Indian Journal of Chemistry Vol. 53A, April-May 2014, pp. 557-560
Vapor phase chemoselective conjugate hydrogenation of isophorone over Pd/SBA-15 catalysts Venkateswarlu Vakati, Saidulu Ganji , Ravi Kumar Marella, Kamaraju Seetha Rama Rao & David Raju Burri* Catalysis Laboratory, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 607, India Email:
[email protected] Received 10 January 2014; revised and accepted 12 March 2014 A series of Pd/SBA-15 catalysts with 1, 2, 3, 4% (by weight) Pd loading has been prepared by a conventional impregnation method and characterized by N2-sorption, low-angle and wide-angle XRD, XPS and TEM techniques. These catalysts exhibit excellent activities for chemoselective hydrogenation of isophorone to dihydroisophorone showing 100% conversion and 100% selectivity in gas phase at atmospheric pressure. The structural and textural characteristics of the catalysts play a crucial role in exhibiting high conversion and selectivity. Keywords: Catalysts, Supported catalysts, Palladium catalysts, Hydrogenation, Isophorone, Carbonyl compounds
The catalytic hydrogenation of α, β-unsaturated carbonyl compounds is a process of great practical importance1-3. The hydrogenation of both C=C and C=O bonds is thermodynamically feasible. The selective hydrogenation of C=C bond of α, β-unsaturated carbonyl compounds, leaving the C=O group intact, for the production of saturated carbonyl compounds is an important catalytic transformation. Numerous transition metal catalysts such as palladium, ruthenium, rhodium, iridium and other metal complexes have been used in the selective hydrogenation of α, β-unsaturated carbonyl compounds4-7. Isophorone (3, 5, 5-trimethyl-2cyclohexen-1-one) is a α, β-unsaturated ketone and its possible hydrogenation products are shown in Scheme 1. All of these products are of industrial significance8,9. In particular, dihydroisophorone (3, 3, 5-trimethylcyclohexanone) is used as a solvent for vinyl resins, lacquers, varnishes, paints and other coatings. However, the boiling points of these important products are close to one another, so that their separation by distillation is complicated and expensive10. Augmenting the selectivity without foregoing the conversion is a challenge. Selectivity control by ceasing all of these by-products is a key issue and overcoming this barrier shall resolve the problem of separation of 3, 3, 5-trimethyl-cyclohexanone from the product mixture. Also, further purification will not be required even for any specific application. Therefore, a highly selective and efficient process is required for the hydrogenation of isophorone.
Cotrupe et al.11 reported the selective hydrogenation of isophorone to dihydroisophorone, but with organic solvents. Pisarek and his coworkers12,13 also reported the selective hydrogenation of isophorone to dihydroisophorone with Cr and Co modified nickel based catalyst in liquid phase using environmentally harmful organic solvents. Hitzler et al.14,15 reported the highly selective hydrogenation of isophorone to dihydroisophorone in supercritical carbon dioxide with an amino polysiloxane supported palladium catalyst in a flow-type reactor in supercritical carbon dioxide. Due to the wide range of product applications in fine chemical processes, hydrogenation reactions are being carried out in organic solvents using supported metal catalysts. However, reaction
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velocities are not so high and the separation of pure products from solvents is difficult. Furthermore, eliminating the use of organic solvents is highly desirable for environmental benign processes.16, 17 Very recently we have reported the chemoselective hydrogenation of isophorone in liquid phase at room temperature in an aqueous medium18, wherein, the selective hydrogenation of various α, β-unsaturated carbonyl compounds to saturated carbonyl compounds with 100% selectivity without disturbing the other functional groups was studied. In comparison with the batch mode operation, the continuous processes are more favorable for commercialization. In particular, the hydrogenation of isophorone, continuous process is highly desirable, since the boiling points of hydrogenation products of isophorone are close to one another and are difficult to separate. In this context, herein, for the first time the selective hydrogenation of isophorone to dihydroisophorone with 100% selectivity in vapor phase at atmospheric pressure with Pd/SBA-15 catalysts is described. Materials and Methods Synthesis of SBA-15
SBA-15 was prepared by literature procedure19. In a typical synthesis, 20 g of P123 triblock copolymer (EO20PO70EO20) was dispersed in 465 g of water and 137.5 g of 35% hydrochloric acid was added with continuous stirring until the formation of a clear homogeneous solution. Stabilizing the reaction temperature at 40 °C, 44 g of tetraethyl orthosilicate (TEOS) was added under constant stirring. Within 15 min of adding TEOS the solution turned into a viscous gel, which again turned into milky white solution within 5 min. This white solution was stirred for 12 h at 40 °C and subjected to hydrothermal treatment at 100 °C for 24 h. The resultant slurry was filtered off, washed with deionized water, dried at 120 °C for 12 h and then calcined at 550 °C for 8 h at a ramping rate of 1 °C/min to remove the template and denoted as SBA-15.
Catalyst characterization
The X-ray diffraction (XRD) patterns were recorded at room temperature using an X-ray diffractometer (Multiflex, Rigaku, Japan) with a nickel filtered CuKα radiation. N2 adsorption– desorption isotherms were recorded on a N2 adsorption unit at -196 °C (Quadrusorb-SI V 5.06, Quantachrome Instruments, USA). The samples were out-gassed at 200 °C for 4 h before the measurement. Transmission electron microscope (TEM) analysis was made using a FEI Technai G2 S-Twin Serial Number D 2083 instrument at an accelerating voltage of 200 kV. The XPS analysis was made on a photoelectron spectrometer (Kratos Axis 165, Shimadzu, Japan) with Mg Kα radiation (1253.6 eV). Catalytic activity test
The catalytic reaction was carried out in a fixed bed reactor (10 mm i.d. and 360 mm long) at atmospheric pressure. The reactor was loaded with 0.1 g of catalyst and reduced in H2 at a flow rate of 20 mL/min at 250 °C for 2 h. Isophorone was fed with a liquid flow rate of 1 mL/h along with H2 gas keeping the total flow constant. Time-on-stream studies for the reaction were carried out at 225 °C. The products were collected in a cold trap at regular intervals and analyzed by GC-17A (Shimadzu Instruments, Japan) equipped with flame ionization detector (FID) and Equity-5 capillary column. The products were confirmed by GCMS-QP-5050 (Shimadzu Instruments, Japan). Results and Discussion The low-angle XRD spectra of SBA-15, Pd/SBA15 catalysts are displayed in Fig. 1. All the samples
Preparation of Pd/SBA-15 catalysts
To prepare mesoporous SBA-15 supported palladium catalysts (Pd/SBA-15), the requisite amounts of aqueous PdCl2 solution (1, 2, 3, 4 wt% as Pd metal) were impregnated. These catalyst samples were dried at 100 °C for 12 h and reduced at 350 °C for 4 h and denoted as xPd/SBA-15, where x indicates the percentage loading of Pd on the SBA-15 support.
Fig. 1—Low angle XRD patterns of SBA-15 and Pd/SBA-15 catalysts.
VAKATI et al.: VAPOR PHASE HYDROGENATION OF ISOPHORONE OVER Pd/SBA-15 CATALYSTS
exhibited three well-resolved XRD diffraction peaks in the range of 2θ = 0.7–2.1°, which can be indexed to the (100), (110) and (200) diffractions, characteristic of the 2D p6mm hexagonal symmetry. The wide-angle XRD patterns of Pd/SBA-15 catalysts shows a broad peak that appears in all the patterns at 2θ = 21.51°–21.85° (Fig. 2) and is responsible for the SiO2 phase of SBA-15. As the loading of Pd increases the intensity of the main peak (2θ = 40.08°) also increases in order, which implies the increment in size of crystalline Pd particles. The XRD reflections that appear at 2θ = 40.08°, 46.5° and 68.01° are characteristic of Pd0 phase according to the card no. JCPDS 88-2335. The very weak and broad
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diffraction peaks in 1 and 2wt% Pd/SBA-15 catalysts, reveal the nanoscale sized Pd particles. Figure 3 shows the N2 adsorption-desorption isotherms of Pd/SBA-15 catalysts together with parent SBA-15. The isotherms obtained for parent SBA-15 and Pd/SBA-15 catalysts are of the type IV and exhibited a hysteresis loop of H1 type in accordance with the IUPAC classification20, which indicates that the parent SBA-15 mesoporous structure is well maintained during the various stages of catalyst preparation. However, there is a gradual decrease in the surface area and total pore volume as the Pd loading increases on the surface of the SBA-15 support (Table 1), which suggests that as the Pd loading increases, the agglomeration of Pd particles increases, leading to partial blockage of pores21,22. As shown in Table 1, there is no substantial variation in the structural parameters, like d spacing, unit cell length and pore wall thickness, indicating the retention of parent SBA-15 mesophase structure in all the Pd/SBA-15 catalysts. XPS spectrum of 2Pd/SBA-15 catalyst is displayed in Fig. 4. The XPS results demonstrate that the Pd species present in 2Pd/SBA-15 catalyst is in the metallic state, corresponding to the binding energies of 335.1 and 340.3 eV in Pd 3d5/2 and Pd 3d3/2 levels respectively 23, 24. Table 1—Textural and structural parameters of SBA-15 and Pd/SBA-15 catalysts obtained from N2 sorption data No. 1 2 3 4 5
Fig. 2—Wide angle XRD patterns of Pd/SBA-15 catalysts.
a
Fig. 3—N2 adsorption-desorption isotherms of SBA-15 and Pd/SBA-15 catalysts.
Catalyst SBA-15 1Pd/SBA-15 2Pd/SBA-15 3Pd/SBA-15 4Pd/SBA-15
SBETa (m2/g) 864 794 622 548 519
Vt b (cc/g) 1.66 1.32 1.2 1.0 0.95
BET surface area; b Total pore volume,
Fig. 4—XPS spectra of 2Pd/SBA-15 catalysts.
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Fig. 5—TEM image of 2Pd/SBA-15 catalyst.
The morphology and particle size distribution of the 2Pd/SBA-15 catalyst was studied by the TEM (Fig. 5). All the palladium particles are spherical in shape and are distributed in the range of 11–13 nm. The catalytic hydrogenation of isophorone to dihydroisophorone has been carried out at 225 °C at atmospheric pressure over different SBA-15 supported Pd catalysts. With increase in Pd metal loading from 1 to 2, the conversion of isophorone increased substantially (from 90% to 99%) but further increase in Pd metal loading beyond 2 wt% decreased the conversion of isophorone marginally. The decrease in conversion at higher loading catalysts like 3Pd/SBA-15 (93%) and 4Pd/SBA-15 (92%) may be due increase in Pd particle size. However, the selectivity of dihydroisophorone was maintained at 100% with all the samples. The time-on-stream study for hydrogenation of isophorone was conducted on the 2 wt% Pd/SBA-15 catalyst at 225 °C for 10 h. Results show that conversion and selectivity are remain constant up to 10 h, which indicates the versatility of the catalyst. Conclusions In summary, Pd/SBA-15 catalysts are highly efficient for vapour phase chemoselective hydrogenation of isophorone to dihydroisophorone with 100% selectivity at atmospheric pressure. High surface area and low particle size of the catalysts are responsible for high catalytic activity.
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