Cordierite honeycomb monoliths coated with Al(III)/ZrO₂ as an efficient and reusable catalyst for the Knoevenagel condensation: A faster kinetics

2.2.1 Preparation of catalysts in powder form (PFM)

5% Al(III)/ZrO₂ (5Z-A), 10% Al(III)/ZrO₂ (10Z-A), 15%Al(III)/ZrO₂ (15Z-A), 20% Al(III)/ZrO₂ (20Z-A) and 25% Al(III)/ZrO₂ (25Z-A) were prepared by impregnation method. In a typical method, 5Z-A (PFM) was prepared by using 11.40 g of zirconyl nitrate and 3.476 g of aluminum nitrate in 40 mL of distilled water and made into a paste. The obtained paste was dried at 120 °C in a hot air oven for 12 h and the obtained solid was finely powdered and calcined for 450 °C for 4 h (Pratap et al., 2015a, 2015b).

2.2.2 Wash coating of CHMs

The CHMs were wash-coated with zirconyl nitrate before coating the active catalyst (Al(III)/ZrO₂). Typically, in the process of wash-coating, 2 g of zirconyl nitrate ZrO(NO₃)₂ was dissolved in 40 mL of de-ionized water. The obtained solution was coated on a bare CHM through ‘dip & dry’ technique in a furnace pre-heated to 400 °C for 8–10 times till ∼0.05 g ZrO₂ was coated.

2.2.3 Coating of active catalysts (5–25%) on wash coated CHMs

In a classic method, for coating 5Z-A (CHM), a dilute solution of 11.40 g of zirconyl nitrate and 3.476 g of aluminum nitrate was made using 40 mL of distilled water. The solution obtained was coated on wash-coated CHM through ‘dip & dry’ technique in a furnace pre-heated to 400 °C for 15 times till ∼0.2 g Z-A was coated finally, CHMs were calcined at 450 °C for 4 h. Compressed air was blown gently through dip coated CHMs to ensure that there were no blockages in the channel. A similar procedure was used to prepare all other solid acid catalysts.

Powder form of 5% Al₂O₃/ZrO₂ abbreviated as 5Z-A (PFM), 10% Al₂O₃/ZrO₂ as 10Z-A (PFM), 15% Al₂O₃/ZrO₂ as 15Z-A (PFM), 20% Al₂O₃/ZrO₂ as 20Z-A (PFM) and 25% Al₂O₃/ZrO₂ as 25Z-A (PFM). CHM coated with 5% Al₂O₃/ZrO₂ abbreviated as 5Z-A (CHM), 10% Al₂O₃/ZrO₂ as 10Z-A (CHM), 15% Al₂O₃/ZrO₂ as 15Z-A (CHM), 20% Al₂O₃/ZrO₂ as 20Z-A (CHM) and 25% Al₂O₃/ZrO₂ as 25Z-A (CHM) respectively. Typical images of CHM catalysts with different (wt %) coatings [Bare CHM, 5Z-A (CHM), 10Z-A (CHM), 15Z-A (CHM), 20Z-A (CHM) and 25Z-A (CHM)] presented in Fig. 1 for the comparison. By observing the figure one can perceptibly discriminate the change in the color contrast between bare CHM as well as different (wt %) coatings of Z-A (CHM) catalysts. Bare CHM is colorless, but as the (wt %) coatings on CHM increases, there is a trivial change in the color contrast which indicates that the surface of the CHMs was entirely coated with the Z-A catalysts which were perceptibly confirmed by both PXRD as well as SEM studies correspondingly.

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Fig. 1 Images of typical CHMs with different (wt %) coatings. [a] Side View: – Bare CHM, 5Z-A (CHM), 10Z-A (CHM), 15Z-A (CHM), 20Z-A (CHM) and 25Z-A (CHM), [b] Top View: – Bare CHM, 5Z-A (CHM), 10Z-A (CHM), 15Z-A (CHM), 20Z-A (CHM) and 25Z-A (CHM), [c] Lateral top View: – Bare CHM, 5Z-A (CHM), 10Z-A (CHM), 15Z-A (CHM), 20Z-A (CHM) and 25Z-A (CHM), [d] Front top View: – Bare CHM, 5Z-A (CHM), 10Z-A (CHM), 15Z-A (CHM), 20Z-A (CHM) and 25Z-A (CHM), [e] Transversal View: – Bare CHM and 15Z-A (CHM), [f], [g], [h] and [i] Cross-sectional View: – 15Z-A (CHM) with different monolithic channels after cutting the monoliths into pieces.

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2.5.1 Conventional method of condensation (CMC)

In a typical condensation reaction, catalytic evaluation of modified forms of zirconia was conducted according to the method described by Pratap et al. (2015a, 2015b). Using a specially designed glass reactor or 30 mL round bottom flask fitted with a hot plate containing stirrer with constant stirring. The total volume of the reaction mixture was always kept at 20 mL. After a fixed reaction time, the reaction mixture was cooled and filtered. The temperature and pressure fluctuation was reported in the range of ±1 °C respectively.

2.5.2 Ultrasonic-assisted condensation (UAC)

Ultrasonic Horns (Labmatrix manufacturing LLP, Digital Sonifier) operating at a frequency of 20 kHz and a maximum rated power output of 400 W was used to perform the condensation reaction. The power amplitude was adjusted from 10% to 50% (40 W–200 W). The ultrasonic horn was inserted into the reaction mixture to provide sonication. The temperature and pressure fluctuation was reported in the range of ±1 °C respectively.

2.5.3 Microwave-assisted condensation (MAC)

Catalytic assessment of modified forms of zirconia was studied under microwave energy by using “Scientific Microwave Synthesis System” microwave lab station for synthesis, Labmatrix manufacturing LLP, Bangalore. The apparatus facilitates the control of reaction mixture temperature with the assistance of infrared sensor through regulation of microwave power output so that the reaction mixture was accurately lined up with infrared sensors that manage the temperature. Variable (Output) power up to 700 W with a frequency of 2450 MHz allowed to apply by microprocessor controlled single magnetron system (Venkatesha et al., 2015). The fluctuation of temperature and pressure were reported in the range of ±1 °C respectively.

3.1.1 Total acidity (TA) by the NH₃-TPD method/ n-butylamine back titration method

Total acidity (TA) and acid site distribution values of all the catalysts measured by the NH₃-TPD technique is given in Table 1. Total acidity of the catalysts was found to follow the order: 5Z-A (PFM) < 5Z-A (CHM) < 10Z-A (PFM) < 10Z-A (CHM) < 25Z-A (PFM) < 25Z-A (CHM) < 20Z-A (PFM) < 20Z-A (CHM) < 15Z-A (PFM) < 15Z-A (CHM). TA values of all the catalysts were also determined by n-butylamine back titration method and their values are in good agreement with the values obtained from the NH₃-TPD technique.

The total acidity values of (PFM) and (CHM) catalysts are given in Table 1. Pure ZrO₂ was found to be less acidic, but the total acidity of Z-A was found to increase when Al³⁺ was inserted into Zr⁴⁺ Lattice, imbalance of positive charge takes place which results in the generation of Lewis acid sites. To maintain the electrical neutrality Brønsted acidity is likely to come out as the charge imbalance is negative. Lewis acid sites may be produced, when the imbalance of charge is positive (Maria et al., 2009). 15Z-A (CHM) and 15Z-A (PFM) possessed higher total acidity when compared to all other catalytic materials.

It is noticed that in the condensation reaction the activity of the catalysts increased with an increase in the Al(III) loadings up to 15 wt (%). But the addition of Al(III) beyond 15 wt (%) decreases the activity of the catalyst which may be due to the decrease in TA which in turn may be due to the excess coverage of Al(III) species over the CHM support (Chary et al., 2004).

3.1.2 Crystallinity by PXRD

In the XRD patterns of (CHM) coated catalysts, no diffraction peaks correspond to bare CHM was detected (Fig. 2). This evidently indicates that the surface of the bare CHM was entirely and efficiently covered with the active catalyst. Pure zirconia consists of monoclinic (M) and tetragonal (T) phases. When pure zirconia was incorporated with 5–10% of Al(III) gradual reduction in the intensity of peak pertaining to monoclinic phase with an augment in the intensity of the tetragonal phase of the zirconia was noticed. This implies a predominant dependence of the tetragonal phase on Al(III) ion loadings. Fascinatingly, for 15% Z-A, peaks pertaining to monoclinic phases were completely eliminated, whereas the peaks pertaining to only tetragonal phase were observed. This may be owing to the strong interaction of Al³⁺ ions with zirconia which inhibits the formation of monoclinic phase, which further may stabilize into a metastable tetragonal phase of zirconia (Chary et al., 2004; Thimmaraju et al., 2016). It is reported that the tetragonal phase of the zirconia catalysts is more catalytically active than its monoclinic phase (Reddy and Sreekanth, 2002). Concurrently, PXRD peaks pertaining to either Al₂O₃ or any other new crystalline phases were not found.

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Fig. 2 PXRD patterns of: [a] ZrO₂, [b] 5Z-A, [c] 10Z-A, [d] 15Z-A, [e] 20Z-A, [f] 25Z-A and [g] Bare CHM. (where M indicates monoclinic and T indicates the tetragonal phase).

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The effect of calcination temperature of 15Z-A (CHM) catalyst on the PXRD pattern was studied by varying the temperature from 250 to 650 °C [Fig. 3]. The PXRD pattern of 15Z-A (CHM) calcined at 250 and 350 °C consists of both monoclinic as well as tetragonal phases. But, at 450 °C monoclinic peaks were completely eliminated and only peaks pertaining to tetragonal phases were observed. From these observations, it could be inferred that the complete phase modification from monoclinic to the catalytically active tetragonal phase occurred. The absence of characteristic peaks of Al₂O₃ implies that metal oxide is highly dispersed in ZrO₂. The dispersion of Al₂O₃ particles which may also impact the higher internal surface area of the CHMs (Haridas et al., 2003). But when the calcination temperature was increased beyond 550 °C, the formation of the stable monoclinic phase from the tetragonal phase was noticed. This clearly indicates the calcination temperature of 450 °C was found to be suitable for the present work.

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Fig. 3 The influence of PXRD patterns of 15Z-A (CHM) catalyst on the effect of calcination temperature over the stability of the tetragonal phase of zirconia: [a] Un-calcined, [b] 250 °C, [c] 350 °C, [d] 450 °C, [e] 550 °C and [f] 650 °C.

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The fraction of the tetragonal phase (f_T) of various Z-A (CHM) catalysts as well as in the case of 15Z-A (CHM) calcined at different temperatures estimated by the following formula suggested by Valgi (Valgi et al., 2002).

It is noticed that as the calcination temperature increases from 250 to 450 °C the value of (f_T) also found to increase, and a further increase of the calcination temperature from 450 to 650 °C resulted in the decrease of f_T value.

The phase composition in the case of the calcined 15Z-A (CHM) catalyst was examined by means of PXRD studies. From the Fig. 3, one can perceptibly distinguish the absence of crystalline phases for calcination temperature <350 °C, signifying the presence of amorphous alumina or zirconia or both <1 nm. Conversely, calcination temperature at 450 °C resulted in characteristic tetragonal reflections pertaining to the 2θ values at 30.5, 35.2, 50.3 and 60° respectively (Osatiashtiani et al., 2014). Table 2 signifies the presence of the tetragonal phase of zirconia crystallites up to ≤550 °C with the volume average crystallite sizes from 18 to 4 nm. Meanwhile, the growth of monoclinic phase of zirconia from 5 to 60 nm observed between the calcination temperatures from 350 to 650 °C (Charaska et al., 2000). This clearly specifies that low calcination temperature favors tetragonal phase, whereas, higher one favors monoclinic phase of zirconia due to crystalline sintering. Higher calcination temperature also results in an increase in the acid site density from 2.8 to 5 which also may be attributable to the vacancy formation through weak acid sites. Such vacancies were vital to impart Lewis acid sites which may play a key role in the Knoevenagel condensation reaction (Wang and Iglesia, 2017).

3.1.3 SEM analysis

SEM images of CHM catalysts are given in Fig. 4. In the case of 15Z-A (CHM), flake-like appearances with larger flower-shaped particles could be noticed, which indicates uniform and strong adherence of active catalytic material on the surface of (CHM). This also specifies that the technique employed to coat the active catalyst on (CHM) is highly efficient. Moreover, at lower loadings of Al₂O₃ the average crystallite sizes of Al₂O₃ are very small but at higher loadings, a slight increase in the size of Al₂O₃ particles was noticed. A similar observation has been reported by Harisekhar et al. (2015).

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Fig. 4 SEM images of: [a] Zirconia, [b] Alumina, [c] Bare CHM, [d] 5Z-A (CHM), [e] 10Z-A (CHM) [f] 15Z-A (CHM), [g] 20Z-A (CHM) and [h] 25Z-A (CHM).

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SEM images of 15Z-A catalysts at various calcination temperatures are represented in Fig. 5. The shape of the catalyst was found to be flat with inflection when the calcination temperature was 250 °C. When the calcination temperature was increased to 350 °C, bean-shaped particles with burr were noticed. Furthermore, when the calcination temperature was 450 °C, flake-like appearances with larger flower-shaped particles could be noticed. More interestingly, when the calcination temperature was increased beyond 450 °C (i.e., 550 °C) well separated and spherical particles were noticed (Lu et al., 2017). High-temperature results in severe aggregation, growth as well as the migration of Z-A particles attributable to high surface energy, the interaction between catalyst particles and cordierite substrates. Therefore 450 °C was found to be an optimum calcination temperature.

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Fig. 5 The influence of calcination temperature on the SEM images of 15Z-A (CHM) catalyst: [a] Uncalcined, [b] 250 °C, [c] 350 °C, [d] 450 °C, [e] 550 °C, and [f] 650 °C.

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3.1.4 TEM analysis

TEM images of 15Z-A (CHM) are given in Fig. 6. It could be observed from the figure that a uniform as well as the compact distribution of particles with the spherical geometry of particles in the size ranging from 60 to 80 nm.

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Fig. 6 TEM images of 15Z-A (CHM).

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3.1.5 ICP-OES analysis

The amount of metal content in the case of Z-A (PFM) samples, was determined by ICP-OES analysis. The amount of aluminum content exchanged over modified zirconia was discussed in Table 1.

3.2.1 Effect of the nature of catalysts on the yield (%) of ECC

In order to find an efficient and the facile catalytic system for the Knoevenagel condensation of benzaldehyde (BD) with ethyl cyanoacetate (ECA), the reactions were carried out over all the catalysts (PFM & CHM) prepared for the present work and the results are given in Table 1.

The (%) yield of ethyl-α-cyanocinnamate (ECC) was found to follow the order: 5Z-A (PFM) < 5Z-A (CHM) < 10Z-A (PFM) < 10Z-A (CHM) < 25Z-A (PFM) < 25Z-A (CHM) < 20Z-A (PFM) < 20Z-A (CHM) < 15Z-A (PFM) < 15Z-A (CHM).

When the reaction was conducted without any catalyst, there was a negligible formation of product (< 2%). This clearly infers that condensation (BD with ECA) is a catalyzed reaction.

3.2.2 The influence of calcination temperature over catalytic activity of 15Z-A (CHM)

A correlation between the f_T value and the yield (%) ECC was observed, i.e., as the calcination temperature increases from 250 to 450 °C the value of (f_T) as well as the yield of ECC increases. Further increase in calcination temperature from 450 to 650 °C results in a decrease in the value of (f_T) and the yield of ECC (%) also found to get decreased. But at the highest calcination temperature (i.e., about 650 °C), the value of (f_T) and catalytic activity was negligible because the phase stability of the catalyst was not satisfactory.

3.2.3 Effect of wash-coating of CHMs on the yield (%) of ECC

The (%) yield of ethyl-α-cyanocinnamate (ECC) in case of CHM catalysts with as well as with-out wash coating was found to follow the order: 5Z-A (CHM) < 10Z-A (CHM) < 25Z-A (CHM) < 20Z-A (CHM) < 15Z-A (CHM).

The above results pertaining to the CHM coated catalysts on the effect of the wash-coating process are comparable with the results of total acidity as well as the PXRD pattern. Interestingly, wash-coated CHMs show the highest yield when compared to without wash-coat CHMs (Fig. 7). An almost twofold increase in the (%) yield of ECC was noticed in the case of wash-coated CHM. This can be because of the fact that, upon a wash-coating process, the internal surface area of CHM increases by completely coating the inner surface of the monolithic material (Arai and Machida, 1991; Patil et al., 2008).

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Fig. 7 Effect of wash coating process on CHM catalysts with different Z-A coatings (wt%) under different modes of heating. [Reaction conditions (1. CMC): molar ratio (BD:ECA) =1:10, reaction time = 150 min, reaction temperature = 40 °C; (2. UAC): molar ratio (BD:ECA) =1:5, reaction time = 75 min, reaction temperature = 40 °C; (3. MAC): molar ratio (BD:ECA) =1:1, reaction time = 15 min, reaction temperature = 40 °C].

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3.2.4 Effect of mode of heating in the yield (%) of ECC

When the condensation reaction was carried out by using different heating methods, the yield (%) of ECC was found to follow the order:

Conventional heating < Ultrasonic-assisted heating < Microwave assisted heating.

The reason for this superiority of microwave heating compared to different heating methods is discussed briefly under section 3.2.7.

3.2.5 Effect of Al₂O₃ loading

A series of Z-A catalysts (both powder and CHM forms) with variations in Al₂O₃ concentrations ranging from 5 to 25 wt% were discussed in the condensation of BD with ECA and the (%) yield of ECC is represented in Table 1.

A correlation between total acidity and the yield of ECC was noticed, i.e., 15Z-A is highly acidic showed the highest (%) yield of ECC and 5Z-A being the least acidic showed the least (%) yield of ECC. Hence, the 15Z-A catalyst was preferred for further optimization studies.

3.2.6 Effect of reaction temperature

Microwave-assisted Knoevenagel condensation (BD with ECA) reaction over 15Z-A pertaining to both (PFM) and (CHM) catalysts was conducted by varying reaction temperature from 20 to 60 °C. The results are given in Fig. 8. It was found that the yield (%) of ECC increases correspondingly as the reaction temperature increases from 20 to 40 °C. Up to 40 °C, the yield of ECC increases as the temperature increases too, suggesting that the rate of the reaction is directly proportional to the temperature. An increase in the temperature beyond 40 °C decreases the (%) yield of ECC which may be attributable to the formation of more condensation by-products. Consequently, 40 °C was preferred as an optimal reaction temperature for the further studies.

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Fig. 8 Effect of reaction temperature on the yield of ECC. [Reaction conditions (MAC): molar ratio (BD:ECA) =1:1, reaction time = 15 min].

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3.2.7 Effect of molar ratio

The results pertaining to the effect of the molar ratio of the Knoevenagel condensation (BD with ECA) reaction over 15Z-A [both (PFM) and (CHM)] are given in Fig. 9. When the molar ratio (BD: ECA = 1:1) the (%) yield of ECC was found to be 75 and 95 in case of PFM and CHM catalysts respectively. However, as the molar ratio increases (BD: ECA = 1:2) the yield (%) of ECC found to decrease signifying that no self-condensation of ECA occurred even if its concentration doubled in the reactant mixture. Additionally, as the molar ratio was reversed, i.e., (BD: ECA = 2:1), the yield of ECC was found to decrease, which may be as a result of the suppression of condensation products due to the decrease in the concentration of ECA.

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Fig. 9 Effect of molar ratio on the yield of ECC. [Reaction conditions (MAC): reaction time = 15 min, reaction temperature = 40 °C].

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3.2.8 Effect of reaction time

Effect of reaction time for the Knoevenagel condensation (BD with ECA) reaction over 15Z-A pertaining to the microwave-assisted heating mode of both (PFM) and (CHM) catalysts was studied by varying the reaction time from 5 to 25 min. The results attained are given in Fig. 10. It was found that the yield of ECC increased when the reaction time was increased gradually from 5 to 15 min. The maximum yield of ECC was observed at 15 min. Beyond 15 min the yield of ECC gets stabilized. As a result, 15 min was preferred as an optimal reaction time for further studies.

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Fig. 10 Effect of reaction time on the yield of ECC. [Reaction conditions: molar ratio (BD:ECA) =1:1, reaction temperature = 40 °C].

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3.2.9 Comparison between MAC, CMC and UAC catalyzed by 15Z-A (CHM)

The effects of variation of the reaction conditions during conventional, ultrasonic and microwave modes of heating on the Knoevenagel condensation reaction are compared in Table 3. As can be seen from the Table 3, the different heating modes with their (%) yield were found to follow the order: Conventional heating < Ultrasonic-assisted heating < Microwave assisted heating.

In case of conventional heating, the reaction proceeded extremely slowly and the (%) yield of ECC = 75 because heat is imparted to the reactants by transferring first all the way through the wall of the vessel by making it an inefficient as well as a slow method for the transportation of energy into the reacting system. Ultrasonic-assisted heating was found to be much better compared to conventional heating with the (%) yield of ECC = 82. This may be due to the fact that ultrasonic irradiation can significantly increase the interface mixing and thus boost the rate of the reaction (Ramachandran et al., 2013). Likewise, in this process, with the support of cavitation phenomenon, the chemical reaction may get accelerated (Samani et al., 2016). But, microwave assisted heating is found to be more superior compared to conventional and ultrasonic heating with the highest yield ECC. In the present study, improvement in the rate of product formation under microwave energy may be attributable to relatively high tan d values of the reactants. However, more interestingly in the absence of the catalyst, even though the reactants are polar, microwave energy fails to initiate the condensation reaction. Further, the polar acid sites on the catalyst also play a vital role. Furthermore, MAC catalyzed by honeycomb monolith i.e., 15Z-A (CHM) demonstrates that the application of microwave energy is easy and the expressway with better reaction rates, reduced reaction times in initiating and attaining the reaction towards equilibrium at a faster rate as compared to thermal radiation (Gedye et al., 1986; Gawande et al., 2014; Giguere et al., 1986; Michael et al., 1991; Lidstrom et al., 2001; Arvela et al., 2005; Galema, 1997; Leadbeater and Marco, 2003; Perreux and Loupy, 2001; Kappe, 2004; De La Hoz et al., 2005; Kappe and Dallinger, 2009).

3.3.1 Effect of poisoning of acid sites of Z-A by adsorption of pyridine on its catalytic activity

In the present work, we have made an effort to elucidate the relative role of the acid surface sites in the Knoevenagel condensation reaction. All the catalytic materials (both PFM and CHM) were poisoned by exposing them to pyridine vapors at 100 °C. Subsequently the catalysts were flushed with N₂ and the temperature was increased to 300 °C to desorb all the pyridine (Aroux et al., 2008). PXRD patterns of CHM catalysts (poisoned by pyridine) are given in Fig. 12. It is fascinating to notice that the PXRD pattern of 15Z-A (CHM) poisoned by pyridine posses more monolithic phase (Fig. 12) but in the case of un-poisoned 15Z-A (CHM), the only tetragonal phase was noticed (Fig. 2). This evidently concludes that poisoning enhances monoclinic phase resulting in a decrease in tetragonal phase of zirconia. Further, the poisoned catalysts were subjected to Knoevenagel condensation reaction under the same set of reaction conditions. It was observed that in case of poisoned catalysts, the (%) yield of ECC was decreased approximately 3 times as compared to that of unpoisoned one since pyridine interacts more with acid sites by neutralizing more number of acid sites present in the catalyst. In addition, when total acidity (TA) was measured in the case of 15Z-A (CHM) catalyst, a decrease in the TA values was noticed which may be owing to loss of acidic sites. This study infers that more tetragonal phase of zirconia was responsible for more TA and conversely more monoclinic phase was accountable for lesser TA. In general, the presence of the tetragonal phase of zirconia is predominantly liable for its high TA. This infers that in the case of Z-A (CHM) catalysts, total acidity (TA), PXRD phases and their catalytic activity were found to be co-relatable. Consequently, a triangular correlation exists between total acidity (TA), PXRD phases as well as catalytic activity.

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Fig. 12 PXRD patterns of CHMs poisoned by pyridine [a] 5Z-A, [b] 10Z-A, [c] 15Z-A, [d] 20Z-A and [e] 25Z-A.

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