Journal of the Japan Petroleum Institute, 64, (2), 103-111 (2021) 103 [Regular Paper] Synthesis of SrO–Al2O3 Solid Base Catalysts from Strontium Hydroxide and Aluminum Alkoxide by a Solid-liquid Interface Reaction Hiromi MATSUHASHI†1)＊, Asako IWAMOTO†1), Misaho SASAKI†1), Kana YOSHIDA†1), and Hirofumi ARITANI†2) †1) Dept. of Science, Hokkaido University of Education, 1-2 Hachiman-cho, Hakodate, Hokkaido 040-8567, JAPAN †2) Dept. of Life Science & Green Chemistry, Faculty of Engineering, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, JAPAN (Received October 19, 2020) _ Highly dispersed SrO in amorphous Al2O3, SrO Al2O3, was synthesized by solid-liquid interface reaction of Sr(OH)2・8H2O in the solid phase with Al(OCH(CH3)2)3 dissolved in 2-propanol. The water of crystallization in _ Sr(OH)2・8H2O was consumed for the hydrolysis of Al(OCH(CH3)2)3. SrO Al2O3 catalyst synthesized by solid-liquid interface reaction of equimolar amounts of Sr(OH)2・8H2O and Al(OCH(CH3)2)3, then heat treated at 673 K, exhibited the highest activity among the prepared catalysts for the base-catalyzed retro-aldol reaction of _ _ diacetone alcohol. Catalytic activity of SrO Al2O3 catalyst calcined at 673 K was twice that of SrO Al2O3 cata- _ lyst prepared by physical mixing of Sr(OH)2・8H2O with Al2O3. Active SrO Al2O3 catalyst was obtained by heating at a temperature just below that of Sr3Al2O6 crystallization. Formation of SrO by heat treatment at 673 K was confirmed using X-ray absorption near edge structure analysis. The SrO particle size was too small for detection by powder X-ray diffraction. We found that interface reaction of a metal hydroxide in the solid phase with an alkoxide in the liquid phase is useful for the preparation of well-dispersed mixed metal oxides under mild conditions. Keywords Strontium oxide, Solid base, Solid-liquid interface, Retro-aldol reaction 1. Introduction been carried out with SrO than MgO as the base cata- lyst, particularly the preparation and application of SrO Alkaline earth metal oxides such as MgO are well as a base catalyst, partially because of the difficulties known to show activities as solid base catalysts. The associated with the preparation of SrO. preparation, active site structure, and application of Alkaline earth metal oxides are usually obtained by MgO to numerous base-catalyzed organic reactions thermal decomposition of the corresponding hydroxide have been extensively investigated1)～4). Strontium or carbonate at elevated temperatures2),3). For exam- oxide (SrO) has a higher base strength among the alka- ple, Mg(OH)2 is converted to MgO by thermal decom- line earth metal oxides, because strontium (Sr) has position around 650 K22),23). However, the melting lower electronegativity within the group of alkaline point of Sr(OH)2 is slightly lower than the decomposi- earth metals. The base strength increases in the order tion temperature of the hydroxide24). Consequently, 5),6) MgO ＜ CaO ＜ SrO ＜ BaO . Therefore, SrO is Sr(OH)2 first melts and then decomposes as the tem- expected to achieve higher catalytic activity in various perature increases. The decomposition of Sr(OH)2 in organic reactions. SrO catalysts are applicable to the liquid phase results in solid SrO with low surface many base-catalyzed reactions, such as aldol condensa- area. In contrast, thermal decomposition of SrCO3 to tion5), nitroaldol reaction7), Michel addition8),9), Canniz- SrO requires very high temperatures (＞1073 K) be- 10) 11)～18) 7),10),25)～29) zaro reaction , transesterification , Tishchenko cause of the high thermal stability of SrCO3 . reaction19),20), diacetone alcohol decomposition6), and Therefore, the conventional thermal decomposition amination of alkene21). However, fewer studies have method is not appropriate for SrO catalyst preparation from the hydroxide or carbonate. To overcome these DOI: doi.org/10.1627/jpi.64.103 problems, we investigated solid-liquid interface reaction ＊ To whom correspondence should be addressed. for the preparation of SrO base catalyst. ＊ E-mail: [email protected] In general, a metal alkoxide with high reactivity to- J. Jpn. Petrol. Inst., Vol. 64, No. 2, 2021 104 ward H2O is converted into the corresponding metal hy- Wako Pure Chemical Corp.) was used as received, or droxide and alcohol by hydrolysis. The water of crys- heated in an oven at 373 K for 12 h to eliminate the tallization contained in the metal hydroxide would be water of crystallization to form Sr(OH)2. Treatment of consumed by this hydrolysis reaction in a nonaqueous Sr(OH)2・8H2O or Sr(OH)2 with Al(OCH(CH3)2)3 was solvent. Water molecules are necessary to maintain performed as follows30). The required amount of the crystalline structure of the metal hydroxide. Sr(OH)2・8H2O or Sr(OH)2 was kneaded in a mortar to- Therefore, the crystalline structure will be disrupted as gether with an equimolar amount of Al(OCH(CH3)2)3 water molecules are removed from the lattice. Metal FUJIFILM Wako Pure Chemical Corp.). An appropri- hydroxide molecules are likely to be displaced from a ate volume of 2-propanol was added to the mixture in solid surface if the surrounding water molecules are the mortar and kneaded for a further 10 min. The consumed by hydrolysis of the metal alkoxide. Such 2-propanol suspension was then transferred to an Erlen- displaced metal hydroxide molecules are insoluble in meyer flask and stirred at 333-343 K for 12 h. The an organic solvent so can be expected to precipitate. precipitate was separated by filtration, to yield the SrO_ Therefore, newly formed metal hydroxide solids will be Al2O3 precursor. The solid product (after filtration) obtained as precipitates as results of both displacement was dried at 373 K for 2 h and then heat treated for ac- and hydrolysis. tivation at 573-1073 K for 1 h. Heat-treated catalysts Displacement of metal hydroxide molecules from the were kept in sealed ampoules until use. solid surface will expose a new hydrated metal hydrox- The mole ratio of Al/Sr was 1 in the sample prepared ide surface, and so further hydrolysis of the metal from anhydrous Sr(OH)2. The mole ratios of Al/Sr alkoxide by the water molecules in the crystal structure was 1, 2, and 3 in samples prepared from Sr(OH)2・ will continue. Both hydroxide precipitates are formed 8H2O. In addition, pure Sr(OH)2 catalyst heat treated simultaneously by the reaction on the hydroxide sur- at 673 K was also used for comparison. face, and no solvation by 2-propanol is expected. The physically mixed sample was prepared as fol- 31) Therefore, the solid-liquid interface reaction would pro- lows . The required quantities of Sr(OH)2・8H2O and vide hydroxide precipitates well mixed on the molecu- γ-Al2O3 (JRC-ALO-6, Catalysis Society of Japan) were lar level. mixed by kneading in a mortar together with a small Water of crystallization contained in Sr(OH)2・8H2O volume of ethanol for 15 min, in a glove box. The can be eliminated by heating at 373 K. Therefore, the mixture was allowed to stand at room temperature until water molecules formed by dehydration of Sr(OH)2 to all solvent had evaporated. The obtained solid was form SrO would also be consumed by hydrolysis of the heated in a muffle furnace at 673-873 K for 1 h. The metal alkoxide. Consequently, dehydration of Sr(OH)2 mole ratio of Al/Sr was fixed at 1.17. _ should be promoted by mixing with a metal alkoxide. The SrO Al2O3 samples prepared from Sr(OH)2, _ We previously reported the synthesis of BaO Al2O3 Sr(OH)2・8H2O (Al/Sr＝1), Sr(OH)2・8H2O (Al/Sr＝2), solid base catalyst under mild conditions using the Sr(OH)2・8H2O (Al/Sr＝3) are named SAA, SAH1, solid-liquid interface reaction of Ba(OH)2・H2O in the SAH2, and SAH3, respectively, and the physical mix- solid phase with Al(OCH(CH3)2)3 dissolved in ethyl ing sample is named SAP. acetate30). The prepared catalyst consisted of catalyti- 2. 2. Retro-aldol Reaction of Diacetone Alcohol cally active BaO highly dispersed in amorphous Al2O3. All reactions were carried out under atmospheric The present investigation applied a catalyst-prepara- conditions. The base-catalyzed decomposition reac- tion technique using a solid-liquid interface reaction to tion of diacetone alcohol (4-hydroxy-4-methyl-2-penta- the following system: Sr(OH)2, with and without water none, FUJIFILM Wako Pure Chemical Corp., DAA) to of crystallization, and Al(OCH(CH3)2)3. This research form acetone (the retro-aldol reaction) was performed aimed to show that the interface reaction of a metal hy- in a batch reaction system at 299 K for 3 h (Scheme 1). droxide in the solid phase with an alkoxide in the liquid First, the catalyst 50 mg was removed from its sealed phase, carried out under mild conditions, is useful for ampoule, then immediately placed in a round-bottom the formation of catalytically active and well-dispersed flask together with the substrate 50 mmol DAA (used as mixed metal oxides. received). Samples of the reaction mixture were re- moved from the reaction flask at intervals of 30 min and 2. Experimental analyzed by gas-liquid chromatography (GLC). 2. 3. Catalyst Characterization 2. 1. Catalyst Preparation Thermogravimetry and differential thermal analysis All catalysts were prepared under atmospheric condi- (TG-DTA) was performed on the prepared samples tions. Strontium hydroxide, with and without water of under nitrogen before heat treatment to observe the crystallization, was used in the solid-liquid interface re- thermal decomposition profiles.