DSpace at VNU: Highly selective oxidation of benzyl alcohol to benzaldehyde catalyzed by nano Au gamma-Al2O3 under environment-friendly conditions

Reac Kinet Mech Cat DOI 10.1007/s11144-014-0773-4 Highly selective oxidation of benzyl alcohol to benzaldehyde catalyzed by nano Au/c-Al2O3 under environment-friendly conditions Nguyen Quang Long • Ngo Anh Quan Received: 27 May 2014 / Accepted: 21 August 2014 Ó Akade´miai Kiado´, Budapest, Hungary 2014 Abstract The liquid phase oxidation of benzyl alcohol to benzaldehyde under environment-friendly conditions has been investigated Water is used as a medium to replace toxic organic solvents The reaction used hydrogen peroxide as the oxidant The novel green reaction was catalyzed by nano Au/c-Al2O3 Gold nanoparticles supported on gamma alumina (c-Al2O3) were prepared by a deposition–precipitation method These synthesized materials were characterized by several techniques such as X-ray diffraction, N2 low temperature adsorption, and transmission electron microscopy The benzaldehyde selectivity changed depending on the supports, the Au loading and the reaction temperature Over 98 % selectivity of benzaldehyde was observed at 60 °C after h on the % Au supported on cAl2O3 which was calcined at 550 °C Keywords Environment-friendly reaction Á Nano gold catalyst Á Selective oxidation Á Benzaldehyde Introduction Over the last decade, many nanotechnology centers have been established worldwide and nanomaterials research has intensified a lot The supported nano gold catalysts have become the most attractive subject since the report of their high catalytic activity in the oxidation reaction of CO by Haruta and other research groups [1–4] Furthermore, alumina has been one of the most well-known catalyst and catalyst support for decades, with its many applications in laboratory and industrial scale process [5] But when it comes to supported gold catalyst, alumina N Q Long (&) Á N A Quan Faculty of Chemical Engineering, Ho Chi Minh City University of Technology, Ho Chi Minh, Vietnam e-mail: nqlong@hcmut.edu.vn 123 Reac Kinet Mech Cat has not been considered a good catalyst support One of the reasons is that the reactivities reported for Au/c-Al2O3 are very variable [6], which cannot be explained solely by differences in the preparation method or gold particle size However, partial research has found that gold supported catalysts have significant activity in many reactions, especially liquid-phase selective oxidation under mild conditions [6–8] Benzaldehyde is the most industrially important compound of the family of aromatic-aldehydes, particularly in perfume production The commercial process for the manufacturing of benzaldehyde involves the hydrolysis of benzyl choride and the oxidation of toluene [9] Other processes, including the oxidation of benzyl alcohol, the reduction of benzyl chloride, and the reaction of carbon monoxide and benzene, have been utilized in the past, but they no longer have any industrial application The vapor- and liquid-phase air oxidation of toluene is the source of the majority of the world’s synthetic benzaldehyde However, the future trend is trying to gradually replace benzyl chloride and toluene, hazardous substances, for designing chemical syntheses that are more environment-friendly [10] In this study, the selective oxidation of benzyl alcohol to benzaldehyde catalyzed by nano Au/c-Al2O3 was investigated The catalysts were prepared by the homogeneous deposition–precipitation (HDP) method The conversions of benzyl alcohol and benzaldehyde were reported in order to understand the effect the nano Au catalysts on the liquid selective oxidation process under environment-friendly conditions Experimental Synthesis and characterization of the catalysts HAuCl4 was synthesized in our laboratory from commercial gold (99.99 % Au) Commercial gold was cleansed using concentrated nitric acid and then heat up to 500 °C in thermal oven After purification, it was dipped in an aqua regia solution and kept until complete dissolution Excessive nitrate was removed by heating slowly with addition of hydrochloric acid to obtain pure HAuCl4.4H2O Alumina used in this study came from two main sources: purchased from Merck (c-Al2O3, 99 %) (denoted as M) and self-synthesized from aluminum triisopropoxide precursor (Sigma Aldrich, 97 %) using a sol–gel method (Yoldas Process) The gel was dried and calcined at two temperature 450 °C C (723 K) and 550 °C (823 K) The synthesized c-Al2O3 samples were denoted as S723 and S823 Gold nanoparticles supported on gamma-alumina were prepared by the homogeneous HDP method In a typical procedure, g of c-Al2O3 was added into 100 mL solution of urea 0.42 M and a proper amount of HAuCl4 The mixture was kept at 80 °C, stirred continuously for 16 h The solid part was filtered, dried at 60 °C for h Finally, the calcination took place at 300 °C for h at the heating rate of °C/min Each of the three different alumina materials was deposited an amount of 0, 1, % wt of gold accordingly 123 Reac Kinet Mech Cat O OH Au/γAl2O3 H H2O2 , temperature Fig The selective oxidation of benzylalcohol to benzaldehyde using supported gold catalyst The structural properties of the catalysts were characterized by X-ray diffraction (XRD) using a Multiflex X-ray Diffractometer (Rigaku Corp.) The specific surface area was determined by the N2 adsorption method at 77.3 K on a Quantachrome Autosorb-1 apparatus The specific surface area is calculated based on BET theory Transmission Electron Microscopy (TEM) was carried out on a JEOL JEM-1400 Catalytic activity test The selective oxidation reaction of benzyl alcohol to benzaldehyde (Fig 1) was carried out in a magnetically stirred flask (250 ml) Oxidizing agent H2O2 was fed gradually in the form of droplets If there is no other note, the reaction conditions are fixed under the following conditions: the temperature is 80 °C, the reaction time is h, the molar ratio benzyl alcohol: H2O2 = 1:1.3, the amount of catalyst is 0.3 g The product mixture was filtered to remove the solid catalyst and extracted using diethyl ether to remove the remaining water The magnesium sulfate powder was then added and kept still for h for further dehydration The organic phase was analyzed by gas chromatography apparatus Varian Series 2,800, with a FID detector and a capillary column FactorFour VF-1 ms length 15 m, diameter 0.25 mm, thickness of 0.25 lm Conversion, selectivity and yield were calculated from the measured values on GC by the following formulas: À Á Benzyl alcohol BnOHị conversion : X%ị ẳ mBnOHinị mBnOHoutị = mBnOHinị 100 Benzaldehyde BALị selectivity : S%ị ẳ mBAL = mBnOHÀðinÞ À mBnOHÀðoutÞ Â 100: Benzaldehyde ðBALÞ yield : H%ị ẳ conversion selectivity 100 Results and discussion Synthesis and characterization of catalyst Fig shows the XRD patterns of c-Al2O3 synthesized from different calcination temperature as well as the Merck sample The standard XRD of c-Al2O3 according to JCPDS No-290063 shows c-Al2O3 characteristic peaks at 2h = 37.6°; 39.5°; 45.8° and 66.8° These peaks have been observed clearly in the XRD pattern of the 123 Reac Kinet Mech Cat Fig XRD patterns of c-Al2O3: a M (Merck) b S823 and c S723 in comparison with standard c-Al2O3 Fig XRD patterns of % Au/M a % Au/S723 b and % Au/S823 c in comparison with standard Au d and standard c-Al2O3 e three types of the supports confirming the formation of c-Al2O3 crystalline structure in the products When varying the calcination temperature in the synthesis, characteristic peaks sharper and narrower lines due to higher crystallinity leading to larger crystalline particles Furthermore, % Au/c-Al2O3 and standard XRD of Au according to JCPDS No-040784 are shown in Fig The XRD patterns changed after the deposition of the very small amount of gold particles The changes in XRD results were due to the appearance of gold deposited on the material surface It is interesting that, although the amount of gold deposited on alumina surface was the same (verified by testing the amount of gold in catalyst using the ICP method), the XRD results differ according to different type of the supports This might refer to 123 Reac Kinet Mech Cat Table Characteristics of c-Al2O3, Au/c-Al2O3 catalysts Specific surface area (by BET) (m2/g) No Support materials Description M Merck, 99 % c-Al2O3 137 S723 Self-synthesized, calcined at 450 °C 314 S823 Self-synthesized, calcined at 550 °C 235 Fig TEM results of % Au/ S823 the interaction between gold particles and support material surface, which cause changes to surface and crystalline energy of gold particles Table summarizes the characteristics of the synthesized samples with various c-Al2O3 supports in the synthesis The crystalline size, which is calculated by the Scherrer equation (column in Table 1) was not significantly different when the calcination of Al2O3 was varied Fig reports the particle morphology of the gold nanoparticles with various Al2O3 carriers The shape of the samples was almost spherical In Table 1, it is noted that specific surface area of these samples by BET method was significantly different The surface area of the synthesized materials was higher than those of the commercial samples Gold particles deposited on c-Al2O3 supports by the DP method are in nano size The gold particle size, which was obtained from XRD results using the Scherrer equation, were 8.8 and 9.4 nm for % Au/S723 and % Au/S823, respectively This is consistent with the TEM result of % Au/S823 (Fig 4) The average particle size, which was determined by measuring the gold particles in the TEM images, was about 11 nm Therefore, it can be concluded that the nano-sized gold has been deposited on the c-Al2O3 by deposition–precipitation method Catalytic activity of Au/c-Al2O3 Figs and report the catalytic activity of the nano Au/c-Al2O3 for the selective oxidation of benzyl alcohol under environment-friendly conditions It is seen that the gold particles are responsible for catalytic activity since in the experiments conducted in similar conditions using no catalyst or c–alumina as catalyst resulted 123 Reac Kinet Mech Cat Fig Effect of nanogold on the oxidation of benzyl alcohol over various support material Reaction conditions: reaction temperature = 80 °C, reaction time = h, molar ratio benzyl alcohol: H2O2 = 1:1.3, benzyl alcohol weight = 5.3 g, catalyst weight = 0.3 g Fig Effect of Au catalyst concentration on the oxidation of benzyl alcohol Reaction conditions: reaction temperature = 80 °C, reaction time = h, molar ratio benzyl alcohol: H2O2 = 1:1.3, benzyl alcohol weight = 5.3 g, catalyst weight = 0.3 g in a very low conversion of benzyl alcohol and yield of benzaldehyde (\1 %) (Fig 6) For all the supports, the conversion of benzyl alcohol increased and the selectivity of benzaldehyde also increased As a result, higher yield of benzaldehyde was obtained for the % Au/c-Al2O3 catalysts (Fig 5) Although the benzaldehyde selectivities were high in the S723-supported catalysts, the conversion of the benzyl alcohol was relatively low On the other hand, the conversion of the reactant were high, but the selectivity of the desired benzaldehyde was low on the M-supported catalysts (Fig 5) Hence, the separation of the benzaldehyde from the products is practically more complicated in both cases The surface area of the S723 was much higher than the M material (Table 1) However, gold particles were in nano size, which may allow the Au to penetrate into 123 Reac Kinet Mech Cat the pore of the support It led to the blockage of the pore or/and increase the diffusion resistance which plays important rule in catalytic reaction in liquid phase The support material S823 seems to be able to balance the surface area and the diffusion resistance problem in this reaction condition The effect of Au loading on S823 on the catalytic activity of Au/S823 samples is shown in Fig The reaction could give more than 61% selectivity within h at % Au catalyst It is observed that a higher Au loading gave a higher conversion and selectivity However, the improvement in the reaction rate was not significant at the loading 4% Au as compared to the case of 2% Au Therefore, it was decided to use 2% Au/S823 catalyst for further investigations Furthermore, the catalytic activity of nanoparticles gold-supported was investigated the effect of temperature on the reaction conversion The reaction temperature was at 60, 80 and 100 °C while other parameters were fixed as mentioned in Sect 2.2 It was found out from Fig that increasing the reaction temperature led to significant enhancements in the conversion benzyl alcohol but decreases in selectivity Indeed, the reaction carried out at 60 °C could give the highest selectivity of 98 % The selectivity reduced to 56% when the catalytic reaction was carried out at 100 °C In the considered reaction, not only benzyl alcohol reacts with the oxidizing agent, but benzaldehyde—the desired product—can also react with oxygen to produce benzoic acid, both of which is kinetically favorable at high temperatures [11, 12] Therefore, more benzoic acid was produced when the reaction was conducted at higher temperatures It led to the fact that the selectivity decreased at high temperature reaction as observed in the Fig From the point view of green chemistry, another point of great concern for heterogeneous catalysts is the ease of separation as well as the reusability of the Fig Effect of temperature on the oxidation of benzyl alcohol by 2% Au/S823 Reaction conditions: reaction time = h, molar ratio benzyl alcohol: H2O2 = 1:1.3, benzyl alcohol weight = 5.3 g, catalyst weight = 0.3 g 123 Reac Kinet Mech Cat Fig Catalyst recycling studies of 2% Au/S823 Reaction conditions: reaction temperature = 80 °C, reaction time = h, molar ratio benzyl alcohol: H2O2 = 1:1.3, benzyl alcohol weight = 5.3 g, catalyst weight = 0.3 g catalyst The supported gold nanoparticles were therefore investigated for recyclability over successive three runs The catalyst was washed with acetone and water to remove products and unreacted materials, dried under vacuum at room temperature overnight, and reused in identical conditions with the fresh catalyst It was found that the catalyst could be recovered after three times without significant decrease in catalytic activity as reported in Fig Conclusions Nano-sized Au supported on c-Al2O3 can effectively catalyze the selective oxidation of benzyl alcohol to benzaldehyde under environment-friendly conditions This study showed that changes in support material, even though they were all the same type of alumina (c-Al2O3), have a significant effect on both physical–chemical and catalytic activity of the catalysts The gold catalyst was prepared and characterized by XRD, TEM, BET The catalysts exhibited high activity for the selective oxidation of benzyl alcohol to form benzaldehyde More than 98 % selectivity being achieved after h reaction at 60 °C The solid catalyst could be reused at least three times without a significant degradation in catalytic activity Acknowledgement This research is funded by Vietnam National University HoChiMinh City (VNUHCM) under grant number C2014-20-33 References Haruta M, Kobayashi T, Sano H, Yamada N (1987) Novel gold catalysts for the oxidation of carbon 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Keggin heteropolyacid catalysts Catal Commun 43:155–158 12 Bal R, Pendem C (2014) Preparation of the CuCr2O4 spinel nanoparticles catalyst for selective oxidation of toluene to benzaldehyde Green chem 16(5):2500–2508 123 ... are more environment-friendly [10] In this study, the selective oxidation of benzyl alcohol to benzaldehyde catalyzed by nano Au/ c-Al2O3 was investigated The catalysts were prepared by the homogeneous... c-Al2O3 by deposition–precipitation method Catalytic activity of Au/ c-Al2O3 Figs and report the catalytic activity of the nano Au/ c-Al2O3 for the selective oxidation of benzyl alcohol under environment-friendly. .. ease of separation as well as the reusability of the Fig Effect of temperature on the oxidation of benzyl alcohol by 2% Au/ S823 Reaction conditions: reaction time = h, molar ratio benzyl alcohol: