) and the highest catalytic efficiency. Ga4B2O9 is also applicable in high-temperature solid-gas catalysis, for example, Ga4B2O9 catalyzed efficiently the dehydrogenation of n-propanol, resulting in a high selectivity to propanal (79%). In contrast, the comparison gallium borate, GaPKU-1, which is a Brönsted acid, preferred to catalyze the dehydration process to obtain propylene with a selectivity of 94%.

KEYWORDS: structure-properties relationships, solid bases catalysis, Lewis base sites location, heterogeneously-catalyzed Strecker reaction, n-propanol catalytic conversion, gallium borates.


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1. INTRODUCTION Heterogeneous solid-base catalysis, acting as a complementary technique to solid-acid catalysis, is valuable to chemical industry.1 Just for example, the acid-catalyzed transformation of alcohols usually leads to the dehydration product (olefins/ethers), while the base-catalyzed process results into the dehydrogenation product (ketones/aldehydes).2-5 Apparently, the acidic or basic nature of the catalyst determines the catalytic pathway and the product selectivity. However, compared to the extensively used solid acid catalysis in industry, the solid base catalysis is significantly less developed due to the lack of satisfied solid bases. Usually, people basify oxides with alkali-metal or other strong bases to obtain surficial vacancies/defects, which provide excessive electron density.6 For example, the first study of heterogeneous basic catalysts was reported by Pines et al.,7 in which alumina with sodium metal dispersed on the surface was found to be an effective catalyst for double bond migration of alkenes. Similarly, K doped MgO was a highly effective catalyst for the isomerization of alkenes even at room temperature.8 The problem of such bases is the poor durability and reusability under catalytic conditions, even the origin of the basicity remained mysterious. Mono-component bulk-type bases would not have this drawback, however the commonly used metal oxides or recently emerged MOF materials suffers from the insufficient electron-withdrawing ability of N or O in their framework.1,9 The irregular/distorted metal-oxygen polyhedra may lead to a large fluctuation of spatial charge distribution, where metal cations or oxygen anions theoretically may behave as acidic or basic sites, respectively. Gallium is an interesting element which has diverse coordination styles to oxygen. Although five polymorph of Ga2O3 are more or less neutral oxides, very interestingly, when Ga2O3 is incorporated with B (especially in 3-coordination), which is usually considered to offer Lewis acidity, the resultant Ga4B2O9 is an intrinsic bulk-type Lewis base. Apparently, the


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basicity is not due to the electronegativity of Ga, but comes from its special structural connectivity between Ga and O.

Figure 1. The schematic framework structure of Ga4B2O9 along the b-axis. The inset shows the amplified connectivity of the µ3-O linked with three 5-coordinated Ga3+.

Figure 2. Structure view of Ga-PKU-1 along the c-axis. Ga4B2O9 has a mullite-type structure.10 As depicted in Figure 1, GaO6 octahedra share edges to form one-dimensional chains along the b-direction, and the chains are further crosslinked by GaO5, BO3 and BO4 groups into a mullite-type structure. It is somehow complex


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that Ga4B2O9 has a local superstructure and is statistically disordered within the ac-plane, for instance, the 3D structure can be constructed by stacking the disordered ac-sheets along the baxis. Regardless of the superstructure, 5-coordinated Ga3+ could be considered as Lewis acidic sites and the O2- neighboring to them could behave as the conjugated Lewis basic sites. For comparison, we also investigated the catalytic performance of another gallium borate, Ga-PKU-1 (Ga9B18O33(OH)15•H3B3O6•H3BO3),11 which is an octahedra-based molecular sieve. In detail, the main structural feature is the 18-ring membered channels along the c-axis, composed of edgesharing GaO6 octahedra (see Figure 2). There exist a number of hydroxyl groups attached to the borate groups, in the forms of BO(OH)2, BO2(OH) and B2O4(OH), apparently, they act as Brönsted acid sites. Herein our work, CO2- and NH3-temperature programmed desorption (TPD) were performed for both Ga4B2O9 and Ga-PKU-1 to prove their basicity and acidity, respectively. Density functional theory calculations reveal that the structural origin to the basicity in Ga4B2O9 is the µ3-O linked exclusively with three 5-coordianted Ga3+ (see the inset of Figure 1). Two model catalytic reactions were employed to justify its strong Lewis basicity and high thermal stability, including the synthesis of α-aminonitriles (Strecker reaction) under mild conditions, and the selective conversion of n-propanol in the fixed-bed micro-reactor. Please note that Strecker reaction can be catalyzed by either solid acid or base with different mechanisms. In contrast to already extensive studies on heterogeneously acidic catalysts,12-22 very limited efforts have been made to the study on heterogeneously basic catalysts.23-25 Herein, Ga4B2O9 is an intrinsic and strong Lewis-base catalyst, exhibiting a higher performances than the Brönsted acid Ga-PKU-1. Importantly, Ga4B2O9 shows a remarkable durability and remains the constant catalytic efficiency. A possible mechanism for basic catalytic Strecker reaction was also


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proposed according to the aforementioned strong basicity of oxygens in GaO5. With regards to the catalytic transformation of n-propanol, Ga4B2O9 exhibits a base-catalyzed approach to yield the dehydrogenation product of n-propanal, while Ga-PKU-1 prefers to yield propylene according to the acid-catalyzed mechanism. 2. EXPERIMENTAL SECTION 2.1 Catalysts preparation. Ga4B2O9 used in this study was prepared by sol-gel method as follows. 1 mmol of β-Ga2O3 (0.1874 g) was dissolved in 2 mL of concentrated HNO3 at 180°C in a closed system. Then the solution, together with 2 mmol of H3BO3, and an appropriate amount of citric acid were charged into 50 mL of water. The mixture was heated gently with continuous stirring for a few hours to remove the excess water. The resulting gel precursor was dried in an oven at 180 °C and then preheated at 500 °C for 10 h with a temperature ramp rate of 1 °C/min. After grinding in an agate mortar, the precursor was further calcined at 620 °C for 10 h in air, followed with another heating at 620 °C for 5 h under dynamic O2 flow. After washing the resultant powder, pure Ga4B2O9 was obtained. Ga-PKU-1 was prepared according to the method reported in literature.11 In detail, 2.5 mmol of β-Ga2O3 (0.4686 g) was first dissolved using the above method. The resultant aqueous solution was kept in a 50 mL autoclave and been evaporated to nearly dry by just opening to air for several hours. Thereafter, this autoclave was charged with H3BO3 (6.1830 g, 100 mmol), sealed and heated at 220 °C for 5 days. After the reaction, white powder of Ga-PKU-1 could be obtained by washing unwanted residues with warm water. The average yield of Ga-PKU-1 was estimated to be >85%. It is somehow interesting that heating Ga-PKU-1 at 610 °C will result into the composite of Ga4B2O9 and B2O3. If heating Ga-PKU-1 at 750 °C, GaBO3 with calcites-type structure would be obtained.


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2.2 Typical procedure for the synthesis of N-tosyl aldimines precursors. Five imines were synthesized to use as the precursors, and their syntheses process were shown in Scheme 1. In a typical run, 4-methylbenzenesulfonamide (12 mmol) was added to a mixture of benzaldehyde (10 mmol) and toluene (25 mL) in a 50 mL flask. After refluxing for 1h, the mixture was kept heating and BF3OEt2 (2 mmol) was added slowly. When the reaction completed (as monitored by thin layer chromatography (TLC), the reaction solution was quenched with NaOH solution (1 M) and extracted with ethyl acetate for three times. The combined organic layers were washed several times with brine, dried over anhydrous Na2SO4, and concentrated under reduced atmosphere to obtain the primary product. In some cases, this primary product was purified using flash column chromatography to afford pure 4-methyl-Nphenylmethylene-benzenesulfonamide (1a). Scheme 1. The procedure for the synthesis of five imine precursors

[a]

Ts = p-toluenesulfonyl

If using 4-methyl-benzaldehyde, 3,4-dimethyl-benzaldehyde, 2,4,6-trimethyl-benzaldehyde, or 4-(1-methylethyl)-benzaldehyde as starting material, the corresponding substrate noted as 1b, 1c, 1d or 1e was obtained. Nuclear magnetic resonance (NMR) spectra (1H NMR and 13C NMR)


7


were employed to identify their structures, and 1H and

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13

C NMR spectra of 5 substrates were

provided in the Supporting Information (SI, Figures S1-S10). 2.3 Typical procedure for the cyanation of N-tosyl aldimines. For a typical run, a mixture of a specified solvent (1 mL), TMSCN (0.4 mmol), imine (0.2 mmol) and Ga4B2O9 (0.06 mmol) were mixed at 45 oC and reacted under N2 atmosphere. After an interval of 4 h, the mixture was quenched with saturated NaHCO3 aq., extracted with dichloromethane for three times, dried with MgSO4 and further concentrated under reduced atmosphere. The synthesis route was shown in Scheme 2, and the NMR spectra (1H and

13

C NMR) of 5 products were also included in SI

(Figures S11-S20). Scheme 2. Ga4B2O9 or Ga-PKU-1 catalyzed Strecker reaction between five imines and TMSCN

2.4 Catalytic conversion of n-propanol in an online micro-reactor system. The catalytic dehydration or dehydrogenation of n-propanol was evaluated on a micro-reactor system equipped with an on-line gas chromatography. Prior to activity measurements, the calcined samples were finely ground in an agate mortar, pelletized, crushed, and finally sieved in the size between 40 and 60 mesh size. After these pre-processing, 0.2 g catalysts were placed in the middle of a stainless steel tubular reactor, and pre-treated at 400°C in a N2 flow of 40 mL/min


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for 5 h. As the reactor temperature was cooled to 200°C, n-propanol was fed by a micro-liquid pump under a certain weight hourly space velocity. An online gas chromatograph (Techcomp GC 7800) equipped with a FID detector and a Porapak Q column (60–80 mesh, ϕ3×3000 mm) was connected to analyze the components in the effluent with a sampling frequency of 1 h−1. The molecular structures of the offline liquid reaction products were analyzed on a gas chromatograph coupled with mass detector (GC-MS). Some operation conditions, the characteristics of the laboratory-scale apparatus and the catalysts used in this study were summarized in Table S1, SI. 2.5 Catalyst Characterizations and products analyses. Powder X-ray diffraction data were collected on a PANalytical X'pert diffractometer equipped with a PIXcel 1D detector and Cu Kα radiation. Tube voltage and current were 45 kV and 40 mA, respectively. Scanning electron microscopy (SEM) was performed on a JSM-7800F electron microscopy at an accelerating voltage of 2 kV and a working distance of 4.0 mm. 1H and

13

C nuclear magnetic resonance

(NMR) spectra were obtained using Agilent VNMRS (600MHz). NH3-TPD method was adopted to measure the acidic properties using NH3 as the probe molecule employing a Quantachrome ChemBET Pulsar. A given amount of sample was outgassed in He gas at 400 oC for 60 min, and then the sample was purged with 5 wt% NH3/He mixture gas at the rate of 50 mL/min at 60 oC for 30 min. After removing most of the weaklyadsorbed NH3 by flushing He for 60 min. The chemisorbed NH3 gases were desorbed by temperature programmed desorption (TPD) at a ramp rate of 15 °C/min and then bubbled into a standard solution of H2SO4 (0.01 M). Thereafter, the NH3 absorbent solution was titrated with standard solution of NaOH (0.001 M) in order to determine the amount of acid sites in presence of the indicator of Phenolphthalein. Similarly, CO2-TPD was employed to measure the basic


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properties using CO2 as the probe molecule. The desorbed CO2 were absorbed in the NaOH standard solution (0.01 M), then was titrated with HCl (0.001 M) standard solution in order to determine the amount of basic sites in presence of the indicator of Methyl Orange. The offline liquid reaction products were analyzed on an Agilent 7890N gas chromatograph coupled with mass detector (Agilent 5975C) using a capillary column (DB-624, length: 30 m, inner diameter: 0.32 mm, heating rate: 20 °C/min starting from 40°C (retained for 8 min) to 80°C. Helium was used as both the carrier (1ml/min) and make-up gas (40 ml/min). The mass spectra of the products were acquired at ionization energy of 70 eV using a scan rate of 1.0 s/decade. The GC-MS spectra were identiﬁed by comparing the m/z value of each peak with the NIST mass spectral database. 2.6 Computation and modeling. All the calculations were performed using Density Functional Theory (DFT) and based on the Projector Augmented Wave (PAW) method,26 as implemented in the Vienna Ab Initio Simulation Package (VASP).27 Generalized Gradient Approximation (GGA) was used for the exchange and correlation potential, and all the calculations were carried out with the PBE functional.28 The Kohn-Sham wave functions for the valence electrons were expanded on a plane-wave basis set within an energy cutoff, which is chosen to be 380 eV. The Monkhorst-Pack scheme κ-point grid sampling was set as 3 × 3 × 1 for the irreducible Brillouin zone. The convergence criteria for structural optimization and energy calculation were 1×10−6 eV/atom for a self-consistent field (SCF). 3. RESULTS AND DISCUSSION 3.1 Catalyst characterizations. XRD patterns of as-synthesized Ga4B2O9 and Ga-PKU-1 are shown in Figure 3, both are consistent with the data reported in literature.11,29 The former one exhibits relatively wide diffraction peaks while the peaks for the latter one are quite sharp,


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indicating the difference in morphology. Indeed, Ga4B2O9 comprises of compact aggregates of nano-rods, and Ga-PKU-1 has a high crystallinity and the average size in length is estimated to be about 10-20 µm (see the inset of Figure 3). The NH3-TPD curve for Ga-PKU-1 presents a wide desorption peak centered at 430 ºC (see Figure 4a). It is reasonably originated from the release of NH3 adsorbed on the acid sites with a moderate intensity, while Ga4B2O9 has no detectable peak. Moreover, the CO2-TPD curves further consolidate the acidity for Ga-PKU-1 and the basicity for Ga4B2O9 (see Figure 4b). Although three-coordinated B atoms in BO3 are usually considered as potential Lewis acid sites, here the Lewis acidity of BO3 in Ga4B2O9 is negligible. Quantitatively, the desorbed probe molecules were confirmed by the inverse titration method. Ga-PKU-1 possesses a much larger number of acidic sites and the higher acidic strength, while Ga4B2O9 possesses much more basic sites than Ga-PKU-1. This means two compounds are completely different in nature and Ga-PKU-1 should be superior in acidcatalyzed reactions, while Ga4B2O9 would be the better one in base-catalyzed reactions.

Figure 3. XRD patterns and SEM images for Ga4B2O9 and Ga-PKU-1.


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Figure 4. (a) NH3- and (b) CO2-TPD profiles for Ga-PKU-1 and Ga4B2O9. 2.2 The ascertainment of active basic sites in Ga4B2O9 by DFT calculations. DFT calculations were applied to unravel the origin of the structure-induced Lewis basicity. A reasonable approximation needs to be applied for simplicity during the calculations. In detail, we built an ordered and superstructure model with a- and c-axes lengths been doubled and the symmetry was selected as P1. Geometry optimizations starting from this model were performed and a fine convergence could be achieved, in the following, the Bader charge distributions were


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calculated. Bader charge analysis was implemented by Henkelamn and his co-workers, which is a widely used approach to analyze the charge transfer and population.30 Table 1. Surrounding environments and average Bader charge distribution for oxygen atoms in Ga4B2O9 Oxygen Surrounding environment a) Average Bader Charge distribution O1a

2Ga(5)+2Ga(6)

-0.87|e|

O1b

1Ga(5)+2Ga(6)

-0.89|e|

O2

2Ga(5)+2Ga(6)

-0.90|e|

O3

2Ga(6)+1B

-0.80|e|

O4

2Ga(6)+1B

-0.81|e|

O5

1Ga(5)+1Ga(6)+1B

-0.82|e|

O6

1Ga(5)+1Ga(6)+1B

-0.82|e|

O7

3Ga(5)

-0.92|e|

O8

2Ga(5) +1B

-0.79|e|

a)

The surround environments of each oxygen atom, for example, the representation of “2Ga(5)+1Ga(6)” means the central O atom bonds to two 5-coordinated and one 6-coordinated Ga3+. The average charge distributions for oxygen atoms, along with the surrounding environment are listed in Table 1. The average charge on oxygen is either ~- 0.80 |e| or ~ -0.90 |e|, respectively, with or without the presence of B atoms as the surrounding atom. O bonded with B has a relatively lower Bader charge because B also has a strong electron-withdrawing ability. Here, the preliminary calculations suggest the Lewis base sites are located at the oxygen atoms linked with 5-coordinnated Ga3+, i.e. O1, O2, and O7.


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Figure 5. (a) Top and (b) side views of the optimized structural model for Ga4B2O9 with an exposed surface along the (100) plane. To simulate the charge distributions in real surface, a new structural model with the (110) plane exposed was built. The b-lattice parameter was doubled, and a vacuum layer was created as shown in Figure 5. Here, we purposely exposed O7 and O8 as the terminal atoms as they are representative atoms with the Bader charge of - 0.90 |e| or ~ -0.80 |e|, respectively. After another round of structural optimization, the Bader charges were again calculated and provided in Table S2, which are generally consistent with the results in Table 1. Then, the calculations of the adsorption to the acidic molecule CO2 on this exposed surface were performed as follows.31 The


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binding energy of adsorbed CO2 is defined as: Eb = Esur + ECO2 – Ead. Esur is the total energy of the bare slab of the surface, ECO2 is the total energy of free CO2, and Ead is the total energy of the slab with the adsorbed CO2 molecule on the surface. Therefore, a positive Eb value corresponds to an exothermic adsorption.32

Figure 6. (a, c) Top and (b, d) side views of two stable configurations of CO2 adsorbed on the Ga4B2O9 (100) surface. The CO2 molecule is perpendicular to the surface plane and the C atom points to either O7 or O8. Table 2. Adsorption energies and structural parameters of CO2 adsorbed in two different configurations on the surface of Ga4B2O9 Adsorption site

C-Ol bond length (Å)

C-Or bond length (Å)

Ol-C-Or bond angle (°)

Eb (eV)

O7

1.18

1.18

175.1

2.74

O8

1.18

1.18

176.8

1.83


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Calculations give two stable configurations for one CO2 molecule adsorbed on the surficial O7 or O8 site. As shown in Figure 6, the binding configurations of the CO2 molecule is perpendicular to the (100) surface and the C atom points to either O7 or O8, respectively. The corresponding binding energies and the structural parameters for both configurations are listed in Table 2. Accordingly, the adsorption mode of CO2 on O7 site is the most favorable state energetically. In detail, the configuration on O7 site provide a suitable distance in a wellcoordinated manner between Ol or Or with adjacent Ga3+ to generate a certain degree of coulomb attraction, which thus significantly enhances the structural stability. Furthermore, the generation of coulomb attraction could allow all three atoms of CO2 molecule (Ol, Or and C) and the other three atoms on the surface (O7 and two Ga) to form a six-membered ring, which is also an important factor to the high stability of the O7 adsorption mode. The above analyses illustrate that the acidic CO2 molecule is inclined to be adsorbed at the O7 position, where a strong interaction will occur and a multiple-atom ring will formed. Therefore it is reasonable in thermodynamics to conclude that the O7 sites on the (100) surface will be approached by substrate molecules during the base-catalyzed reactions. 2.3 Strecker reaction catalyzed by Ga4B2O9. Strecker reaction is an effective route to synthesize α-aminonitriles, which are very useful intermediate products serving as precursors for the synthesis of versatile α-amino acids, various nitrogen-containing heterocyclic compounds and some useful biologically molecules.33-36 Here, Ga4B2O9-catalyzed Strecker reactions using imine 1a (4-methyl-N-phenylmethylene-benzenesulfonamide) as the substrate molecule were performed by varying the reaction parameters, including the solvents type, catalysts equiv., TMSCN equiv., and reaction temperatures, in order to acquire the optimal condition. As summarized in Table 3, first, chloroform was found to be best medium with a maximum yield of


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81.0%. Second, the chemical equivalents of both Ga4B2O9 and TMSCN have a strong influence on the yield of α-aminonitriles and the optimal parameters were 0.3 and 2 equiv., respectively. Third, the optimal reaction temperature was determined to be 45 oC, above which a negative effect was observed because the imine precursors would become unstable and easily decomposed into other byproducts at over-high temperatures. When extending the scope of the substrate to 4 more different aromatic imines, most aromatic imines reacted efficiently with TMSCN to form the corresponding products with high yields when catalyzed by Ga4B2O9 (see Table 3, Entry 1316). Most importantly, the reusability and stability of Ga4B2O9 was also confirmed by the cycling experiments, where the Ga4B2O9 powder catalyst can be easily recycled by a simple filtration, solvent-washing and drying. As shown in Figure 7, the yields of α-aminonitriles in all five runs were almost unchanged and the identical XRD patterns before and after the catalysis reactions consolidated the high stability of Ga4B2O9. In addition, the base-catalyzed mechanism was also proposed according to the aforementioned strong basicity of µ3-O as shown in Figure 8. In this mechanism, the µ3-O on distorted GaO5 polyhedra is able to adopt Si4+ from TMSCN due to its strong ability to offering electrons as well as the small steric hindrance. In detail, when Strecker reaction was ongoing, Ga4B2O9 was speculated to facilitate the dual-activation of both substrates, in which one is the activation of silicon in TMSCN by the Lewis base moiety (O2-), the other one is the activation of nitrogen in imine by the Lewis acid moiety (Ga3+). As illustrated in Figure 8, TMSCN was activated by O2- site and thereafter coordinated with it to form a penta-coordinate silicate species, thus the cyanide group can be polarized to acquire a high reactivity. The resultant intermediate species of penta-coordinate silicate was proved elsewhere by solid state 29Si MAS-NMR spectra.25,37 Thereafter, the activated imine by Ga3+ was attracted by the highly reactive cyanide ion to form an N-TMS intermediate.


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It was subsequently hydrolyzed to the Strecker product due to the cleavage of the rather labile NSi bond by a trace amount of moisture present in the solvent, which is important to accelerate the transformation of the substrate, as observed earlier by Feng et al.38 Table 3. Summarized results for Strecker reaction over Ga4B2O9 catalyst a)

Catalyst

TMSCN

Temp.

Yield b)

TOF c) (h-

equiv.

equiv.

(ºC)

(%)

1

Entry

Solvent

Imine

1

Chloroform

1a

0.3

2

45

81.0

0.68

2

Toluene

1a

0.3

2

45

56.9

0.49

3

Acetonitrile

1a

0.3

2

45

67.4

0.58

4

Chloroform

1a

0

2

45

1.2

0.01

5

Chloroform

1a

0.1

2

45

50.7

0.43

6

Chloroform

1a

0.2

2

45

67.7

0.58

7

Chloroform

1a

0.4

2

45

80.9

0.69

8

Chloroform

1a

0.3

1

45

63.4

0.54

9

Chloroform

1a

0.3

3

45

77.7

0.67

10

Chloroform

1a

0.3

2

25

56.9

0.49

11

Chloroform

1a

0.3

2

35

67.4

0.58

12

Chloroform

1a

0.3

2

55

58.1

0.50

)


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13

Chloroform

1b

0.3

2

45

64.1

0.55

14

Chloroform

1c

0.3

2

45

35.5

0.30

15

Chloroform

1d

0.3

2

45

53.5

0.46

16

Chloroform

1e

0.3

2

45

58.5

0.50

a)

Reaction conditions: 0.2 mmol imines, Ga4B2O9 catalyst, 1 mL solvent, nitrogen atmosphere, 4h;

b)

Yield was calculated by 1H NMR measurements;

c)

TOF = turnover frequency (h−1), it means moles of reactants converted per moles of catalysts per unit time.

Figure 7. Reusability of Ga4B2O9 and its structural stability identified by XRD patterns after 5 cycles.


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Figure 8. The proposed mechanism for the Strecker reaction catalyzed by Ga4B2O9. 2.4 Structure evolution from Ga-PKU-1, to Ga4B2O9, GaBO3 and the catalytic performances. It is interesting that appropriate heating on Ga-PKU-1 would lead to the transformations to Ga4B2O9 and GaBO3, though they have completely different structural characteristics. First, we start with Ga2O3 (i.e. the β-polymorph), which is a close-to-neural oxide. Once it reacts with H3BO3 under hydrothermal condition, the Brönsted acid, Ga-PKU-1 will form. The framework of Ga-PKU-1 features the exclusive 6-coordinated Ga3+, and a large amount of B-OH groups are distributed throughout the surface, offering the Brönsted acidic centers to promote acid-catalyzed reactions. Upon heating at ~610 °C, a proportion of GaO6 groups will undergo an atomic rearrangement and change to 5-fold coordination style of GaO5, in other words, this structure transformation makes the µ3-O linked to 5-coordinated Ga3+ be the


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strong Lewis basic sites in Ga4B2O9. Further heating at 750 °C, all gallium will change back to 6fold coordination and form the calcite-type GaBO3. It is very interesting that these three gallium borates have a similar elemental components, while they exhibit quite different activities in catalytic Strecker reactions. As shown in Table S3, Ga4B2O9 exhibited generally higher catalytic activities for all five imine substrates than other two gallium borates. For example, when employing imine 1a as reactant, the yield of αaminonitrile catalyzed by Ga4B2O9 is 5 times of that catalyzed by Ga-PKU-1. However, when using GaBO3 as the catalyst, all the yields of α-aminonitrile are very poor. In literature, people proposed that the acid- and base-catalyzed Strecker reactions had undergone two completely different reaction pathways. Acid active sites catalyzed the cyanosilylation of imine in Strecker reaction by simultaneously activating the nitrogen-atom of imines and cyano group of TMSCN.12,13,21 On the other hand, basic active sites usually activated silicon atom of TMSCN to produce more reactive hypercoordinated silicon intermediates.39,40 Although the amount of basic sites in Ga4B2O9 is only a third of the acidic sites in Ga-PKU-1, as previously indicated in Figure 4, Ga4B2O9 did exhibit a higher catalytic efficiency in Strecker reaction. Lewis basic sites in Ga4B2O9 are more efficient in this catalytic reaction than the Brönsted acid sites in Ga-PKU-1. 2.5 Comparison with other basic oxides in the catalytic Strecker reaction. Seven typical mono-component basic oxides were selected as the comparisons and the CO2-TPD measurements provided their total basic amounts and strength distributions (see Table 4). For example, MgO contains the maximal amount of basic sites, however, all of them belong to the weak or moderates basic sites. Ga4B2O9 has a less amount of basic sites, while assigned


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exclusively to the strong basic sites. Again, the catalytic Strecker reaction was employed to evaluate their performances as shown in Figure 9a. Ga4B2O9 exhibited the highest catalytic activity among all the solid basic catalysts, despite of the much less basic sites of Ga4B2O9 than some other oxides. Reasonably, the strength of the basicity is more important in the Strecker reactions than the number of basic sites on the surface. Table 4. The amount and strength distributions of basic sites on the surface of different metal oxides according to CO2-TPD measurements Amount of Lewis base (µmol/g) Oxides

Total amount Weak base a) Moderate base b) Strong base c)

MgO

182.5

74.3

108.2

0

CeO2

54.3

28.8

14.4

11.1

Al2O3

43.2

19.2

24.0

0.0

ZnO

24.1

14.6

9.5

0.0

SnO2

13.0

5.2

5.4

2.4

TiO2

8.4

3.4

2.4

2.6

SiO2

2.2

2.2

0

0

0

0

23.1

Ga4B2O9 23.1 a)

peaks located in the range of 100-190 oC;

b)

peaks located in the range of 191-310 oC;

c)

peaks located in the range of 311-550 oC.

2.6 Catalytic conversion of n-propanol by Ga4B2O9 at high temperature. A commonly accepted catalytic scheme is that the dehydration of n-propanol will occur mainly with the involvement of acidic centers, whereas the dehydrogenation will preferentially involve basic or redox ones.4,5 Due to the different nature of Ga-PKU-1 and Ga4B2O9, they indeed gave


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completely different catalytic products. As indicated in Figure 9b, Using Ga4B2O9 as the base catalyst led to the high selectivity of propanal (79%), compared to the very high selectivity of propylene (94%) when using Ga-PKU-1. According to the solid acid-base theory, 41,42 Ga-PKU-1 has a large amount of Brönsted acid sites, n-propanol is absorbed on the B-OH in the surface of Ga-PKU-1, probably by hydrogen bonding, to form protonated species C3H7OH2+ by the transfer of a proton from Brönsted acid site in Ga-PKU-1. Subsequently, the absorbed n-propoxide C3H7OH2+ slowly splits to give water and carbocation C3H7+ ion, which in turn loses a βhydrogen and gives propylene in synchronization with the hydroxyl group. In a similar way, the formation of dehydrogenation product would involve with the intermediate n-propoxidecarbanion species.43,44 Instead of adsorbing on -OH groups, n-propanol interacts as electron-pair donor with the Lewis acidic sites (5-coordinated Ga3+) in Ga4B2O9, while oxygen atoms neighboring to 5-coordinated Ga5+ will act as Lewis basic sites to adsorb hydrogen atoms coming from -OH and -CH groups of n-propanol. Subsequently, the adsorbed acetone is generated by the elimination of these two activated hydrogen atoms giving a H2 molecule.


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Figure 9. (a) Yields of product in the Strecker reaction catalyzed over some different basic oxides. Reaction condition: imine (0.2 mmol), TMSCN (0.4 mmol), catalyst (0.3 equip.), chloroform (1 mL), 4h, 318 K, N2 atmosphere. (b) Catalytic conversion of n-propanol using both Ga4B2O9 and Ga-PKU-1 catalysts. 4. CONCLUSION We proposed that the distorted GaO5 polyhedra could behave as the Lewis acidic-basic groups, and thus discovered that Ga4B2O9 with the mullite-type structure exhibited an intrinsic Lewis basicity despite of its local superstructure. Its basicity as well as the acidity of its


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comparison gallium borate, Ga-PKU-1, have been proved by the CO2- and NH3-TPD experiments. Furthermore, the DFT calculations supported our conjecture that the origin of the strong basicity is the presence of the µ3-O neighboring with 5-coordinated Ga3+ exclusively and these oxygen atoms are likely the adsorption sites in base-catalyzed reactions. Experimentally, the synthesis of α-aminonitriles by Strecker reaction, which can be catalyzed by either acid or base catalysts, was first selected as the model reaction. Interestingly, although Ga4B2O9 has fewer basic sites (23.1 µmol/g) than the acidic sites (68.9 µmol/g) in Ga-PKU-1, Ga4B2O9 exhibited a superior catalytic efficiency than Ga-PKU-1, suggesting that the base-catalytic process is more efficient. Moreover, the catalytic efficiency of Ga4B2O9 is also higher than other selected mono-metal basic oxides, indicating that the strength of the basicity is more favorable than amount of the active sites in this Strecker reaction. To approach the industry applications, a solid-gas catalytic reaction at high temperature was also attempted using Ga4B2O9 as the base catalyst, indeed, Ga4B2O9 catalyzed efficiently the dehydrogenation of n-propanol, resulting in a high selectivity to propanal (79%), while Ga-PKU-1 catalyzed the dehydration process to obtain propylene with a selectivity of 94%. Future work could be proceed in the morphology controllable synthesis and transition metal-doping induced bi-functionality. Most importantly, the discovery of structural origin to enhance the basicity of O is enlightening for further development of bulk-type Lewis bases and the applications in chemistry industry.

ASSOCIATED CONTENT Supporting Information.


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Tables for Bader charge distribution for Ga4B2O9, Strecker reactions catalyzed over Ga4B2O9, Ga-PKU-1 and GaBO3. 1H and 13C NMR spectra for substrates and products in Strecker reactions. AUTHOR INFORMATION Corresponding Author *[email protected]. *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 91222106, 21671028, 21771027) and the Natural Science Foundation of Chongqing (Grant Nos. CSTC 2014jcyjA50036, 2016jcyjA0291). REFERENCES (1) Hattori, H. Heterogeneous Basic Catalysis. Chem. Rev. 1995, 95, 537-558. (2) Larmier, K.; Chizallet, C.; Maury, S.; Cadran, N.; Abboud, J.; Lamic-Humblot, A. F.; Marceau, E.; Lauron-Pernot, H. Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols. Angew. Chem. Int. Ed. 2017, 56, 230-234.


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SYNOPSIS.

A clear mechanism to enhance the Lewis basicity of oxygen has been revealed which could enlighten the further discovery of strong and durable solid bases. Ga4B2O9 is such a case to clearly show the structure-induced strong basicity from a special connectivity between Ga and O. Solid base-catalyzed reactions were employed to evaluate its strong and durable basicity.


33

Strong Lewis Base Ga4B2O9: Ga-O Connectivity Enhanced Basicity

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