- Email: [email protected]
Effects of post-thermal treatments of ball-milled boron nitrides on solid base catalysis
Catalysis Today xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Effects of post-thermal treatments of ball-milled boron nitrides on solid base catalysis Atsushi Takagakia,b,* a b
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0395, Japan
ARTICLE INFO
ABSTRACT
Keywords: Boron nitride Knoevenagel condensation Solid base catalyst
Ball-milled hexagonal boron nitride was investigated as a solid base catalyst. The ball-milled boron nitrides were heated in a vacuum or air and used for Knoevenagel condensation. Measurements by X-ray diffraction, nitrogen adsorption, thermogravimetry-mass spectrometry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy indicated that the heat treatment under vacuum hardly changed the physicochemical properties including crystal structure and surface composition whereas the heat treatment in air significantly altered the properties due to the formation of B2O3. The number of basic sites increased with the increase of the evacuation temperature. The boron nitrides after evacuation gave moderate product yield for Knoevenagel condensation of benzaldehyde with malononitrile at 40 °C. The boron nitrides after calcination showed different activity, dependent on the calcination temperature. The boron nitride after ball-milling, followed by calcination at 300 °C exhibited the highest activity. The catalytic activity was not directly related to the base amount and the surface areas, suggesting that acid-base bifunctionality plays an important role in Knoevenagel condensation using the ball-milled boron nitride catalysts.
1. Introduction Solid acid and base catalysts offer the opportunity to reduce the impact on the environment and increase profits because they are reusable and readily separable from the reaction product in the liquid phase [1]. Solid base catalysts are useful to produce fine chemicals via CeC bond formation reactions, including aldol condensation, nitroaldol reaction, Knoevenagel condensation, and Michael addition [2,3]. These catalysts are also necessary to transform biomass into chemicals and fuels. For example, isomerization of glucose to fructose is an important reaction to form 5-hydroxymethylfurfural [4,5], and aldol condensation of 5-hydroxymethylfurfural with acetone [6] is a typical reaction to produce sustainable aviation fuels. Several solid bases such as alkaline earth metal oxides, hydrotalcites [7,8], zeolites [9], and aminosilicas [10,11] have been investigated so far. It is known that the coexistence of acid and base sites significantly accelerates typical base-catalyzed reactions including aldol reaction, nitroaldol reaction, Knoevenagel condensation [12–14]. Such acid–base cooperative catalysts include amine-tethered silica materials (mesoporous silica [10], and silica-alumina [11]) which have weak acid silanol groups. For aldol reaction, the weak acid sites could stabilize the carbonyl groups of reactants, resulting in enhancement of the reaction ⁎
rate. The distance between acid-base pairs, the ratio of acid and base concentrations, the strengths of acid and base sites are critical to increase the reactivity. The emergence of the acid-base pair was also possible via cleavage of SieOeSi bond on a mesoporous silica by treatment with ammonia at high temperature [15]. A well-defined pair of Si−OH and Si–NH2 was formed, which could catalyze Knoevenagel condensation. Hexagonal boron nitride has a similar structure as graphite and studied for applications outside of catalysis because of its high chemical and thermal stability. However, recently, hexagonal boron nitride was found to be an efficient catalyst for oxidative dehydrogenation of light alkanes [16–18]. Also, we found that ball-milled hexagonal boron nitride functioned as a new solid base catalyst [19,20]. The ball-milling treatment results in a significant increase in the surface areas and the emergence of hydroxyl and amino groups on the surface. It was suggested that these functional groups were formed via cleavage of BeN bond by moisture. The acid and base sites could be formed at adjacent positions on the surface. The ball-milled boron nitride exhibited high activity for base-catalyzed reactions, including the nitroaldol reaction, Knoevenagel condensation, and isomerization of glucose. The activity for the nitroaldol reaction was higher than those of conventional solid bases such as MgO and hydrotalcite because of its acid-base
Correspondence to: Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. E-mail address: [email protected].
https://doi.org/10.1016/j.cattod.2019.11.007 Received 1 July 2019; Received in revised form 24 October 2019; Accepted 7 November 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Atsushi Takagaki, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.11.007
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 1. X-ray diffraction patterns of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air.
Fig. 2. N2 isotherms of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air.
bifunctionality [19]. The processing conditions of ball-mill greatly influenced the physicochemical properties and the solid base catalytic activity. Ball-milling at 400 rpm gave the highest surface area while higher rotation speeds decreased the surface areas. The number of base sites increased with the increase of rotation speeds, which is a good agreement with the formation of amino groups. The boron nitride ballmilled at 400 rpm exhibited the highest catalytic activity for the nitroaldol reaction. To further improve the catalytic activity, the effects of post-thermal treatment were investigated in this study. The ball-milled boron nitrides were heated at several temperatures in a vacuum (evacuation) or air (calcination). The samples were characterized by a variety of techniques and tested for Knoevenagel condensation.
of 40 kV and a current of 80 mA with 2θ values from 5° to 70°. The surface area of the catalysts was evaluated by the Brunauer–Emmett–Teller (BET) method using nitrogen adsorption (BELSORP mini, Microtrac-BEL). The sample was pretreated by evacuation at the temperatures of the post-treatment of the ball-milled boron nitride for 1 h before the measurement. The pore size distribution of the sample was obtained from the analysis of the adsorption branches of the isotherms using the Barrett, Joyner, and Halenda (BJH) method. The surface functional groups of the catalysts were characterized by Fourier transform infrared spectroscopy (FTIR, FT/IR-6600, JASCO) measurements. The sample was pressed into pellets with KBr for the measurements. The oxidation state of elements and their compositions over the surface were analyzed by X-ray photoelectron spectroscopy (XPS, KRATOS Ultra 2, Shimadzu). The binding energies in each measurement were referenced to the core level of the C1s peak (284.8 eV). The thermal stability of the ball-milled BN sample in N2 and air was examined using thermogravimetry-mass spectrometry (TG-MS, Rigaku)．About 10 mg of sample was heated from room temperature to 550 °C at a heating rate of 2 °C min-1 under N2 or air. The base strength of the catalysts was evaluated by using a color indicator reagent [21]. A quantity of 50 mg of sample was added to 1 mL of a toluene solution containing 0.4 mg of bromothymol blue (pKa = 7.2) for 1 day. The base amounts of the catalysts were determined by titration using 0.01 M benzoic acid in toluene for the bromothymol blue-adsorbed BN. The acid strength and amounts of the catalysts were also examined by using 4-(phenylazo)diphenylamine (pKa = 1.5) as a color indicator and nbutylamine as a titrant.
2. Experimental 2.1. Catalyst preparation Layered hexagonal boron nitride (Wako) was ball-milled at 400 rpm for 12 h using a planetary ball mill (Pulverisette 7, Fritsch, zirconia vessel (12 mL) with six zirconia balls (diameter 10 mm). Ball milling was conducted for 24 cycles in which each cycle was carried out for 30 min with 5 min intervals. The rotation direction was reversed in each cycle. The ball-milled boron nitride was heated at 150−400 °C in a vacuum or air for 1 h. The samples prepared were denoted as BNbm150-400vac and BNbm-150−400 cal, respectively. 2.2. Characterization
2.3. Knoevenagel condensation
The crystal structure of the catalysts was determined by X-ray diffraction (XRD) (RINT-2500, Rigaku) with Cu Kα radiation at a voltage
Knoevenagel condensation was conducted using a BN catalyst 2
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 3. Pore size distributions of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air. Table 1 BET surface areas and pore volumes of BN after ball-milling followed by heating at various temperatures in a vacuum or air. Sample
Heat treatment Temp./oC
BNbm BNbm-150vac BNbm-200vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
in a vacuum
in air
− 150 200 300 400 150 200 300 400
BET surface area/ m2 g-1
Pore volume/ mL g-1
215 250 243 301 309 46 61 145 141
0.33 0.33 0.42 0.39 0.39 0.26 0.31 0.45 0.44
Fig. 5. FTIR spectrum of ball-milled BN.
samples heated in a vacuum, indicating that bulk crystal structure remained unchanged by evacuation at 150−400 °C. In contrast, additional peaks at 14.6 and 27.9° were observed for the samples heated in air at 200−400 °C. These peaks were attributed to B2O3 (PDF #60297), indicating that the BN samples after ball-milling was partly oxidized in air. These peaks became apparent for the sample calcined at 400 °C. Our previous study also showed that a further increase of calcination temperature to 700 °C in air resulted in a clear formation of B2O3 [17]. Figs. 2 and 3 show the N2 adsorption isotherms and pore size distributions of the BN samples after ball-milling followed by the heat treatment in a vacuum or air. Table 1 lists the surface areas and pore volumes of the BN samples. The BET surface area of the ball-milled BN before the post-treatment was 215 m2 g-1. The surface area was slightly increased to 250 m2 g-1 for BNbm-150vac. A large increase of the surface areas was observed for BNbm-300vac and BNbm-400vac, which were 301 and 309 m2 g-1, respectively. The increase of surface areas of the samples was attributed to the removal of water. In contrast, a drastic change of the surface areas was found for the BN samples by calcination. The surface areas were decreased to 46 m2 g-1 for BNbmcal150 and 61 m2 g-1 for BNbm-cal200, and then increased to 145 m2 g-1 for BNbm-cal300 and 141 m2 g-1 for BNbm-cal400. The pore size distributions of the calcined samples indicated that micropores were significantly decreased by calcination below 200 °C. The decrease of the surface areas and micropores could be due to the formation of amorphous boron (hydro)oxide on the surface of the ball-milled BN. The further increase of calcination temperature results in an increase in the surface areas and micropores. It seems that the transformation of
Fig. 4. TG-MS curves of ball-milled BN sample in N2 and air.
(50 mg) in toluene (4 mL) with benzaldehyde (1.0 mmol, Wako), malononitrile (1.0 mmol, Wako) and n-decane (0.2 mmol, Wako) as an internal standard. The reaction was performed at 40 °C for 60 min. Aliquots were taken using a syringe and analyzed by gas chromatography (GC-FID; GC-2014, column DB-1MS, Shimadzu). 3. Results and discussions Fig. 1 shows the XRD patterns for BN samples after ball-milling, followed by evacuation or calcination at several temperatures. The ballmilled BN before the post heat treatment showed five peaks at 26.7, 41.5, 43.8, 50.1, and 55.1 degrees, which were attributed to the (002), (100), (101), (102), (004) reflections of hexagonal boron nitride (PDF #34-0421), respectively. No apparent change was found for the 3
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 6. FTIR spectra of BN after ball-milling followed by in situ evacuation at various temperatures. (a) 3800–3000 cm-1. (b) 1800–700 cm-1.
Fig. 7. FTIR spectra of BN after ball-milling followed by in situ calcination at various temperatures in air. (a) 3800–3000 cm-1. (b) 1800–700 cm-1.
Fig. 8. B1s XPS spectra of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air, and (c) h-BN and B2O3.
amorphous boron (hydro)oxide to crystal B2O3 occurred because the oxygen contents on the surface of the samples were monotonically increased as observed by XPS (see below). Fig. 4 shows the results of TGMS of the ball-milled BN sample under N2 and air flow. A continuous weight loss was observed from room temperature to ca. 300 °C in N2, which was attributed to the removal of
water adsorbed and the dehydration of B–OH groups as monitored by the mass signal of 18. A noticeable weight loss was also found to 500 °C in air with the decrease of the mass signal of 18. It seems that there are three steps with different slope of weight loss during the dehydration in air. First is the removal of physisorbed water from room temperature to 100 °C. Second is the dehydration of B–OH groups on the surface to 4
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 9. Distribution of boron species in BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air. B–N: a peak centered at 190.4 eV. B–O(I): a peak centered around 192 eV. B–O(II): a peak centered around 193.2 eV.
Fig. 10. N1s XPS spectra of BN after ball-milling followed by heating at various temperatures in (a) a vacuum, and (b) air.
Fig. 11. Distribution of nitrogen species in BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air. N-B: a peak centered around 398.0 eV. Amino: a peak centered around 399.1 eV.
temperature to 400 °C. The OeH stretching band around 3400 cm-1 became weak with the increase of evacuation temperature from room temperature to 300 °C, indicating that adsorbed water was removed, and also a part of the OeH groups on the surface were dehydrated. The NeH stretching band around 3160 cm-1 also became a little weak, suggesting that some of the amino groups were dehydrated with the hydroxyl groups, leaving the remaining amino groups intact. A weak band around 1200−1300 cm-1 became intense with the increase of evacuation temperature, which could be assigned to B–O [26] because an absorption at 1190 cm-1 was observed for B2O3 [27,28]. Fig. 7 shows FTIR spectra of BN samples after ball-milling followed by in situ
form amorphous boron (hydro)oxide from 100 °C to 250 °C. Third is the successive dehydration of OH groups to transform to B2O3. Fig. 5 shows the FTIR spectrum for the ball-milled BN. Four absorption bands were observed, which is in a good agreement with the previous studies [19,20]. Strong absorption at 1382 cm-1 corresponds to B–N stretching [22] and a band at 815 cm-1 to B–N-B bending [23]. A broad band around 3400 cm-1 is assigned to OeH stretching vibration, and a shoulder around 3160 cm-1 can be ascribed to NeH stretching vibration [24,25]. The former band includes not only OH groups on the surface but also adsorbed water. Fig. 6 shows FTIR spectra of BN samples after ball-milling followed by in situ evacuation from room 5
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Table 2 Surface compositions of BN after ball-milling followed by heating at various temperatures calculated from XPS. Sample
Surface compositions
BNbm BNbm-150vac BNbm-200vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
B/mol%
N/mol%
O/mol%
39.2 38.3 39.1 38.8 39.8 40.9 39.3 37.3 34.3
41.2 41.1 40.2 39.9 41.7 33.9 35.8 33.8 28.6
19.7 20.6 20.8 21.3 18.5 25.2 24.9 28.9 37.1
N/B
O/B
1.05 1.07 1.03 1.03 1.05 0.83 0.91 0.91 0.83
0.50 0.54 0.53 0.55 0.46 0.62 0.63 0.78 1.08
Fig. 13. Time-courses of Knoevenagel condensations using ball-milled BN catalysts. Reaction conditions: benzaldehyde (1.0 mmol), malononitrile (1.0 mmol), catalyst (20 mg), toluene (4 mL), 40 °C.
1200−1300 cm-1 were significant, which are due to the formation of B2O3. Fig. 8 shows core-level spectra of B1s of BN samples after ballmilling followed by heating at several temperatures in a vacuum and air. As for comparisons, the spectra of B1s of B2O3 and h-BN are also shown. The sample without post-treatment, BNbm, had two peaks centered at 190.4 and 192.1 eV. The peak position of 190.4 eV is ascribed to B–N, which is in a good agreement with those reported for bulk h-BN [29,30]. The peak around 192 eV was not observed for bulk h-BN. Thus the peak at 192.1 eV in BNbm is assigned to oxidized boron, which is related to the formation of B–OH groups as reported previously [31,19,20,32]. The post-treatment in a vacuum did not change the spectra. In contrast, the post-treatment in air gave an additional peak around 193.2 eV. The peak of 193.2 eV was also observed for B2O3. Thus the peak is ascribed to B–O related to B2O3. Fig. 9 shows a fraction of boron species in BN after the post-treatment in vacuum and air. For BN samples after evacuation (Fig. 9(a)), no apparent change of the distribution was found. For BN samples after calcination (Fig. 9(b)), the fraction of B–O(II) corresponding to the peak of 193.2 eV increased with the increase of calcination temperature, which is in a good agreement with XRD results. Fig. 10 shows core-level spectra of N1s of BN samples after ballmilling followed by heating in a vacuum and air. The spectra for the samples could be deconvoluted into two components. One is a peak around 398.0 eV, which is ascribed to NeB. The other is a peak around 399.1 eV, which is likely attributable to amino groups. Fig. 11 shows a fraction of nitrogen species in BN. For BN samples after evacuation (Fig. 11(a)), the distribution remained unchanged as observed for B1s spectra. For BN samples after calcination (Fig. 11(b)), however, the fraction of amino groups decreased. Table 2 and Fig. 12 show the surface compositions, N/B ratio, and O/B ratio of BN samples before and after the heat treatments. The ratios of N/B and O/B for BNbm were 1.05 and 0.50, respectively. The compositions of the BN samples after evacuation remained unchanged, indicating that no additional formation of boron oxide species. In contrast, the O/B ratio of the BN samples after calcination largely increased from 0.50 to 1.08 with the increase of calcination temperature. The increase in oxygen contents was apparent for BNbm-300 cal and BNbm-400 cal. The solid base property was evaluated using bromothymol blue as a color indicator reagent (Table 3). Treatment of the samples after evacuation (BNbm-vac) and calcination at a lower temperature (BNbm150 cal and BNbm-200 cal) with a solution of toluene containing bromothymol blue (pKa = 7.2) resulted in green color whereas the samples
Fig. 12. N/B and O/B ratios of BN after ball-milling followed by heating at various temperatures in a vacuum or air. Table 3 Coloration by bromothymol blue (pKa = 7.2) and titration of benzoic acid. Sample
Coloration by Bromothymol blue
Base amount (H_ ≥ +7.2)/mmol g-1
BNbm BNbm-150vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
+ + + + + + − −
0.60 0.20 0.44 0.61 0.04 0.12 0 0
Table 4 Coloration by 4-(phenylazo)diphenylamine (pKa = 1.5) and titration of n-butylamine. Sample
Coloration by 4-(phenylazo) diphenylamine
Acid amount (H0 ≤ +1.5)/mmol g-1
BNbm BNbm-150vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
− + + + − − + +
0 0.28 0.37 0.29 0 0 1.81 0.69
calcination from room temperature to 400 °C. The tendency for the change of the spectra by calcination was the same as that by evacuation. However, it should be noted that the decrease in the intensity of OeH bands around 3400 cm-1 and the increase of BeO around 6
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Table 5 Results of Knoevenagel condensation using BN catalysts after ball-milling followed by heat treatment.
Sample
Benzalmalononitrile yield/%
SBET/m2 g-1
Base amount/mmol g-1
BNbm BNbm-150vac BNbm-300vac BNbm-400vac BNbm-200 cal BNbm-300 cal BNbm-400 cal
86 58 82 67 63 94 1
215 250 301 309 61 145 141
0.60 0.20 0.44 0.61 0.12 0 0
a
Acid amount/mmol g-1 0 0.28 0.37 0.29 0 1.81 0.69
b
O/B ratio 0.50 0.54 0.55 0.46 0.63 0.78 1.08
Reaction conditions: benzaldehyde (1.0 mmol), malononitrile (1.0 mmol), catalyst (20 mg), toluene (4 mL), 40 °C, 60 min. a Weak base. H_≥+7.2. b Very weak acid. H0 ≤ +1.5.
after calcination at a higher temperature (BNbm-300 cal and BNbm400 cal) gave a bright yellow color. This indicates that the samples evacuated had weak basic sites (H_ ≥ +7.2) but the samples calcined over 300 °C did not have such basic sites stronger than H_ ≥ +7.2. The base amount was estimated by titration using benzoic acid. The number of basic sites for BNbm-vac increased with the increase of the evacuation temperature. The BNbm-400vac had the greatest number of basic sites, 0.61 mmol g-1. The numbers of basic sites for the samples calcined were small, 0.04 mmol g-1 for BNbm-150 cal and 0.12 mmol g-1 for BNbm-200 cal. The solid acid property was examined using 4-(phenylazo)diphenylamine as a color indicator reagent (Table 4). Treatment of the samples after evacuation (BNbm-vac) with a solution of toluene containing 4-(phenylazo)diphenylamine (pKa = 1.5) resulted in purple color, indicating that the samples evacuated had very weak acid sites (H0 ≤ +1.5). The samples after calcination at higher temperatues (BNbm-300 cal and BNbm-400 cal) also showed very weak acidity. The numbers of acid sites for BNbm-vac were around 0.30 mmol g-1. The samples after calcination at higher temperatures had more acid sites in which the BNbm-300 cal had the greatest number of acid sites, 1.81 mmol g-1. In contrast, the samples after calcination at lower temperatures (BNbm-150 cal and BNbm200 cal) did not have such weak acid sites. The catalytic activity of BN was investigated by Knoevenagel condensation of benzaldehyde with malononitrile. While the reaction occurred even at room temperature [19], the reaction temperature was set to 40 °C in this study due to precise heat control. Fig. 13 shows the timecourses of the reaction using three ball-milled BN samples. BNbm could catalyze the reaction with the conversion of 24 % for 10 min and 100 % for 60 min. While BNbm-300vac showed lower activity than BNbm, BNbm-300 cal gave the conversion of 51 % for 10 min, two times higher than BNbm. Table 5 lists the results of Knoevenagel condensation using BN catalysts after post heat treatment. Other related physicochemical properties are also added. The BN samples after evacuation gave the product yields from 58 to 82 % for 60 min. Although BNbm-400vac had the greatest number of basic sites with the highest surface areas among the samples evacuated, BNbm-300vac showed the highest activity. A difference between these samples was that the surface O/B ratio of BNbm-400vac is smaller than others. It seems that the presence of OH groups plays a vital role in the activity, which could be related to the number of acid sites because BNbm-300vac had the greatest number of acid sites amount for the samples evacuated. Like aldol reaction [33] and nitroaldol reaction [34], the improvement of catalytic activity by acid-base bifunctionality for Knoevenagel condensation was also reported [35,36]. When comparing BNbm-150vac and BNbm-200 cal, the activity of BNbm-200 cal was close to that of BNbm-150vac whereas a large difference of surface areas for the two samples was observed, indicating that the surface areas did not affect the activity. Surprisingly,
while BNbm-300 cal had no apparent basic sites stronger than H_ ≥ +7.2, the sample exhibited the highest activity among the samples in this study. Also, a drastic change of the activity was found between BNbm-300 cal and BNbm-400 cal. The negligible activity for BNbm-400 cal is likely due to the increase of B2O3 phase, as shown in XRD and XPS. FTIR results of calcined BN sample showed that NeH stretching remained even at 400 °C, suggesting that BNbm-300 cal still has weak basic sites. In addition, the BNbm-300 cal had the greatest number of very weak acid sites. A possible explanation for the highest activity for BNbm-300 cal is likely due to the acid-base cooperative effect by hydroxyl groups and amino groups on the surface of ballmilled boron nitride. 4. Conclusions The heat treatment of the ball-milled boron nitrides under vacuum hardly changed their crystal structures and the surface compositions. The number of basic sites increased with the increase of the evacuation temperature. The heat treatment in air significantly altered their crystal structures, surface areas, and surface compositions, which were due to the formation of B2O3. The surface areas and the number of basic sites decreased with the increase of calcination temperature. The BN catalysts after evacuation gave moderate product yield. The BN catalysts after calcination showed different activity, dependent on the calcination temperature. BNbm-300 cal exhibited the highest activity, whereas BNbm-400 cal showed negligible activity. The catalytic activity was not directly related to the base amount and surface areas. It seems that acidbase bifunctionality plays an important role in Knoevenagel condensation using the ball-milled boron nitride catalysts. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by a Grand-in-Aid for Science Research (B) (No. 18H01785) of JSPS, Japan. References [1] [2] [3] [4] [5] [6] [7]
7
K. Tanabe, W.F. Hölderich, Appl. Catal. A Gen. 181 (1999) 399. Y. Ono, H. Hattori, Solid Base Catalysis, Springer-Verlag, Berlin: Heidelberg, 2011. W. Bing, M. Wei, J. Solid State Chem. 269 (2019) 184. C. Moreau, R. Durand, A. Roux, D. Tichit, Appl. Catal. A Gen. 193 (2000) 257. A. Takagaki, M. Ohara, S. Nishimura, K. Ebitani, Chem. Commun. (2009) 6276. A. Bohre, B. Saha, M. Abu-Omar, ChemSusChem 8 (2015) 4022. S. Nishimura, A. Takagaki, K. Ebitani, Green Chem. 15 (2013) 2026.
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
G. Lee, Y. Jeong, A. Takagaki, J.C. Jung, J. Mol. Catal. A Chem. 393 (2014) 289. K. Sugino, N. Oya, N. Yoshie, M. Ogura, J. Am. Chem. Soc. 133 (2011) 20030. N.A. Brunelli, C.W. Jones, J. Catal. 308 (2013) 60. K. Motokura, M. Tada, Y. Iwasawa, Angew. Chem. Int. Ed. 47 (2008) 9230. J.D. Bass, A. Solovyov, A.J. Pascall, A. Katz, J. Am. Chem. Soc. 128 (2006) 3737. R.K. Zeida, M.E. Davis, J. Catal. 247 (2007) 379. A.E. Fernandes, A.M. Jonas, Catal. Today 334 (2019) 173. Y. Inaki, Y. Kajita, H. Yoshida, K. Ito, T. Hattori, Chem. Commun. (2001) 2358. J.T. Grant, C.A. Carrero, F. Goeltl, J. Venegas, P. Mueller, S.P. Burt, S.E. Specht, W.P. McDermott, A. Chieregato, I. Hermans, Science 354 (2016) 1570. Y. Honda, A. Takagaki, R. Kikuchi, S.T. Oyama, Chem. Lett. 47 (2018) 1090. Y. Fang, X. Wang, Angew. Chem. Int. Ed. 56 (2017) 15506. S. Torii, K. Jimura, S. Hayashi, R. Kikuchi, A. Takagaki, J. Catal. 355 (2017) 176. S. Namba, A. Takagaki, K. Jimura, S. Hayashi, R. Kikuchi, S.T. Oyama, Catal. Sci. Technol. 9 (2019) 302. J.R. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, B. Maestro-Mdurga, A. PesqueraRodríguez, C. Ramírez-López, L. Lorenzo-Ibarreta, J. Torrecilla-Soria, M.C. VillaránVelasco, Appl. Catal. A Gen. 366 (2009) 315. X. Li, X. Wang, J. Zhang, N. Hanagata, X. Wang, Q. Weng, A. Ito, Y. Bando, D. Golberg, Nat. Commun. 8 (2017) 13936. J. Li, K. Gao, J. Mater. Chem. 13 (2003) 628. T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y.K. Gun’ko, J. Coleman, J.
Am. Chem. Soc. 134 (2012) 18758. [25] C. Huang, C. Chen, X. Ye, W. Ye, J. Hu, C. Xu, X. Qiu, J. Mater. Chem. A 1 (2013) 12192. [26] Y. Zhou, J. Lin, L. Li, X. Pan, X. Shun, X. Wang, J. Catal. 365 (2018) 14. [27] O.M. Moon, B.-C. Kang, S.-B. Lee, J.-H. Boo, Thin Solid Film 464–465 (2004) 164. [28] C. Zhi, N. Hanagata, Y. Bando, D. Golberg, Chem. Asian J. 6 (2011) 2530. [29] C.-Y. Su, Z.-Y. Juang, K.-F. Chen, B.-M. Cheng, F.-R. Chen, K.-C. Leou, C.-H. Tsai, J. Phys. Chem. C 113 (2009) 14681. [30] Z. Xu, Y. Chen, W. Li, J. Li, H. Yu, L. Liu, G. Wu, T. Yang, L. Luo, RSC Adv. 8 (2018) 17944. [31] L. Fu, G. Chen, N. Jiang, J. Yu, C.-T. Lin, A. Yu, J. Mater. Chem. 4 (2016) 19107. [32] J.P.L. Perez, B.W. McMahon, J. Yu, S. Schneider, J.A. Boatz, T.W. Hawkins, P.D. McCrary, L.A. Flores, R.D. Rogers, S.L. Anderson, ACS Appl. Mater. Interfaces 6 (2014) 8513. [33] R.K. Zeidan, M.E. Davis, J. Catal. 247 (2007) 379. [34] V.E. Collier, N.C. Ellebracht, G.I. Lindy, E.G. Moschetta, C.W. Jones, ACS Catal. 6 (2016) 460. [35] M.J. Climent, A. Corma, S. Iborra, A. Velty, J. Mol. Catal. A Chem. 182–183 (2002) 327. [36] G. Postole, B. Chowdhury, B. Karmakar, K. Pinki, J. Banerji, A. Auroux, J. Catal. 269 (2010) 110.
8
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Effects of post-thermal treatments of ball-milled boron nitrides on solid base catalysis Atsushi Takagakia,b,* a b
Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan International Institute for Carbon-Neutral Energy Research (I2CNER), Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0395, Japan
ARTICLE INFO
ABSTRACT
Keywords: Boron nitride Knoevenagel condensation Solid base catalyst
Ball-milled hexagonal boron nitride was investigated as a solid base catalyst. The ball-milled boron nitrides were heated in a vacuum or air and used for Knoevenagel condensation. Measurements by X-ray diffraction, nitrogen adsorption, thermogravimetry-mass spectrometry, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy indicated that the heat treatment under vacuum hardly changed the physicochemical properties including crystal structure and surface composition whereas the heat treatment in air significantly altered the properties due to the formation of B2O3. The number of basic sites increased with the increase of the evacuation temperature. The boron nitrides after evacuation gave moderate product yield for Knoevenagel condensation of benzaldehyde with malononitrile at 40 °C. The boron nitrides after calcination showed different activity, dependent on the calcination temperature. The boron nitride after ball-milling, followed by calcination at 300 °C exhibited the highest activity. The catalytic activity was not directly related to the base amount and the surface areas, suggesting that acid-base bifunctionality plays an important role in Knoevenagel condensation using the ball-milled boron nitride catalysts.
1. Introduction Solid acid and base catalysts offer the opportunity to reduce the impact on the environment and increase profits because they are reusable and readily separable from the reaction product in the liquid phase [1]. Solid base catalysts are useful to produce fine chemicals via CeC bond formation reactions, including aldol condensation, nitroaldol reaction, Knoevenagel condensation, and Michael addition [2,3]. These catalysts are also necessary to transform biomass into chemicals and fuels. For example, isomerization of glucose to fructose is an important reaction to form 5-hydroxymethylfurfural [4,5], and aldol condensation of 5-hydroxymethylfurfural with acetone [6] is a typical reaction to produce sustainable aviation fuels. Several solid bases such as alkaline earth metal oxides, hydrotalcites [7,8], zeolites [9], and aminosilicas [10,11] have been investigated so far. It is known that the coexistence of acid and base sites significantly accelerates typical base-catalyzed reactions including aldol reaction, nitroaldol reaction, Knoevenagel condensation [12–14]. Such acid–base cooperative catalysts include amine-tethered silica materials (mesoporous silica [10], and silica-alumina [11]) which have weak acid silanol groups. For aldol reaction, the weak acid sites could stabilize the carbonyl groups of reactants, resulting in enhancement of the reaction ⁎
rate. The distance between acid-base pairs, the ratio of acid and base concentrations, the strengths of acid and base sites are critical to increase the reactivity. The emergence of the acid-base pair was also possible via cleavage of SieOeSi bond on a mesoporous silica by treatment with ammonia at high temperature [15]. A well-defined pair of Si−OH and Si–NH2 was formed, which could catalyze Knoevenagel condensation. Hexagonal boron nitride has a similar structure as graphite and studied for applications outside of catalysis because of its high chemical and thermal stability. However, recently, hexagonal boron nitride was found to be an efficient catalyst for oxidative dehydrogenation of light alkanes [16–18]. Also, we found that ball-milled hexagonal boron nitride functioned as a new solid base catalyst [19,20]. The ball-milling treatment results in a significant increase in the surface areas and the emergence of hydroxyl and amino groups on the surface. It was suggested that these functional groups were formed via cleavage of BeN bond by moisture. The acid and base sites could be formed at adjacent positions on the surface. The ball-milled boron nitride exhibited high activity for base-catalyzed reactions, including the nitroaldol reaction, Knoevenagel condensation, and isomerization of glucose. The activity for the nitroaldol reaction was higher than those of conventional solid bases such as MgO and hydrotalcite because of its acid-base
Correspondence to: Department of Applied Chemistry, Faculty of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan. E-mail address: [email protected].
https://doi.org/10.1016/j.cattod.2019.11.007 Received 1 July 2019; Received in revised form 24 October 2019; Accepted 7 November 2019 0920-5861/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Atsushi Takagaki, Catalysis Today, https://doi.org/10.1016/j.cattod.2019.11.007
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 1. X-ray diffraction patterns of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air.
Fig. 2. N2 isotherms of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air.
bifunctionality [19]. The processing conditions of ball-mill greatly influenced the physicochemical properties and the solid base catalytic activity. Ball-milling at 400 rpm gave the highest surface area while higher rotation speeds decreased the surface areas. The number of base sites increased with the increase of rotation speeds, which is a good agreement with the formation of amino groups. The boron nitride ballmilled at 400 rpm exhibited the highest catalytic activity for the nitroaldol reaction. To further improve the catalytic activity, the effects of post-thermal treatment were investigated in this study. The ball-milled boron nitrides were heated at several temperatures in a vacuum (evacuation) or air (calcination). The samples were characterized by a variety of techniques and tested for Knoevenagel condensation.
of 40 kV and a current of 80 mA with 2θ values from 5° to 70°. The surface area of the catalysts was evaluated by the Brunauer–Emmett–Teller (BET) method using nitrogen adsorption (BELSORP mini, Microtrac-BEL). The sample was pretreated by evacuation at the temperatures of the post-treatment of the ball-milled boron nitride for 1 h before the measurement. The pore size distribution of the sample was obtained from the analysis of the adsorption branches of the isotherms using the Barrett, Joyner, and Halenda (BJH) method. The surface functional groups of the catalysts were characterized by Fourier transform infrared spectroscopy (FTIR, FT/IR-6600, JASCO) measurements. The sample was pressed into pellets with KBr for the measurements. The oxidation state of elements and their compositions over the surface were analyzed by X-ray photoelectron spectroscopy (XPS, KRATOS Ultra 2, Shimadzu). The binding energies in each measurement were referenced to the core level of the C1s peak (284.8 eV). The thermal stability of the ball-milled BN sample in N2 and air was examined using thermogravimetry-mass spectrometry (TG-MS, Rigaku)．About 10 mg of sample was heated from room temperature to 550 °C at a heating rate of 2 °C min-1 under N2 or air. The base strength of the catalysts was evaluated by using a color indicator reagent [21]. A quantity of 50 mg of sample was added to 1 mL of a toluene solution containing 0.4 mg of bromothymol blue (pKa = 7.2) for 1 day. The base amounts of the catalysts were determined by titration using 0.01 M benzoic acid in toluene for the bromothymol blue-adsorbed BN. The acid strength and amounts of the catalysts were also examined by using 4-(phenylazo)diphenylamine (pKa = 1.5) as a color indicator and nbutylamine as a titrant.
2. Experimental 2.1. Catalyst preparation Layered hexagonal boron nitride (Wako) was ball-milled at 400 rpm for 12 h using a planetary ball mill (Pulverisette 7, Fritsch, zirconia vessel (12 mL) with six zirconia balls (diameter 10 mm). Ball milling was conducted for 24 cycles in which each cycle was carried out for 30 min with 5 min intervals. The rotation direction was reversed in each cycle. The ball-milled boron nitride was heated at 150−400 °C in a vacuum or air for 1 h. The samples prepared were denoted as BNbm150-400vac and BNbm-150−400 cal, respectively. 2.2. Characterization
2.3. Knoevenagel condensation
The crystal structure of the catalysts was determined by X-ray diffraction (XRD) (RINT-2500, Rigaku) with Cu Kα radiation at a voltage
Knoevenagel condensation was conducted using a BN catalyst 2
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 3. Pore size distributions of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air. Table 1 BET surface areas and pore volumes of BN after ball-milling followed by heating at various temperatures in a vacuum or air. Sample
Heat treatment Temp./oC
BNbm BNbm-150vac BNbm-200vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
in a vacuum
in air
− 150 200 300 400 150 200 300 400
BET surface area/ m2 g-1
Pore volume/ mL g-1
215 250 243 301 309 46 61 145 141
0.33 0.33 0.42 0.39 0.39 0.26 0.31 0.45 0.44
Fig. 5. FTIR spectrum of ball-milled BN.
samples heated in a vacuum, indicating that bulk crystal structure remained unchanged by evacuation at 150−400 °C. In contrast, additional peaks at 14.6 and 27.9° were observed for the samples heated in air at 200−400 °C. These peaks were attributed to B2O3 (PDF #60297), indicating that the BN samples after ball-milling was partly oxidized in air. These peaks became apparent for the sample calcined at 400 °C. Our previous study also showed that a further increase of calcination temperature to 700 °C in air resulted in a clear formation of B2O3 [17]. Figs. 2 and 3 show the N2 adsorption isotherms and pore size distributions of the BN samples after ball-milling followed by the heat treatment in a vacuum or air. Table 1 lists the surface areas and pore volumes of the BN samples. The BET surface area of the ball-milled BN before the post-treatment was 215 m2 g-1. The surface area was slightly increased to 250 m2 g-1 for BNbm-150vac. A large increase of the surface areas was observed for BNbm-300vac and BNbm-400vac, which were 301 and 309 m2 g-1, respectively. The increase of surface areas of the samples was attributed to the removal of water. In contrast, a drastic change of the surface areas was found for the BN samples by calcination. The surface areas were decreased to 46 m2 g-1 for BNbmcal150 and 61 m2 g-1 for BNbm-cal200, and then increased to 145 m2 g-1 for BNbm-cal300 and 141 m2 g-1 for BNbm-cal400. The pore size distributions of the calcined samples indicated that micropores were significantly decreased by calcination below 200 °C. The decrease of the surface areas and micropores could be due to the formation of amorphous boron (hydro)oxide on the surface of the ball-milled BN. The further increase of calcination temperature results in an increase in the surface areas and micropores. It seems that the transformation of
Fig. 4. TG-MS curves of ball-milled BN sample in N2 and air.
(50 mg) in toluene (4 mL) with benzaldehyde (1.0 mmol, Wako), malononitrile (1.0 mmol, Wako) and n-decane (0.2 mmol, Wako) as an internal standard. The reaction was performed at 40 °C for 60 min. Aliquots were taken using a syringe and analyzed by gas chromatography (GC-FID; GC-2014, column DB-1MS, Shimadzu). 3. Results and discussions Fig. 1 shows the XRD patterns for BN samples after ball-milling, followed by evacuation or calcination at several temperatures. The ballmilled BN before the post heat treatment showed five peaks at 26.7, 41.5, 43.8, 50.1, and 55.1 degrees, which were attributed to the (002), (100), (101), (102), (004) reflections of hexagonal boron nitride (PDF #34-0421), respectively. No apparent change was found for the 3
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 6. FTIR spectra of BN after ball-milling followed by in situ evacuation at various temperatures. (a) 3800–3000 cm-1. (b) 1800–700 cm-1.
Fig. 7. FTIR spectra of BN after ball-milling followed by in situ calcination at various temperatures in air. (a) 3800–3000 cm-1. (b) 1800–700 cm-1.
Fig. 8. B1s XPS spectra of BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air, and (c) h-BN and B2O3.
amorphous boron (hydro)oxide to crystal B2O3 occurred because the oxygen contents on the surface of the samples were monotonically increased as observed by XPS (see below). Fig. 4 shows the results of TGMS of the ball-milled BN sample under N2 and air flow. A continuous weight loss was observed from room temperature to ca. 300 °C in N2, which was attributed to the removal of
water adsorbed and the dehydration of B–OH groups as monitored by the mass signal of 18. A noticeable weight loss was also found to 500 °C in air with the decrease of the mass signal of 18. It seems that there are three steps with different slope of weight loss during the dehydration in air. First is the removal of physisorbed water from room temperature to 100 °C. Second is the dehydration of B–OH groups on the surface to 4
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Fig. 9. Distribution of boron species in BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air. B–N: a peak centered at 190.4 eV. B–O(I): a peak centered around 192 eV. B–O(II): a peak centered around 193.2 eV.
Fig. 10. N1s XPS spectra of BN after ball-milling followed by heating at various temperatures in (a) a vacuum, and (b) air.
Fig. 11. Distribution of nitrogen species in BN after ball-milling followed by heating at various temperatures in (a) a vacuum and (b) air. N-B: a peak centered around 398.0 eV. Amino: a peak centered around 399.1 eV.
temperature to 400 °C. The OeH stretching band around 3400 cm-1 became weak with the increase of evacuation temperature from room temperature to 300 °C, indicating that adsorbed water was removed, and also a part of the OeH groups on the surface were dehydrated. The NeH stretching band around 3160 cm-1 also became a little weak, suggesting that some of the amino groups were dehydrated with the hydroxyl groups, leaving the remaining amino groups intact. A weak band around 1200−1300 cm-1 became intense with the increase of evacuation temperature, which could be assigned to B–O [26] because an absorption at 1190 cm-1 was observed for B2O3 [27,28]. Fig. 7 shows FTIR spectra of BN samples after ball-milling followed by in situ
form amorphous boron (hydro)oxide from 100 °C to 250 °C. Third is the successive dehydration of OH groups to transform to B2O3. Fig. 5 shows the FTIR spectrum for the ball-milled BN. Four absorption bands were observed, which is in a good agreement with the previous studies [19,20]. Strong absorption at 1382 cm-1 corresponds to B–N stretching [22] and a band at 815 cm-1 to B–N-B bending [23]. A broad band around 3400 cm-1 is assigned to OeH stretching vibration, and a shoulder around 3160 cm-1 can be ascribed to NeH stretching vibration [24,25]. The former band includes not only OH groups on the surface but also adsorbed water. Fig. 6 shows FTIR spectra of BN samples after ball-milling followed by in situ evacuation from room 5
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Table 2 Surface compositions of BN after ball-milling followed by heating at various temperatures calculated from XPS. Sample
Surface compositions
BNbm BNbm-150vac BNbm-200vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
B/mol%
N/mol%
O/mol%
39.2 38.3 39.1 38.8 39.8 40.9 39.3 37.3 34.3
41.2 41.1 40.2 39.9 41.7 33.9 35.8 33.8 28.6
19.7 20.6 20.8 21.3 18.5 25.2 24.9 28.9 37.1
N/B
O/B
1.05 1.07 1.03 1.03 1.05 0.83 0.91 0.91 0.83
0.50 0.54 0.53 0.55 0.46 0.62 0.63 0.78 1.08
Fig. 13. Time-courses of Knoevenagel condensations using ball-milled BN catalysts. Reaction conditions: benzaldehyde (1.0 mmol), malononitrile (1.0 mmol), catalyst (20 mg), toluene (4 mL), 40 °C.
1200−1300 cm-1 were significant, which are due to the formation of B2O3. Fig. 8 shows core-level spectra of B1s of BN samples after ballmilling followed by heating at several temperatures in a vacuum and air. As for comparisons, the spectra of B1s of B2O3 and h-BN are also shown. The sample without post-treatment, BNbm, had two peaks centered at 190.4 and 192.1 eV. The peak position of 190.4 eV is ascribed to B–N, which is in a good agreement with those reported for bulk h-BN [29,30]. The peak around 192 eV was not observed for bulk h-BN. Thus the peak at 192.1 eV in BNbm is assigned to oxidized boron, which is related to the formation of B–OH groups as reported previously [31,19,20,32]. The post-treatment in a vacuum did not change the spectra. In contrast, the post-treatment in air gave an additional peak around 193.2 eV. The peak of 193.2 eV was also observed for B2O3. Thus the peak is ascribed to B–O related to B2O3. Fig. 9 shows a fraction of boron species in BN after the post-treatment in vacuum and air. For BN samples after evacuation (Fig. 9(a)), no apparent change of the distribution was found. For BN samples after calcination (Fig. 9(b)), the fraction of B–O(II) corresponding to the peak of 193.2 eV increased with the increase of calcination temperature, which is in a good agreement with XRD results. Fig. 10 shows core-level spectra of N1s of BN samples after ballmilling followed by heating in a vacuum and air. The spectra for the samples could be deconvoluted into two components. One is a peak around 398.0 eV, which is ascribed to NeB. The other is a peak around 399.1 eV, which is likely attributable to amino groups. Fig. 11 shows a fraction of nitrogen species in BN. For BN samples after evacuation (Fig. 11(a)), the distribution remained unchanged as observed for B1s spectra. For BN samples after calcination (Fig. 11(b)), however, the fraction of amino groups decreased. Table 2 and Fig. 12 show the surface compositions, N/B ratio, and O/B ratio of BN samples before and after the heat treatments. The ratios of N/B and O/B for BNbm were 1.05 and 0.50, respectively. The compositions of the BN samples after evacuation remained unchanged, indicating that no additional formation of boron oxide species. In contrast, the O/B ratio of the BN samples after calcination largely increased from 0.50 to 1.08 with the increase of calcination temperature. The increase in oxygen contents was apparent for BNbm-300 cal and BNbm-400 cal. The solid base property was evaluated using bromothymol blue as a color indicator reagent (Table 3). Treatment of the samples after evacuation (BNbm-vac) and calcination at a lower temperature (BNbm150 cal and BNbm-200 cal) with a solution of toluene containing bromothymol blue (pKa = 7.2) resulted in green color whereas the samples
Fig. 12. N/B and O/B ratios of BN after ball-milling followed by heating at various temperatures in a vacuum or air. Table 3 Coloration by bromothymol blue (pKa = 7.2) and titration of benzoic acid. Sample
Coloration by Bromothymol blue
Base amount (H_ ≥ +7.2)/mmol g-1
BNbm BNbm-150vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
+ + + + + + − −
0.60 0.20 0.44 0.61 0.04 0.12 0 0
Table 4 Coloration by 4-(phenylazo)diphenylamine (pKa = 1.5) and titration of n-butylamine. Sample
Coloration by 4-(phenylazo) diphenylamine
Acid amount (H0 ≤ +1.5)/mmol g-1
BNbm BNbm-150vac BNbm-300vac BNbm-400vac BNbm-150 cal BNbm-200 cal BNbm-300 cal BNbm-400 cal
− + + + − − + +
0 0.28 0.37 0.29 0 0 1.81 0.69
calcination from room temperature to 400 °C. The tendency for the change of the spectra by calcination was the same as that by evacuation. However, it should be noted that the decrease in the intensity of OeH bands around 3400 cm-1 and the increase of BeO around 6
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki
Table 5 Results of Knoevenagel condensation using BN catalysts after ball-milling followed by heat treatment.
Sample
Benzalmalononitrile yield/%
SBET/m2 g-1
Base amount/mmol g-1
BNbm BNbm-150vac BNbm-300vac BNbm-400vac BNbm-200 cal BNbm-300 cal BNbm-400 cal
86 58 82 67 63 94 1
215 250 301 309 61 145 141
0.60 0.20 0.44 0.61 0.12 0 0
a
Acid amount/mmol g-1 0 0.28 0.37 0.29 0 1.81 0.69
b
O/B ratio 0.50 0.54 0.55 0.46 0.63 0.78 1.08
Reaction conditions: benzaldehyde (1.0 mmol), malononitrile (1.0 mmol), catalyst (20 mg), toluene (4 mL), 40 °C, 60 min. a Weak base. H_≥+7.2. b Very weak acid. H0 ≤ +1.5.
after calcination at a higher temperature (BNbm-300 cal and BNbm400 cal) gave a bright yellow color. This indicates that the samples evacuated had weak basic sites (H_ ≥ +7.2) but the samples calcined over 300 °C did not have such basic sites stronger than H_ ≥ +7.2. The base amount was estimated by titration using benzoic acid. The number of basic sites for BNbm-vac increased with the increase of the evacuation temperature. The BNbm-400vac had the greatest number of basic sites, 0.61 mmol g-1. The numbers of basic sites for the samples calcined were small, 0.04 mmol g-1 for BNbm-150 cal and 0.12 mmol g-1 for BNbm-200 cal. The solid acid property was examined using 4-(phenylazo)diphenylamine as a color indicator reagent (Table 4). Treatment of the samples after evacuation (BNbm-vac) with a solution of toluene containing 4-(phenylazo)diphenylamine (pKa = 1.5) resulted in purple color, indicating that the samples evacuated had very weak acid sites (H0 ≤ +1.5). The samples after calcination at higher temperatues (BNbm-300 cal and BNbm-400 cal) also showed very weak acidity. The numbers of acid sites for BNbm-vac were around 0.30 mmol g-1. The samples after calcination at higher temperatures had more acid sites in which the BNbm-300 cal had the greatest number of acid sites, 1.81 mmol g-1. In contrast, the samples after calcination at lower temperatures (BNbm-150 cal and BNbm200 cal) did not have such weak acid sites. The catalytic activity of BN was investigated by Knoevenagel condensation of benzaldehyde with malononitrile. While the reaction occurred even at room temperature [19], the reaction temperature was set to 40 °C in this study due to precise heat control. Fig. 13 shows the timecourses of the reaction using three ball-milled BN samples. BNbm could catalyze the reaction with the conversion of 24 % for 10 min and 100 % for 60 min. While BNbm-300vac showed lower activity than BNbm, BNbm-300 cal gave the conversion of 51 % for 10 min, two times higher than BNbm. Table 5 lists the results of Knoevenagel condensation using BN catalysts after post heat treatment. Other related physicochemical properties are also added. The BN samples after evacuation gave the product yields from 58 to 82 % for 60 min. Although BNbm-400vac had the greatest number of basic sites with the highest surface areas among the samples evacuated, BNbm-300vac showed the highest activity. A difference between these samples was that the surface O/B ratio of BNbm-400vac is smaller than others. It seems that the presence of OH groups plays a vital role in the activity, which could be related to the number of acid sites because BNbm-300vac had the greatest number of acid sites amount for the samples evacuated. Like aldol reaction [33] and nitroaldol reaction [34], the improvement of catalytic activity by acid-base bifunctionality for Knoevenagel condensation was also reported [35,36]. When comparing BNbm-150vac and BNbm-200 cal, the activity of BNbm-200 cal was close to that of BNbm-150vac whereas a large difference of surface areas for the two samples was observed, indicating that the surface areas did not affect the activity. Surprisingly,
while BNbm-300 cal had no apparent basic sites stronger than H_ ≥ +7.2, the sample exhibited the highest activity among the samples in this study. Also, a drastic change of the activity was found between BNbm-300 cal and BNbm-400 cal. The negligible activity for BNbm-400 cal is likely due to the increase of B2O3 phase, as shown in XRD and XPS. FTIR results of calcined BN sample showed that NeH stretching remained even at 400 °C, suggesting that BNbm-300 cal still has weak basic sites. In addition, the BNbm-300 cal had the greatest number of very weak acid sites. A possible explanation for the highest activity for BNbm-300 cal is likely due to the acid-base cooperative effect by hydroxyl groups and amino groups on the surface of ballmilled boron nitride. 4. Conclusions The heat treatment of the ball-milled boron nitrides under vacuum hardly changed their crystal structures and the surface compositions. The number of basic sites increased with the increase of the evacuation temperature. The heat treatment in air significantly altered their crystal structures, surface areas, and surface compositions, which were due to the formation of B2O3. The surface areas and the number of basic sites decreased with the increase of calcination temperature. The BN catalysts after evacuation gave moderate product yield. The BN catalysts after calcination showed different activity, dependent on the calcination temperature. BNbm-300 cal exhibited the highest activity, whereas BNbm-400 cal showed negligible activity. The catalytic activity was not directly related to the base amount and surface areas. It seems that acidbase bifunctionality plays an important role in Knoevenagel condensation using the ball-milled boron nitride catalysts. Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by a Grand-in-Aid for Science Research (B) (No. 18H01785) of JSPS, Japan. References [1] [2] [3] [4] [5] [6] [7]
7
K. Tanabe, W.F. Hölderich, Appl. Catal. A Gen. 181 (1999) 399. Y. Ono, H. Hattori, Solid Base Catalysis, Springer-Verlag, Berlin: Heidelberg, 2011. W. Bing, M. Wei, J. Solid State Chem. 269 (2019) 184. C. Moreau, R. Durand, A. Roux, D. Tichit, Appl. Catal. A Gen. 193 (2000) 257. A. Takagaki, M. Ohara, S. Nishimura, K. Ebitani, Chem. Commun. (2009) 6276. A. Bohre, B. Saha, M. Abu-Omar, ChemSusChem 8 (2015) 4022. S. Nishimura, A. Takagaki, K. Ebitani, Green Chem. 15 (2013) 2026.
Catalysis Today xxx (xxxx) xxx–xxx
A. Takagaki [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
G. Lee, Y. Jeong, A. Takagaki, J.C. Jung, J. Mol. Catal. A Chem. 393 (2014) 289. K. Sugino, N. Oya, N. Yoshie, M. Ogura, J. Am. Chem. Soc. 133 (2011) 20030. N.A. Brunelli, C.W. Jones, J. Catal. 308 (2013) 60. K. Motokura, M. Tada, Y. Iwasawa, Angew. Chem. Int. Ed. 47 (2008) 9230. J.D. Bass, A. Solovyov, A.J. Pascall, A. Katz, J. Am. Chem. Soc. 128 (2006) 3737. R.K. Zeida, M.E. Davis, J. Catal. 247 (2007) 379. A.E. Fernandes, A.M. Jonas, Catal. Today 334 (2019) 173. Y. Inaki, Y. Kajita, H. Yoshida, K. Ito, T. Hattori, Chem. Commun. (2001) 2358. J.T. Grant, C.A. Carrero, F. Goeltl, J. Venegas, P. Mueller, S.P. Burt, S.E. Specht, W.P. McDermott, A. Chieregato, I. Hermans, Science 354 (2016) 1570. Y. Honda, A. Takagaki, R. Kikuchi, S.T. Oyama, Chem. Lett. 47 (2018) 1090. Y. Fang, X. Wang, Angew. Chem. Int. Ed. 56 (2017) 15506. S. Torii, K. Jimura, S. Hayashi, R. Kikuchi, A. Takagaki, J. Catal. 355 (2017) 176. S. Namba, A. Takagaki, K. Jimura, S. Hayashi, R. Kikuchi, S.T. Oyama, Catal. Sci. Technol. 9 (2019) 302. J.R. Ochoa-Gómez, O. Gómez-Jiménez-Aberasturi, B. Maestro-Mdurga, A. PesqueraRodríguez, C. Ramírez-López, L. Lorenzo-Ibarreta, J. Torrecilla-Soria, M.C. VillaránVelasco, Appl. Catal. A Gen. 366 (2009) 315. X. Li, X. Wang, J. Zhang, N. Hanagata, X. Wang, Q. Weng, A. Ito, Y. Bando, D. Golberg, Nat. Commun. 8 (2017) 13936. J. Li, K. Gao, J. Mater. Chem. 13 (2003) 628. T. Sainsbury, A. Satti, P. May, Z. Wang, I. McGovern, Y.K. Gun’ko, J. Coleman, J.
Am. Chem. Soc. 134 (2012) 18758. [25] C. Huang, C. Chen, X. Ye, W. Ye, J. Hu, C. Xu, X. Qiu, J. Mater. Chem. A 1 (2013) 12192. [26] Y. Zhou, J. Lin, L. Li, X. Pan, X. Shun, X. Wang, J. Catal. 365 (2018) 14. [27] O.M. Moon, B.-C. Kang, S.-B. Lee, J.-H. Boo, Thin Solid Film 464–465 (2004) 164. [28] C. Zhi, N. Hanagata, Y. Bando, D. Golberg, Chem. Asian J. 6 (2011) 2530. [29] C.-Y. Su, Z.-Y. Juang, K.-F. Chen, B.-M. Cheng, F.-R. Chen, K.-C. Leou, C.-H. Tsai, J. Phys. Chem. C 113 (2009) 14681. [30] Z. Xu, Y. Chen, W. Li, J. Li, H. Yu, L. Liu, G. Wu, T. Yang, L. Luo, RSC Adv. 8 (2018) 17944. [31] L. Fu, G. Chen, N. Jiang, J. Yu, C.-T. Lin, A. Yu, J. Mater. Chem. 4 (2016) 19107. [32] J.P.L. Perez, B.W. McMahon, J. Yu, S. Schneider, J.A. Boatz, T.W. Hawkins, P.D. McCrary, L.A. Flores, R.D. Rogers, S.L. Anderson, ACS Appl. Mater. Interfaces 6 (2014) 8513. [33] R.K. Zeidan, M.E. Davis, J. Catal. 247 (2007) 379. [34] V.E. Collier, N.C. Ellebracht, G.I. Lindy, E.G. Moschetta, C.W. Jones, ACS Catal. 6 (2016) 460. [35] M.J. Climent, A. Corma, S. Iborra, A. Velty, J. Mol. Catal. A Chem. 182–183 (2002) 327. [36] G. Postole, B. Chowdhury, B. Karmakar, K. Pinki, J. Banerji, A. Auroux, J. Catal. 269 (2010) 110.
8