ARTICLE pubs.acs.org/IECR
CoAl-CrO4 Layered Double Hydroxides as Selective Oxidation Catalysts at Room Temperature Jinesh Cherukattu Manayil, Sivashunmugam Sankaranarayanan, Deep Singh Bhadoria, and Kannan Srinivasan* Discipline of Inorganic Materials & Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR), GB Marg, Bhavnagar-364 002, India
bS Supporting Information ABSTRACT: A new combined approach of using layered double hydroxides (LDHs) for both environmental remediation and the derived material as heterogeneous catalyst for selective oxidation is presented. A series of CoAl LDHs with carbonate or nitrate anion in the interlayers was synthesized by two different methods, namely, constant-pH precipitation using NaOH as precipitating agent and homogeneous precipitation by hexamine hydrolysis. The samples were optimized for chromate removal and an uptake of 45 mg Cr/g was obtained for CoAl2-N (CoAl-LDH with Co/Al atomic ratio of 2 and nitrate as interlayer anion by constant-pH precipitation) which was significantly higher than its corresponding carbonate sample that showed 15 mg Cr/g. The thus derived materials after uptake of chromate were explored for solvent free selective oxidation of benzyl alcohol using tert-butyl hydroperoxide as oxidant at room temperature. Among the catalysts screened, CoAl2-N sample after chromate uptake gave 23% conversion with 100% selectivity for benzaldehyde using a catalyst/substrate weight ratio 1:20. The catalyst was recyclable up to four cycles, and the reaction was heterogeneous. The studies revealed that the chromate ion-exchanged sample was more stable and selective as oxidation catalyst than that of surface adsorbed sample. The observed activity of this catalyst was ascribed to the cooperative contribution of Co and Cr.
1. INTRODUCTION Selective oxidation of benzyl alcohol to benzaldehyde is an important reaction as the product finds application in industrial processes, especially in fragrances, pharmaceuticals, and dyes. Different methods like carbonylation of benzene,1 hydrolysis of benzal chloride,2 oxidation of toluene,3,4 benzyl chloride,5 and benzyl alcohol6 are traditionally pursued for making benzaldehyde. Most of these processes have certain disadvantages such as contamination by chorine, formation of benzoic acid as side products, and so forth. Conventional oxidizing agents such as dichromate7 and permanganate8 are also employed for this reaction under homogeneous condition. However, benzoic acid was coproduced by further oxidation along with cogeneration of heavy metal waste, making this process environmentally unfriendly. Previous reports on oxidation of benzyl alcohol selectively to benzaldehyde involve catalysts such as precious metal nano particles,9 metal complexes,10 and transition metal supported heterogeneous catalysts.11 Different oxidants like molecular oxygen,12 H2O2,13 and tert-butyl hydroperoxide (TBHP) as oxidants at higher temperatures14 were also reported. Layered double hydroxides (LDHs; otherwise referred as hydrotalcite-like (HT-like) materials) are layered materials with the general formula [M(II)1xM(III)x(OH)2] Ax/nn 3 mH2O, where M(II) and M(III) are divalent and trivalent cations, An is the interlayer anion, and x can generally have the values between 0.20 and 0.40.15 Structurally, they possess a brucite-like (Mg(OH)2) layered network wherein a partial substitution of bivalent ion by trivalent ion, say Al3+, occurs, and the resulting excess positive charge is compensated by anions which occupy r 2011 American Chemical Society
space between the sheets along with water molecules. LDHs have remarkable application potential in remediation of environmentally undesirable anions (both inorganic and organic) owing to their facile ion-exchange property and eco-friendly nature.16 They usually exhibit high anion exchange capacity (AEC) of around 23 meq/g depending upon the interlayer anion, which are comparable to many commercial anion exchangers.1719 Different techniques are employed for the removal of toxic Cr(VI) such as ion exchange,20 magnetic separation,21 bio sorption,22 and adsorption,23 and so forth. Among them, adsorption over solid materials is versatile and largely accepted because of its low cost and practicability. LDHs as adsorbents have also been explored for the removal of chromate.24 It is also wellknown that LDHs and their derived forms are potential catalytic materials. LDHs by themselves are used as oxidation catalysts.25 LDHs were explored previously for benzyl alcohol oxidation however at relatively higher temperature (around 90 °C) or at high catalyst loading.26 With this knowledge and background, the objectives of the present work are (i) to remove noxious Cr (VI) anions in aqueous solution using a series of CoAl LDHs having nitrate or carbonate as interlayer anion, (ii) to use the exchanged adsorbent after chromate uptake as oxidation catalyst for the solvent free selective oxidation of benzyl alcohol at moderate temperatures Received: July 5, 2011 Accepted: October 27, 2011 Revised: October 11, 2011 Published: October 27, 2011 13380
dx.doi.org/10.1021/ie201436b | Ind. Eng. Chem. Res. 2011, 50, 13380–13386
Industrial & Engineering Chemistry Research
ARTICLE
using TBHP as an oxidant, and (iii) to find how the variation in the interlayer anion influences the chromate uptake and sustained catalytic activity.
2. EXPERIMENTAL SECTION 2.1. Material Synthesis. The carbonate containing CoAl LDHs was prepared by constant pH precipitation under low supersaturation whose details are given elsewhere.27 For the synthesis of nitrate containing samples, Co(NO3)2 3 6H2O and Al(NO3)3 3 9H2O were used as metal precursors, and NaNO3 and NaOH were used as precipitating agents. During the synthesis, pH was maintained constant at 9 ( 0.1 using an auto titrator assembly (Schott). The precipitation and aging were done under N2 atmosphere, and the resultant slurry was filtered, washed thoroughly with decarbonated water, and dried under vacuum at room temperature. Synthesis of nitrate containing LDHs was also carried out using hexamine hydrolysis that involved slow and homogeneous precipitation of metal cations with a controlled release of ammonia.28 Samples thus prepared are named as M(II)M(III)x-Y for constant pH precipitated samples and M(II)M(III)x-Y-Hex for hexamine samples where x stands for M(II)/M(III) atomic ratio and Y represents the interlayer anion, namely, N for nitrate and C for carbonate. The samples after chromate adsorption are named here as M(II)M(III)x-Y-CrO4. CoAl2-N and CoAl2-C refer to CoAl LDH with nitrate and carbonate anion in the interlayer respectively with Co/Al atomic ratio of 2.0, and CoAl2-N-CrO4 and CoAl2C-CrO4 are corresponding samples after chromate adsorption. 2.2. Characterization Techniques. The chemical compositions of the materials were measured through inductively coupled plasma optical emission spectrometry (ICP-OES PerkinElmer, OES, Optical 2000 DV). The samples were characterized by various physicochemical techniques such as powder X-ray diffraction (PXRD; Rigaku-MiniFlex), FT-IR (Perkin-Elmer FT1730), and scanning electron microscopy (SEM; Leo Series VP1430) equipped with EDX (Oxford Instruments). Brunauer EmmettTeller (BET) specific surface area and pore size analysis of the samples were measured by nitrogen adsorption at 196 °C using a sorptometer (ASAP-2010, Micromeritics), and the data were analyzed using built-in software. 2.3. Chromate Uptake Studies. Adsorption of CrO42 on LDH materials was carried out by taking 25 mL of 150 ppm (150 mg Cr/L, 2.88 mmol/L) potassium chromate (K2CrO4) solution, into which 25 mg of the material was added. All reactions were carried out in closed reaction vessel under N2 atmosphere. The residual CrO42 in the filtrate after the reaction was determined spectrophotometrically (Shimadzu UV-3101PC) using the diphenylcarbazide method.29 All experiments were conducted at stock solution pH (7.5). Chromate uptake was calculated by using the formula:
Q e ¼ ððCi -Ce ÞV =mÞ 1000 where Qe is chromate removed (mg Cr/g); V is volume of solution (L); Ci is the initial concentration of chromate (mg Cr/L), Ce is the equilibrium concentration (mg Cr/L), and m is the mass of the adsorbent. For recycle studies, 0.5 M NaNO3 was used as desorption medium. 2.4. Oxidation of Benzyl Alcohol. The adsorbent after chromate removal was washed thoroughly with water to remove the physically adsorbed chromate and then dried in vacuum desiccators before used as catalyst. The chromate adsorbed CoAl
Figure 1. Powder X-ray diffraction patterns of (a) CoAl2-N-Hex; (b) CoAl2-N; (c) CoAl2-N-CrO4; (d) CoAl2-C; (e) CoAl2-C-CrO4 (the inset is expanded region of 003 reflection).
catalyst used in this study was prepared from 3 g batch adsorption experiments in 3 L of 150 ppm chromate solution for 2 h. The catalytic reactions were carried out in a two neck 25 mL roundbottom flask to which 1 g (9.24 mmol) of benzyl alcohol and 50 mg (5 wt %) of catalyst were added. Calculated amount of tertbutyl hydroperoxide (70 wt % in water; 0.388 mmol (0.05 mL) to 15.52 mmol (2 mL)) was added as oxidant. The reaction mixture was stirred for 6 h at room temperature and analyzed using gas chromatography (Varian 450-GC) with a capillary column (Factor Four VF-1) and a FID detector using n-decane as external standard. The experiments were done in duplicate and results (conversion and selectivity reported here are based on GC) have variation of (2%.
3. RESULTS AND DISCUSSION 3.1. Materials Characterization. The PXRD patterns of assynthesized CoAl LDHs are given in Figure 1. The profiles confirmed the presence of a pure HT-like phase without the cocrystallization of any detectable impurity. The reflections close to 2θ = 1011.6° and 2024° (d value close to 8.87.6 Å, and 4.43.7 Å, respectively) are ascribed to diffraction by basal planes (003) and (006) respectively while the broad and less intense peak around 2θ = 60° is assigned to diffraction by nonbasal (110) plane. The diffraction profiles showed broad reflections in mid 2θ range indicative of turbostatic disorder. Relatively, sharp basal and ab plane reflections were seen for carbonate containing samples as reported in literature [ICDD-051:0045, 27]. The (003) reflection appeared around 7.6 Å for carbonate containing sample (CoAl2-C) while at 8.8 Å for nitrate containing sample obtained through hexamine hydrolysis (CoAl2-N-Hex).28 For the nitrate containing sample prepared through constant pH precipitation (CoAl2-N), the (003) reflection was relatively broad with the peak centering around 8.4 Å suggests the copresence of both nitrate and carbonate. The lattice parameters “a” and “c” were calculated and included in Supporting Information, Table 1S. The “a” parameter (calculated as 2d110) refers to cationcation distance within the brucite-like layer and was in the range (3.063.08 Å), similar to the values reported in literature for LDHs.30,31 The parameter “c” (c = ((3d003) + (6d006))/2) relates to the interlayer distance and sheet thickness suggest the extent of electrostatic interaction between the layers. In the case of nitrate containing samples the parameter “c” was 13381
dx.doi.org/10.1021/ie201436b |Ind. Eng. Chem. Res. 2011, 50, 13380–13386
Industrial & Engineering Chemistry Research
Figure 2. Chromate uptake over different materials (chromate solution: 25 mL (150 mg Cr/L); material: 25 mg; time: 2 h; temperature: 30 °C (room temperature); stirring speed: 600 rpm).
slightly higher (∼26 Å) suggesting weaker electrostatic interaction compared to carbonate samples that showed around 22.5 Å. Irrespective of the synthesis methodology (constant pH precipitation or hexamine hydrolysis), a nearly similar “c” parameter value was observed for nitrate containing samples at equivalent Co/Al atomic ratio. The crystallite size was calculated by the DebyeScherrer equation (Supporting Information, Table 1S) that showed a larger size for the carbonate sample (115 Å) compared to the nitrate sample (40 Å). FT-IR analysis (Supporting Information, Figure 1S-A) showed a sharp band around 1381 cm1 for the sample prepared through hexamine hydrolysis (CoAl2-N-Hex) confirmed the presence of nitrate in the interlayers while a band around 1362 cm1 was noted for carbonate containing sample (CoAl2-C) attributed to ν3 antisymmetric stretching. For the nitrate containing sample obtained through constant pH precipitation (CoAl2-N), a split in the ν3 band was noticed corroborating the copresence of both nitrate and carbonate anions in the interlayer for this sample. The textural properties of the samples (given in Supporting Information, Table 1S) showed a Type II isotherm and exhibited very low specific surface area (
