Fly ash based effective solid base catalyst (KF/Al2O3/fly ash473, KF/Al2O3/fly ash673, and KF/Al2O3/fly ash873) was synthesized by loading KF over chemically and thermally activated fly ash. The chemical activation was done by treating fly ash with aluminum nitrate via precipitation method followed by thermal activation at 650°C to increase the alumina content in fly ash. The increased alumina content was confirmed by SEM-EDX analysis. The alumina enriched fly ash was then loaded with KF (10 wt%) and calcined at three different temperatures 473 K, 673 K and 873 K. The amount of loaded KF was monitored by XRD, FTIR spectroscopy, SEM-EDX, TEM and Flame Atomic Absorption Spectrophotometer. The catalytic activities of the catalysts were tested in the Claisen-Schmidt condensation of benzaldehyde and 4-methoxybenzaldehyde with 2′-hydroxyacetophenone to produce 2′-hydroxychalcone and 4-methoxy-2′-hydroxychalcone respectively. Higher conversion (83%) of benzaldehyde and (89%) of 4-methoxybenzaldehyde reveals that among these heterogeneous catalysts KF/Al2O3/fly ash673 is very active.
Fluoride ion is useful as weakly basic, nonnucleophilic in many organic chemical processes involving hydrogen abstraction or hydrogen bond formation [
The Claisen-Schmidt condensation between acetophenone and benzaldehyde derivatives is a valuable C–C bond-forming reaction which produces
Fly ash (SiO2, Al2O3, Fe2O3, CaO, and MgO) by appropriate activation has been converted into solid acids and solid bases. H2SO4 treated fly ash has been used as Bronsted acid catalyst for synthesis of aspirin and oil of wintergreen as solid support for loading of cerium triflate and sulphated zirconia [
Fly ash (Class F type with SiO2 and Al2O3 > 70%) used as solid support for the solid base was collected from Kota Thermal Power Plant (Rajasthan, India).
The three catalysts KF/fly ash, KF/Al2O3/fly ash and KF/Al2O3 were prepared by using aqueous solution of potassium fluoride for wet impregnation of fly ash, alumina enriched fly ash and
As received fly ash was preheated for 3 h at 900°C under static conditions. 10 g of thermally activated fly ash was added into a glass reactor containing aqueous solution of 0.166 g potassium fluoride (10 wt %). The reactor was equipped with a reflux condenser and a magnetic stirrer bar. The slurry was refluxed at 110°C for 24 h then filtered and washed to eliminate excess KF on the fly ash surface.
An aqueous solution of 0.09 mol of Al(NO3)3·3H2O was added into thermally activated fly ash with constant stirring. Solution of 0.16 mol of (NH4)2CO3 was added dropwise in the above solution and the pH was maintained close to 8.0 by the addition of appropriate amounts of NH4OH (35% aqueous ammonia solution). The resulting gel-like slurry was washed with deionized water until pH = 7. Then the precipitate was dried at 373 K in air for approximately 12 h. The resulting solid was calcined at 673 K for 4 h under static conditions.
The chemically activated fly ash was heated at 400°C for 4 h prior to its introduction in a glass reactor containing aqueous solution of 0.166 g potassium fluoride (10 wt%). The reactor was equipped with a reflux condenser and a magnetic stirrer bar. The slurry was refluxed at 110°C for 24 h then filtered and washed to eliminate any excess KF on the fly ash surface.
10 g of alumina milled to a fine powder was added into deionized water containing desired amount of KF (0.166 g for 10 wt% loading). Thereafter the resulting solid products of all three catalysts were further dried at 110°C for 24 h and calcined at three different temperatures 473 K, 673 K and 873 K for 2 h.
The samples were characterized by Fourier transform infrared spectroscopy (FTIR), powder X-ray diffraction study (XRD), BET surface area analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The loading of KF on the chemically activated fly ash was confirmed by FTIR study using FTIR spectrophotometer (IRPrestige-21, Shimadzu) having a Diffuse Reflectance Scanning technique by mixing the sample with dried KBr (in 1/20 wt ratio) in the range of 400–4000 cm−1 with a resolution of 4 cm−1. X-ray diffraction studies were carried out by using X-ray diffractometer (Philips X’pert) with monochromatic CuK
Catalytic activity of prepared catalysts was tested by Claisen-Schmidt condensation of benzaldehyde and 4-methoxybenzaldehyde with 2′-hydroxyacetophenone in solvent free liquid phase reaction shown in Scheme
Claisen-Schmidt condensation of (a) benzaldehyde and (b) 4-methoxybenzaldehyde with 2-hydroxyacetophenone over fly ash supported solid base catalysts.
The condensation was performed in liquid phase batch reactor, which consists of 50 mL round bottom flask, magnetic stirrer and condenser. A mixture of benzaldehyde (0.106 g, 1 mmol) or 4-methoxybenzaldehyde (0.136 g, 1 mmol) and hydroxyacetophenone (0.136 g, 1 mmol) was taken in round bottom flask. The catalysts, activated at different temperatures for 2 h (substrate to catalyst ratio = 10), were added in the reaction mixture. At the end of the reaction, the catalyst was filtered and the reaction mixture was analyzed by Gas Chromatography (Dani Master GC) having a flame ionization detector and HP-5 capillary column of 30 m length and 0.25 mm diameter, programmed oven temperature of 50–280°C and N2 (1.5 mL/min) as a carrier gas.
The used catalyst (KF/Al2O3/fly ash673) was washed with acetone and dried in oven at 110°C for 12 h followed by activation at 450°C for 2 h before reuse in next reaction cycle under similar reaction conditions as earlier.
Elemental analysis of fly ash and all three catalysts (KF/fly ash, KF/Al2O3/fly ash and KF/Al2O3) was done by EDX, the results are summarized in Table
EDX analysis of prepared catalysts at different pre-treatment temperatures.
Catalyst | Pre-treatment |
K (at.%) | F (at.%) | Al (at.%) | O (at.%) |
---|---|---|---|---|---|
Fly ash | NIL | 0.09 | NIL | 4.10 | 50.2 |
Al2O3/fly ash | 673 K | 0.09 | NIL | 9.2 | 57.6 |
KF/fly ash | 473 K | 2.3 | 1.7 | 4.7 | 56.7 |
673 K | 2.7 | 1.8 | 4.8 | 56.9 | |
873 K | 2.2 | 1.3 | 4.8 | 56.8 | |
KF/Al2O3/fly ash | 473 K | 8.5 | 11.3 | 9.2 | 52.4 |
673 K | 9.1 | 12.1 | 9.4 | 51.9 | |
873 K | 8.1 | 11.7 | 9.3 | 52.0 | |
KF/Al2O3 | 473 K | 20.8 | 19.6 | 12.7 | 46.9 |
673 K | 22.1 | 20.8 | 12.9 | 44.2 | |
873 K | 18.8 | 17.7 | 17.2 | 46.3 |
[at.%: atomic percentage].
The FTIR spectrum of fly ash Figure
FTIR of (a) raw fly ash, (b) thermally activated fly ash, (c) KF/fly ash catalyst, (d) KF/Al2O3/fly ash873 catalyst, (e) KF/Al2O3/fly ash673 vatalyst, (f) KF/Al2O3/fly ash473 catalyst, and (g) regenerated KF/Al2O3/fly ash673 catalyst.
X-ray diffraction pattern of raw fly ash is shown in Figure
X-ray diffraction pattern of (a) raw fly ash (b) KF/Al2O3/fly ash473 catalyst (c) KF/Al2O3/fly ash873 and (d) KF/Al2O3/fly ash673 catalyst.
SEM micrograph of raw fly ash Figure
SEM micrograph of (a) Raw fly ash and (b) KF/Al2O3/fly ash673 catalyst.
TEM image of (a) Raw fly ash and (b) KF/Al2O3/fly ash673 catalyst.
There are several controversies on possibilities of formation of basic sites on KF/Al2O3 catalytic system, yet all the previous investigations [
In KF/Al2O3/fly ash catalyst, the formation of K3AlF6 is confirmed by XRD, while hydroxyl groups and carbonate ion are confirmed by intense peaks at 3500 cm−1and 1550 cm−1 in FTIR spectrum Figure
Conversion (%) of benzaldehyde and 4-methoxybenzaldehyde with KF/Al2O3/fly ash catalyst at (a) different pretreatment temperature (b) different reaction temperature (c) different time (d) different reaction cycles and (e) different molar ratios of benzaldehyde and 4-methoxybenzaldehyde.
The catalytic activity of KF/Al2O3/fly ash depends very much on drying condition of alumina after loading KF by impregnation. In this work we examined the dependence of catalytic activity for condensation on the heating temperature of the catalyst.
The catalytic activity strongly depends on the evacuation temperature and reached around a maximum temperature of 673 K Figure
In the light of the above inferences, the reaction conditions such as reaction temperature, reaction time, molar ratio of reactants and substrate to catalyst ratio for condensation of benzaldehyde and 4-methoxybenzaldehyde with 2′-hydroxyacetophenone were optimized using only KF/Al2O3/fly ash673 catalyst.
The reaction for condensation of benzaldehyde and 4-methoxybenzaldehyde is carried out at temperatures ranging from 40°C to 140°C for 4 h. The effect of temperature on the condensation activity of KF/Al2O3/fly ash673 and kinetics is shown in Figure
The optimum reaction time required, achieving maximum conversion of benzaldehyde and 4-methoxybenzaldehyde is carried out at 40 and 140°C for different time intervals from 30 min to 8 h. The conversion of benzaldehyde and 4-methoxybenzaldehyde gradually increases with time giving 83% and 89% after 4 h respectively as given in Figure
The effect of the molar ratio of benzaldehyde to 2′-hydroxyacetophenone and 4-methoxybenzaldehyde to 2′-hydroxyacetophenone on conversion and yield of products is also studied at a reaction temperature of 120°C and 140°C and reaction time 4 h. At the molar ratio of 1 : 1, maximum conversion of benzaldehyde and 4-methoxybenzaldehyde with yield of their respective products was obtained (Figure
The reaction between 2′-hydroxyacetophenone and benzaldehyde has been carried out over different catalysts as reported in the literature. The results obtained from the reported catalysts showed that when zeolite NaX and a sepiolite partially exchanged with Cs were used as catalysts, their activity was very low [
In order to find the optimum for the studied reactions, we have synthesized highly basic KF/Al2O3/fly ash catalyst and it is found that with this catalyst the conversion value (83%) of benzaldehyde and (89%) of 4-methoxybenzaldehyde was increased. The prepared catalyst is also reused for up to three reaction cycles. The regenerated catalyst showed similar catalytic activity till 3rd reaction cycle giving approximately similar conversion of benzaldehyde (81%) and 4-methoxybenzaldehyde (86%), which indicates that the sites are not deactivated in the regenerated catalyst as confirmed by FTIR of regenerated catalyst (Figure
NMR of 2′-hydroxychalcone: H1-NMR (200 MHz, CDCl3),
The study provides KF/Al2O3/fly ash as an efficient solid base catalyst possessing significant amount of basicity. The chemical activation of fly ash by alumina results in increased amorphous alumina content and thus surface hydroxyl contents produced due to the reaction of KF and alumina on fly ash support. Among all catalysts, KF/Al2O3/fly ash catalyst shows high catalytic activity towards condensation reaction. The experimental results indicate that the basicity of supported KF can be significantly increased by a proper choice of support. In KF/Al2O3/fly ash catalysts, KF reacts with alumina of fly ash and forms potassium hexafluoroaluminate and hydroxide, as the major phase which increases with increase in calcination temperature from 473 K to 673 K, responsible for enhancement of reactivity of catalyst. Along with these species, bidentate carbonate species exist on the surface of KF/Al2O3/fly ash673 catalyst which is formed by the formation of K2CO3. The KF/Al2O3/fly ash catalysts were found active for condensation of benzaldehyde and 4-methoxybenzaldehyde with 2′-hydroxyacetophenone, in which KF/Al2O3/fly ash pretreated at 673 K gave higher conversion of benzaldehyde (87%) with 91% yield of 2′- hydroxychalcone and 4-methoxybenzaldehyde (93%) with 93% yield of 4-methoxy-2′-hydroxychalcone. Experimental results also indicate that the basicity generated over only KF/fly ash catalyst was not sufficient to get higher yield of condensation products, but when fly ash was chemically activated by loading alumina through precipitation using aluminium nitrate as precursor, KF/Al2O3/fly ash catalyst has shown increased catalytic activity for the same reaction. The conversion (83% and 89%) of benzaldehyde and 4-methoxybenzaldehyde in KF/Al2O3/fly ash catalytic system was comparable to the only KF/Al2O3 system (conversion 61% and 59%) however, the cost of the KF/Al2O3/fly ash is lower than the KF/Al2O3, due to the replacement of
Recycling experiments showed that the catalyst is very stable up to three cycles. A composite material which can combine the advantages of fly ash, alumina and KF can expand the catalytic capabilities of the material especially in applications as strong base catalysts for industrially important reactions.
The prepared KF/Al2O3/fly ash catalyst is novel and cost effective. The cost of the KF/Al2O3/fly ash is lower than the commercially available KF/Al2O3, due to the replacement
XRD and TGA support was provided by Dharmsinh Desai University, Nadiad, Gujarat and TEM was performed at UGC-DAE Consortium for Scientific Research, Indore. Fly ash Mission, Department of Science and Technology, New Delhi, India provided the financial support through research Project no. FAU/DST/600(23)/2009-10 sanctioned to the corresponding author.