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Surface Functionalization for Synergistic Catalysis: Silica-Alumina-Supported Cationic Indium and Organic Base for Cyanoethoxycarbonylation [ChemPlusChem](ChemPlusChem Via Acquire Media NewsEdge) A silica-alumina (SiO2 -Al2 O3) surface possessing both In3+ cations and an amine base has been prepared. Surface-functionalized SiO2 -Al2 O3 was characterized by solid-state 29 Si MAS, 13 C CP/MAS, and 27 Al 2D MQ/MAS NMR spectroscopy; X-ray photoelectron spectroscopy; and elemental analysis. The bifunctional surface demonstrates synergistic catalysis for cyanoethoxycarbonylation of aldehyde. Keywords : tertiary amines · cyanation · heterogeneous catalyst · indium · supported catalysts (ProQuest: ... denotes formulae omitted.) Introduction Synergistic catalysis on a func- tionalized solid surface realizes significant acceleration of organ- ic reactions.[1] Immobilization of amine bases with well-defined structures through a silane-cou- pling reaction is an effective ap- proach to construct acid-base bifunctional surfaces on silica or other oxides. Not only Brønsted acids, such as carboxylic acids,[2] and silanol (SiOH) groups on the support surface,[3] but also metal complexes and metal nanoparticles have been reported as counterparts of organic bases for synergistic catalysis.[4] Recently, our group reported Brønsted acid or metal com- plex-organic base synergistic catalysis of tertiary-amine-immo- bilized silica and silica-alumina (SiO2-Al2O3).[5] On these catalyt- ic surfaces, appropriate positioning of the Brønsted acid site and the base site for catalytic reactions was achieved. These re- sults encouraged us to introduce cationic metal species to the catalyst surface through ion exchange between H + and metal cations to increase the catalytic performance in acid-base reac- tions. SiO2-Al2O3 is a solid acid with SiOH groups located on the amorphous silica phase and a strong Brønsted acid site com- posed of a pair of SiOH groups and a tetrahedrally coordinated aluminum.[6] The SiOH group and the strong Brønsted acid site should act as immobilization sites for organic molecules and metal cations through silane-coupling and cation-exchange re- actions, respectively, as shown in Scheme 1. Herein, we focus on an indium cation as part of the acid site, because In3 + , which is a hard acid, is a well-known acid catalyst for a variety of organic reactions, including acid-base reactions.[7] In this study, a SiO2-Al2O3 surface possessing both In3 + cations and amine base was prepared and characterized, and its catalytic performance was evaluated. Results and Discussion The catalyst preparation pathway is shown in Scheme 2. SiO2- Al2O3 was treated with an aqueous solution of InCl3. The result- ing slurry was filtered and dried at 120 8C under air to afford In3+ -loaded silica-alumina (In-SiO2-Al2O3). The tertiary amine group was immobilized by a silane-coupling reaction between 3-(diethylamino)propyltrimethoxysilane and In-SiO2-Al2O3 to give In-SiO2-Al2O3-supported tertiary amine (In-SiO2-Al2O3/ NEt2). SiO2-Al2O3-supported tertiary amine (SiO2-Al2O3/NEt2), without indium, was also prepared through a silane-coupling reaction. Elemental analysis results for SiO2-Al2O3 and the prepared materials are summarized in Table 1. ICP analysis of In-SiO2- Al2O3 revealed an indium loading of 0.16 mmol g1. This value is close to the amount of strong acid sites present in the parent SiO2-Al2O3, as determined by NH3 temperature-pro- grammed desorption (TPD) analysis (0.154 mmol g1). The Si/Al ratio of both the bulk and surface did not change significantly after the loading of indium onto SiO2-Al2O3. The presence of organic amine in SiO2-Al2O3/NEt2 and In-SiO2-Al2O3/NEt2 was suggested by elemental analysis of carbon and nitrogen. The amount of indium and the Si/Al ratio were almost completely retained after the immobilization of tertiary amine. To determine the chemical state of indium on SiO2-Al2O3, XPS analysis of In-SiO2-Al2O3 and In-SiO2-Al2O3/NEt2 was con- ducted. The In 3 d spectra are shown in Figure 1. The binding energy value of In-SiO2-Al2O3 (Figure 1a; 3d5/2:446.5 eV) was higher than that of In2O3 (Figure 1 c; 3 d5/2:443.8 eV), In(OH)3 (3 d5/2:444.6 eV),[8] and even InCl3 (3 d5/2:445.7 eV).[8] This result indicates that the indium species on the SiO2-Al2O3 surface is more cationic than that of In2O3, In(OH)3, and InCl3. The ab- sence of chlorine was confirmed by elemental analysis, which indicated that the In3 + cation was connected to an oxygen atom. Zamaro and co-workers reported that the binding energy value of (InO) + species in mordenite was around 446 eV.[9] This indicates that the higher binding energy value of In3+ species on SiO2-Al2O3 (3 d5/2: 446.5 eV) is assignable to (InO)+. The binding energy of In 3 d was not changed after im- mobilization of the tertiary amine (Figure 1 b), which indicated retention of the (InO) + structure after immobilization of the terti- ary amine. Next, to determine the bind- ing site of (InO) + and tertiary amine on the SiO2-Al2O3 surface, solid-state 29Si and 27Al MAS NMR spectroscopy measure- ments were conducted. The 29Si dd/MAS NMR spectra of SiO2-Al2O3 and the prepared ma- terials are shown in Figure 2. The spectrum of SiO2-Al2O3 was well separated, with four signals at 103, 99, and 90 ppm, which were assigned to Si(OSi)4, Si(OSi)3(OAl)1, Si- (OSi)3(OH)1, and Si(OSi)2(OH)2, re- spectively.[10] The fraction of the signal area relative to the total SiO2 and the Si/Al ratio calculated based on 29Si NMR spectroscopy are summarized in Table 2. After the loading of indium on SiO2-Al2O3, the signal areas of Si(OSi)3(OH)1 and Si(OSi)2(OH)2 did not decrease ; this indicates that indium is not immobilized through re- action with surface silanol groups. After immobiliza- tion of the tertiary amine group, the signal assigned to Si(OSi)2(OH)2 disappeared (Figure 2 c and d), where- as the signal assigned to Si(OSi)4 increased. Addition- ally, new signals appeared at d 50 to 60 ppm, which were assigned to silicon atoms with one alkyl group and three hydroxyl or alkoxy groups (T sites).[11] These observations indicated that the tertiary amine group was immobilized through the silane- coupling reaction with surface SiOH groups on SiO2-Al2O3. 29Si NMR spectroscopy and ICP analysis revealed the Si atom/ SiOH group ratio (100 :32) and the silicon content (11.4 mmol g1), respectively, of SiO2-Al2O3. Based on these values, the amount of SiOH groups in SiO2-Al2O3 was calculat- ed to be 3.6 mmol g1. This value is sufficient for the immobili- zation of about 0.6 mmol g1 of amine (Table 1) by silane-cou- pling reaction. The silicon/aluminum ratio values of all samples, as deter- mined by 29Si NMR spectroscopy (Table 2), were higher than those determined by ICP (Table 1). Two signals were observed at d0 and 60ppm by 27Al MAS NMR spectroscopy measure- ments (Figure S1 in the Supporting Information). These results indicate the presence of both tetrahedrally (AlIV) and octahe- drally (AlVI) coordinated aluminum species. The silicon/alumi- num ratios determined by NMR spectroscopy for all materials were almost the same (Table 2), which indicated retention of the ratio of AlIV and AlVI species during indium loading and amine immobilization. Further information on the site upon which indium was sit- ting was obtained by 27Al MQ MAS NMR spectroscopy mea- surement. The spectra are shown in Figure 3. Resonance posi- tions, isotropic chemical shifts (diso), and second-order quadru- polar effect (SOQE) parameters obtained by 27Al MQ MAS NMR spectroscopy are summarized in Table 3. In all cases, both AlIV and AlVI resonances were detected. Additionally, in the case of In-SiO2-Al2O3 and In-SiO2-Al2O3/NEt2, a new resonance was observed at d 67-69 and 40-43 ppm in the F1 and F2 direc- tions, respectively, defined as AlIVb. These signals were as- signed as distorted AlIV species. As reported by van Bokhoven and co-workers, the introduction of a metal cation into USY zeo- lite through ion-exchange-in- duced distortion of a portion of AlIV through interactions be- tween aluminum and the metal cation.[12] These aluminum spe- cies showed a larger quadrupo- lar effect than that of the AlIV site before the introduction of the metal cation. As shown in Table 3, AlIVb sites in In-SiO2-Al2O3 and In-SiO2-Al2O3/NEt2 showed larger SOQE values (5.6-5.9 MHz) than the AlIVa sites (4.3-4.7 MHz). These 27Al MQ MAS NMR spectroscopy results in- dicate that the indium cation is close to the tetrahedrally coor- dinated aluminum site, which is connected to a bridging oxygen (AlOSi), with H + in SiO2-Al2O3, before indium load- ing. The retention of the tertiary amine structure on SiO2-Al2O3 was confirmed by solid-state 13C CP/MAS NMR spectroscopy measurements (Figure 4). There was no significant change in the 13C NMR spectra in the presence and absence of In3 + cat- ions. It is expected that the catalytic activity of the tertiary amine itself may not differ between SiO2-Al2O3/NEt2 and In- SiO2-Al2O3/NEt2. The surface structure of In-SiO2-Al2O3/NEt2 is shown in Scheme 2c. Cyanation of carbonyl compounds is a well-known reaction that is promoted by acid-base synergistic catalysis. Metal com- plexes and organic amines act as Lewis acids and bases to acti- vate the carbonyl group and the cyanide source, respective- ly.[13] To confirm acid-base synergistic catalysis of In-SiO2- Al2O3/NEt2, cyanoethoxycarbonylation of p-chlorobenzaldehyde was performed. The results are summarized in Table 4. In-SiO2- Al2O3/NEt2 showed the highest catalytic activity, giving the cor- responding product in 95% yield in 30 min at 408C. The prod- uct yield decreased to 80% without indium. In the case of silica-supported tertiary amine (SiO2/NEt2), the product yield was 26% under the reaction conditions. Without immobiliza- tion of the tertiary amine, the product scarcely formed in the presence of In-SiO2-Al2O3. The reaction did not proceed with only triethylamine. These results indicate that the reaction was significantly enhanced in the presence of both the acidic sup- port, In-SiO2-Al2O3 or SiO2-Al2O3, and tertiary amine group. Additionally, the (InO)+ site showed greater promotion ability than H+ for cyanoethoxycarbonylation. To evaluate the effect of the immobilization of tertiary amine on catalytic performance, cyanoethoxycarbonylation by using In-SiO2-Al2O3 with triethylamine was conducted as a con- trol experiment. In this case, the reaction rate was lower than that of the reaction using In-SiO2-Al2O3/NEt2, as shown in Fig- ure S2 in the Supporting Information. This result indicates that the presence of (InO)+ and tertiary amine groups at an appro- priate distance in In-SiO2-Al2O3/NEt2 is required for the catalyt- ic reaction. It has already been reported that the tertiary amine group on the SiO2-Al2O3 surface acts as a nucleophilic activator for ethyl cyanoformate.[5b] To determine the activation mode of p- chlorobenzaldehyde, FTIR spectroscopy measurements of ad- sorbed p-chlorobenzaldehyde on the In-SiO2-Al2O3 and SiO2- Al2O3 surfaces were performed (Figure S3 in the Supporting In- formation). In both cases, the C=O stretching vibration mode of physisorbed p-chlorobenzaldehyde and the aldehyde ad- sorbed on the silanol group were observed at ñ=1693 and 1672 cm1, respectively.[14] Furthermore, new signals appeared at ñ=1656 and 1641 cm1 for In-SiO2-Al2O3, and ñ= 1652 cm1 for SiO2-Al2O3. The signal at ñ= 1641 cm1 may be assignable to the C=O stretching vibration of p-chlorobenzal- dehyde adsorbed on (InO) + itself, or a hydroxyl group con- nected to (InO) + .[14] These differences in the activation mode of p-chlorobenzaldehyde result in different catalytic performan- ces in the cyanoethoxycarbonylation of p-chlorobenzaldehyde (Table 4). These observations suggest synergistic catalysis through activation of ethyl cyanoformate and aldehyde at the tertiary amine group and In3 + site, respectively, on the SiO2- Al2O3 surface. A proposed reaction mechanism is shown in Fig- ure S4 in the Supporting Information. Conclusion A SiO2-Al2O3 surface was functionalized with cationic indium species and tertiary amine groups by ion-exchange and silane- coupling reactions, respectively. These two functions on same solid surface acted as acidic and basic units in the catalytic cya- noethoxycarbonylation of aldehyde. The SiO2-Al2O3 surface has potential usefulness as a suitable stage to accumulate catalytic functions, including metal cations and organic molecules, for organic transformations. Experimental Section Materials Amorphous SiO2-Al2O3 N633L (SiO2, 82.5; Al2O3, 12.6%, 429 m2g1) was purchased from Nikki Chemical Co. The type of SiO2 used was Aerosil 300. Unless otherwise noted, all other materials were pur- chased from Wako Pure Chemicals, Tokyo Kasei Co. , Kanto Kagaku Co. , and Aldrich Inc, and were used without further purification. 1,3,5-Triisopropylbenzene or mesitylene was used as an internal standard in the catalytic reactions. XPS measurements XPS analyses were performed on an ESCA1700R system equipped with a dual Mg/Al X-ray source and a hemispherical analyzer oper- ating in fixed analyzer transmission mode. The spectra were ob- tained with a pass energy of 58.7 eV; an AlKa X-ray source was op- erated at 350 W and 14 kV. Excess charges on the samples were neutralized by argon ion sputtering. The analysis area was 0.8 2 mm. The working pressure in the analyzing chamber was less than 1 107 Pa. Spectra were acquired in the In 3 d, O 1 s, C 1 s, Si 2 p, and Al 2p regions. The Si 2 p peak at a binding energy (BE) of 103.0 eV was taken as an internal reference. Measurement of NMR spectra Solution 1H and 13C NMR spectra were recorded in CDCl3 by using an Avance 400 spectrometer operated at 400 and 100.6 MHz, re- spectively. Solid-state 13C and 29Si MAS NMR spectra (MAS rate=5 kHz) were recorded by using an Avance III spectrometer (9.4 T) operated at 100.6 and 79.5 MHz, respectively. The cross-polarization (CP) con- tact time for the 13C CP/MAS NMR measurements was 1.0 ms. The accumulation number and delay time were about 20 000 and 20 s, respectively, for 13C, and 10 000 and 15 s, respectively, for 29Si. Ada- mantane (d= 38.52 and 29.47 ppm) and hexamethylcyclotrisiloxane (d=9.66 ppm) were used as external standards for the calibration of chemical shifts. Both 27Al MAS NMR spectroscopy and MQ MAS experiments were performed by using a Bruker Avance III spectrometer (9.4 T) operat- ing at a 104.1 MHz for aluminum using a 4.0 mm MAS probe. Magic angle spinning (MAS) was performed at a rotation speed of 14 kHz. To allow quantitative evaluation of the single-pulse excita- tion (SPE) spectra, p/3 pulses with an rf field strength of 104 kHz were used. Chemical shifts were referenced relative to an aqueous solution of Al(NO3)3. The relaxation delays were 2 s. The 27Al 2D 3Q MAS NMR spectra were recorded by three-pulse z-filtering with rotor synchronization at a spinning rate of 14 kHz. The excitation pulse was a p pulse, and the conversion pulse was a p/3 pulse. The conversion delay was 2 s. For each t1 increment, 768 scans were used to accumulate signals. Fourier transform with respect to t1 and t2 led to pure adsorption 2D spectra. The spectra were sheared, so that the orthogonal projection of the 2D spectrum on the isotropic axis gave a high-resolution 1D spectrum free of any anisotropic broadening. The NMR spectroscopy parameters were determined from a simula- tion of the 27Al 2D 3Q MAS NMR spectra. The isotropic chemical shifts (diso) and SOQE were calculated according to Equations (1) ... (1) ... (2) in which Cq and h are the quadrupolar coupling constant and asymmetry parameter, respectively. IR spectroscopy measurements In-SiO2-Al2O3 or SiO2-Al2O3 was preheated at 120 8C under vacuum. In-SiO2-Al2O3 or SiO2-Al2O3 (0.10 g) was placed in a 20 mL Pyrex glass reactor under an argon atmosphere. Then, a solution (1.0 mL) of p-chlorobenzaldehyde (1.6 102 mmol) in toluene was added, and the solution was stirred at 25 8C for 0.5 h. The solvent was removed by evaporation and the product was dried under vacuum at 25 8C. The resulting solid was used for FTIR spectrosco- py measurements without further treatment. FTIR spectra were re- corded on a FTIR 6100 spectrometer with an MCT detector and ATR apparatus. Preparation of catalysts SiO2-Al2O3 (1.0 g) was added to an aqueous solution of InCl3 (50 mL 2.0 102m) and heated at 908C for 1 h. A solid was ob- tained by filtration, which was washed with deionized water and dried at 110 8C under air. The resulting In-SiO2-Al2O3 sample was stored under argon at room temperature. Before immobilization of tertiary amine, In-SiO2-Al2O3 was preheated to 120 8C under vacuum. In-SiO2-Al2O3 (3.75 102 g) was placed in a 20 mL of Pyrex glass reactor under an argon atmosphere. Then, a solution (0.7 mL) of 3-(diethylamino)propyltrimethoxysilane (2.6 102 mmol) in toluene was added, and the mixture was stirred at 90 8C for 1 h. The solvent was removed by evaporation and the product was dried under vacuum at 25 8C. The resulting In-SiO2- Al2O3/NEt2 sample was stored under an argon atmosphere. SiO2- Al2O3/NEt2 and SiO2/NEt2 were prepared by using similar proce- dures. The supports were preheated to 120 8C under vacuum. Cyanoethoxycarbonylation p-Chlorobenzaldehyde (0.50 mmol), ethyl cyanoformate (0.60 mmol), and toluene (1.0 mL) were added to the Pyrex glass reactor containing the In-SiO2-Al2O3/NEt2 catalyst, as mentioned above, and the resulting mixture was stirred at 40 8C. After 30 min, the catalyst was separated by filtration, and 1H NMR spectroscopic analysis of the filtrate showed 95 % conversion of p-chlorobenzal- dehyde and a 95 % yield of the cyanoethoxycarbonylation product. Acknowledgements This study was supported by JSPS KAHENHI (grant nos. 24686092 and 25630362). [1] a) E. L. Margelefsky, R. K. Zeidan, M. E. Davis, Chem. Soc. Rev. 2008, 37, 1118- 1126; b) K. Motokura, M. Tada, Y. Iwasawa, Chem. Asian J. 2008, 3, 1230-1236; c)S. Shylesh,W.R. Thiel,ChemCatChem 2011, 3, 278-287. 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Ed. 2002, 41, 3636-3638; b)J. Tian, N. Yamagiwa, S. Matsunaga, M. Shibasaki, Org. Lett. 2003, 5, 3021 - 3024 ; c) Y. N. Belokon', A. J. Blacker, P. Carta, L. A. Clutterbuck, M. North, Tetrahedron 2004 , 60, 10433 - 10447; d) S. Lundgren, E. Wingstrand, M. Phenhoat, C. Moberg, J. Am. Chem. Soc. 2005, 127, 11592 - 11593 ; e) Y. N. Belokon, E. Ishibashi, H. Nomura, M. North, Chem. Commun. 2006, 1775 - 1777. [14] In FTIR spectroscopy measurements, 20 - 30 and approximately 50 cm 1 redshifts were observed for crotonaldehyde adsorbed on silanol groups and cationic hydroxyl groups (water adsorbed on metal cations), re- spectively, see : J. R. Sohn, J. S. Han, J. S. Lim, Appl. Surf. Sci. 2005, 252, 858-865. Received: February 5, 2014 Published online on April 15, 2014 Ken Motokura, Yasuhiro Ito, Hiroto Noda, Akimitsu Miyaji, Sho Yamaguchi, and Toshihide Baba*[a] [a] Dr. K. Motokura, Y. Ito, H. Noda, Dr. A. Miyaji, Dr. S. Yamaguchi, Prof. Dr. T. Baba Department of Environmental Chemistry and Engineering Interdisciplinary Graduate School of Science and Engineering Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku Yokohama, 226-8502 (Japan) E-mail : [email protected] * Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cplu.201402004. (c) 2014 Blackwell Publishing Ltd. |