Mesoporous Borated Zirconia: A Solid Acid–Base Bifunctional Catalyst

Abstract

The development and use of reusable solid catalysts for the selective organic transformation in solvent-free or environmentally benign solvent media is the key interest of modern frontier science. Herein a facile low temperature aqueous solution based chemical route for the synthesis of mesoporous borated zirconia, an acid–base bifunctional solid catalyst, using aqueous zirconium ammonium carbonate complex and borax in presence of cetyltrimethylammonium bromide is presented. The material has a very high surface area and acidity with weak basicity. The catalytic activity of the material was investigated for the solvent-free Knoevenagel condensation reaction of benzaldehyde/substituted benzaldehyde and malononitrile/cyano ethylacetate to confirm the acid–base bifunctionality. High yield (>90 %) of the corresponding benzylidene was obtained within 15–30 min at room temperature. The evidence that the high catalytic activity is a result of acid–base bifunctionality of the synthesized borated zirconia material was further supported by performing Claisen–Schmidt condensation of benzaldehyde and acetophenone. The methodology was also extended for targeted synthesis of cinnamyl ethyl ester and coumarin or coumarin ester; and resulted in a good yield.

Introduction

During the last two decades, researchers have focused on sustainable chemistry because of ever increasing environmental pollution. Chemical industries including fine chemicals, pharmaceuticals, bulk chemicals and petrochemicals are some of the most important production segments. At the same time these industries are a significant source of pollution and are also energy intensive. The main aim of sustainable chemistry is to develop a cost effective and environment friendly chemical route through waste minimization through routes such as solvent-free or green solvent utilization, room temperature (RT)/low temperature transformation and use of reusable catalysts. The application of reusable solid catalysts for selective organic transformation in solvent-free media is one of the supreme contributions towards sustainable chemistry.1–4 In lieu of the demand of replacing the liquid homogeneous catalyst with reusable solid catalysts, development of solid multifunctional catalysts such as the acid–base bifunctional catalyst is gaining impetus.

In the last few decades, lots of research has been undertaken to develop potential solid catalysts. Of the developed catalysts, zirconia is one of the most promising and potential solid catalysts and offers properties including acidity, basicity, redox reactivity and an amphoteric character. However, the low surface area of the reported zirconia based catalyst is the main drawback and restricts its use as a solid catalyst. The modification of the zirconia by an anion is an important strategy to overcome the aforementioned shortcoming. Different anion modified zirconia have showed enhanced acidic character, improved surface area, and demonstrate increased catalytic activity.5–34 Borate modified zirconia is the least studied among the anion modified zirconia catalysts, although it seems to have high catalytic activity for various organic transformations.20–29 Madje et al., have reported the trans-esterification of β-ketoesters under solvent free condition by borated zirconia.21 Xu et al. have studied the gas phase Beckmann rearrangement of cyclohexanone oxime.22 Urbano et al. reported the Meerwein–Ponndorf–Verley reduction of α,β-unsaturated carbonyl compounds.23 Osiglio et al. studied the alcohol acetylation with acetic acid using borated zirconia as a catalyst.24 Patil et al.25 used borated zirconia for the benzoylation of anisole. Mashle et al.26 reported selective C-methylation of phenol with methanol over borated zirconia. All the reports on borated zirconia concentrate on its increasing acidity resulting from the presence of borate ions in the zirconia framework.

In continuation of our efforts to develop efficient mesoporous solid catalysts and environmentally friendly synthetic methodology for different organic reactions,30–34 we report the synthesis of mesoporous borated zirconia (B₂O₃-ZrO₂, BZ) using in situ incorporation of borate species into the zirconia framework using cetyltrimethylammonium bromide (CTAB) as the pore directing agent. The zirconium carbonate complex30 was used as a source of zirconia and borax for the borate species. Interestingly, the synthesized borate modified zirconia showed high acidity with weak basic sites. The synthesized BZ catalyst was used as a catalyst in a Knoevenagel condensation reaction and a Claisen–Schmidt condensation reaction. We have also used the developed catalyst for the targeted synthesis of cinnamyl ethyl ester and coumarin/coumarin ester.

Results and Discussion

Catalyst characterization

The characterization data of the synthesized BZ catalysts are summarized in Table 1. From the ICP results, it is evident that the amount of boron in the sample gradually increases with the increased amount of borax in the precursor solution.

Table 1. Textural properties and catalytic activity of the synthesized BZ samples.

Sample^[a]	B [mol %]^[b]	SBET [m^2 g^−1]^[c]	Vtotal [cm^3 g^−1]^[d]	Dave [Å]^[e]
[a] BZ-0, 0.5, 1, 1.5, in which 0, 0.5, 1 &1.5 stands for the molar ratio of borax to zirconium in precursor solution. [b] Amount of B in BZ samples in mol % with respect to Zr determined from the ICP-OES. [c] BET surface area. [d] Single point adsorption total pore volume at P/P0=0.97. [e] Average pore width (4 V Å^−1 by BET).
BZ-0	0	162±1.6	0.15	28
BZ-0.5	3.4	223±2.8	0.12	25
BZ-1	7.7	235±1.6	0.16	23
BZ-1.5	9.5	192±1.3	0.09	20

The N₂ adsorption–desorption analysis was performed for the surface area measurement to understand the porous framework of the synthesized materials. The synthesized BZ catalysts show three adsorption stages (at P/P₀<0.2, 0.2<P/P₀>0.8 and P/P₀>0.8) in the nitrogen adsorption–desorption isotherms (Figure 1 a). The isotherms are similar to those of Type I and Type IV with little hysteresis. The continuous nitrogen adsorption at P/P₀>0.2 suggests a contribution in both Type I and Type IV isotherms obtained from the materials containing pores at the borderline of mesopore and micropore regions. The average pore size distributions obtained from BJH (Barrett–Joyner–Halenda) cylindrical pore approximation of all the samples varies in the range of 2–2.9 nm. Molina et al.35 has termed these as super microporous materials for zirconium based mesoporous materials. Previously, we have also observed a similar pattern of isotherm and pore size distributions for sulfated zirconia.32 The total BET (Brunauer–Emmett–Teller) surface area of the samples gradually increases with an increase in the quantity of boron and reaches a maximum at BZ-1 (S_BET=235±1.6 m² g⁻¹). Further increments of boron (BZ-1.5), display a decrease in surface area (192±1.3 m² g⁻¹). The pore size distributions of the synthesized samples are in a narrow range (Figure 1 b), and with the increased amount of boron, the pore size also decreases. This indicates the formation of a layer of boron oxide in the pore wall.

Figure 1. a) N₂ adsorption–desorption isotherm and b) pore size distribution of BZ-1 calcined at 550 °C. Q. Ads.: quantity of adsorbed gas; PV: pore volume.

X-ray diffraction pattern of all synthesized samples was recorded in two different 2 θ range; small angle (1–7°) and wide angle (10–80°). A single broad peak at around 2 θ=1.9–2.4° in the small angle XRD pattern of all the calcined samples implies the formation of pores in the mesoporous range in the BZ framework (Figure 2 a). The presence of only one broad peak indicates the existence of ordered pores in short range. The wide angle powder XRD patterns of calcined samples (BZ-0.5–1.5) exhibit two broad peaks in 2 θ ranges of 10–40° and 40–70° (Figure 2 b) indicating their amorphous nature. The BZ-0 showed well resolved broad peaks, characteristic of tetragonal phase of zirconia (JCPDS, CAS number 14-0534).The incorporation of the borate ion in the zirconia system suppresses the crystallization and leads to the formation of amorphous material. Matsuhashi et al.20 and Patil et al.25 also reported the amorphous nature of borated zirconia calcined at 500 °C. It is noteworthy that the amorphous nature of the BZ samples convert to a crystalline nature as the calcination temperature increases above 500 °C (Figure S 1, Supporting Information).

Figure 2. a) Low angle and b) high angle XRD patterns for the synthesized samples calcined at 550 °C.

The morphology of the synthesized borated zirconia was investigated by using SEM. The SEM image depicts the presence of spherical particles and the particle size of the synthesized spheres varied in the range of 300–1000 nm (Figure 3 a). TEM micrograph was also recorded to image the porous structure of the synthesized material. The TEM image confirms the presence of worm-like pores in the spherical particles (as obtained from SEM) (Figure 3 b). The pores are highly mono-dispersed with an average pore size of 2–3 nm, supporting the average pore size distribution data obtained from BET analysis.

Figure 3. a) SEM and b) TEM images of BZ-1 calcined at 550 °C.

The FTIR spectra (Figure 4 a) of the synthesized boron containing samples (BZ-0.5, 1, 1.5) depicts a broad band in the range of =1290–1440 cm⁻¹ with a peak at =1379 cm⁻¹_, corresponding to tetrahedral BO₄ ion,22–25 which is absent in the spectrum of pure zirconia, BZ-0. This confirms the formation of the tetrahedral BO₄ species in the synthesized BZ. The absence of a peak at ≈1190 or 885 cm⁻¹ confirms the non-existence of trigonal BO₃. The band intensities corresponding to ZrO₂ decrease because of the existence of BO₄ units. This phenomenon confirms that the BO₄ units are highly intermingled with the zirconia framework, affect the crystallization of zirconia and produce amorphous BZ samples as observed in XRD analysis. Additionally, broad bands at =3600–3000 and 1626 cm⁻¹, assigned to physisorbed and coordinated water, were also observed. In ¹¹B MAS NMR spectra (Figure 4 b) of BZ, the presence of only one sharp peak (responsible for tetrahedral BO₄) instead of quadrupolar “doublet” (responsible for trigonal BO₃) confirms the presence of only tetrahedral BO₄ ions.36

Figure 4. a) FTIR and b) ¹¹B solid MAS NMR spectra of the synthesized samples calcined at 550 °C. B(OCH₃)₃ in chloroform (δ=18.1 ppm) was used as reference for ¹¹B solid MAS NMR.

The NH₃ and CO₂-TPD of the synthesized samples (calcined at 550 °C) were performed to investigate acid–base bifunctional nature of the BZ materials. NH₃-TPD was measured to evaluate the total acidity and the strength of acid sites of the synthesized samples. CO₂-TPD was measured to evaluate the total basicity and the strength of base sites of the synthesized samples. The total acidity and basicity are summarized in Figure 5 and Table S 1, Supporting Information.

Figure 5. Total acidity and basicity of the synthesized materials calcined at 550 °C determined by NH₃ and CO₂-TPD experiment, respectively. Q. Des.: quantity of desorbed gas.

The characteristic NH₃-TPD curves of all the synthesized samples were very similar (Figure 6 a). The total acidity of BZ increases gradually with an increased amount of boron. There is an increase in the weak strength acid sites (Lewis sites) after the incorporation of boron in the zirconia system. The acidity decreases on increasing amounts of borate (BZ-1.5) resulting from a decrease in surface area. The characteristic CO₂-TPD curves of all the borated samples were also very similar but with some differences from pure ZrO₂ (Figure 6 b). In a similar manner to the acidic properties, the total basicity of BZ samples also increases gradually with an increased amount of borate and BZ-1 possesses maximum basicity. The nominal decrease in the basicity observed for BZ-1.5 may be because of a slight decrease in the surface area. The weak basicity in the BZ originates from the bridged oxygen (ZrOZr) of the zirconia cage. Climent et al. suggested37 that the bridged oxygen is weakly basic for ALPO.

Figure 6. a) NH₃ and b) CO₂-TPD patterns of the synthesized materials calcined at 550 °C.

The above results reveal that the total acidity and basicity of the borate modified zirconia is improved compared to pure zirconia (Figures 5and 6 and Table S1 Supporting Information). The acidity increases from 0.94 to 1.55 mmol g⁻¹ and basicity from 0.61 to 0.81 mmol g⁻¹. The surface area of the samples also increases from 162±1.6 to 235±1.6 m² g⁻¹ because of borate modification (vide supra). The acid–base character of zirconia is directly correlated to the surface area. It increases with surface area because of the increment of accessible active sites. However, in the synthesized BZ samples the total acidity per square meter also increases from 0.0058 to 0.0066 mmol m⁻². It confirms that the borate species are responsible for the enhancement of acidity. In the borated zirconia, boron with an empty orbital pulls the electron cloud from the oxygen of ZrO₂. The negative charge generated on boron then diffuses into boron oxide cage by the resonance between the lone pair of oxygen and the empty orbital of boron, which enhances the Lewis acidity.20However, no significant change was observed in basicity per square meter. The basicity of the synthesized BZ samples originates only from the bridged oxygen (ZrOZr) of the zirconia cages. Borate species do not have any contribution to the basicity except for the fact that increased surface area leads to an increase in basicity.

To gather information about the pK_a and pK_b of the synthesis of BZ acid–base bifunctional catalyst, the Hammett indicator experiment was performed with different indicators (pK_a=−3.3 to 13.75, Table S2, Supporting Information). It shows that the acidity of the synthesized BZ-1 lies between pK_a values of −11.8 and −12.7. Unfortunately, no information about the base sites was possible as the mesoporous borated zirconia possess strong acidity and weak basicity. We also tried few common indicators such as methyl red (pK_a=5.1), neutral red (pK_a=6.8), phenolphthalein (pK_a=8.2), nile blue (pK_a=10.1), and tropaeolin O (pK_a=11) that are generally used for the determination of basicity. The indicators all showed the relevant acid color. Although the above results show the presence of acid–base sites on the synthesized catalyst, the best tool to characterize acid–base properties is the catalytic reaction.

Catalytic activity

Knoevenagel condensation reaction between benzaldehyde (10 mmol) and malononitrile (Scheme 1) was studied under solvent-free condition at room temperature (≈27 °C) using the synthesized catalysts to evaluate their catalyst efficiency. Among the synthesized catalysts, (Figure 7 a) BZ-1 gave highest activity (yield: 98 %) and BZ-0 shows the lowest activity (yield: 51 %). Whereas BZ-0.5 and BZ-1.5 give 76 % and 52 % yield, respectively after 30 min. The initial rate of the reaction was calculated (Figure 7 b) for all the catalysts. The maximum initial rate of reaction is 25.2×10⁻⁵ mol g⁻¹ s⁻¹ obtained with the BZ-1 catalyst. Thus, the catalytic activity is improved on borate modified zirconia relative to zirconia. The high catalytic activity of BZ-1 is a reflection of its high surface area, acid strength and basicity. Further catalytic study was performed with the BZ-1 catalyst to understand other features of the Knoevenagel condensation reaction over BZ.

Scheme 1. Knoevenagel condensation reaction of benzaldehyde and malononitrile over BZ.

Figure 7. a) Knoevenagel condensation reaction of benzaldehyde and malononitrile over different synthesized catalysts at RT and b) initial reaction rate (10 min) for different catalysts.

The solvent effect was studied using different polar and non-polar solvents (Table 2). Among the used solvents, ethanol gives a better yield (91 %), probably because of the stabilization of a polar intermediate of Knoevenagel condensation reaction by ethanol. However, the reaction in solvent-free condition ended up with higher yield (98 %) in a short reaction time (30 min). To optimize the minimum amount of catalyst required for the maximum yield, the reaction was performed with varying catalyst amounts (2.5–7.5 wt. % with respect to the benzaldehyde) at room temperature using BZ-1 (Figure S2 Supporting Information). It was observed that 5 wt. % catalyst loading was the optimum for maximum yield (98 %) for the performed reaction.

Table 2. Catalytic activity of the BZ-1 for the Knoevenagel reaction in different solvents.^[a]

Solvent	t [h]	Yield [%]
[a] Reaction conditions: benzaldehyde (10 mmol), malononitrile (12 mmol), 5 wt. % of catalyst with respect to aldehyde, RT.
ethanol	0.5	91
acetonitrile	10	75
dichloromethane	10	30
toluene	10	32
benzene	10	38
DMF	10	56
without solvent	0.3	98

As the synthesized catalyst (BZ-1) was highly active for malononitrile, the study was extended for reaction of benzaldehyde with another active methylene compound, ethyl cyanoacetate (Scheme 2). The BZ-1 catalyst was highly effective for ethyl cyanoacetate and ended up with 99 % yield within 15 min reaction time. The good yield was obtained with malononitrile and ethyl cyanoacetate in a short reaction time because of the presence of the electron withdrawing group, which assists the easy release of protons from active methylene groups and stabilizes the negative charge through resonance (enolate formation). The effect of the substitution in the benzaldehyde was studied with both malononitrile and ethyl cyanoacetate in the optimized reaction conditions (Table 3). All the substituted benzaldehyde with both electron withdrawing as well as donating group resulted in remarkably good yields for both malononitrile and ethyl cyanoacetate. A single geometric isomer as shown in Scheme 2 was predominantly obtained in each substituted benzaldehyde studied; although a detailed study to determine the selectivity was not performed.

Scheme 2. Knoevenagel condensation reaction of benzaldehyde and ethyl cyanoacetate over BZ.

Table 3. Results of the solvent-free Knoevenagel condensation reaction of substitute benzaldehyde and malononitrile or ethyl cyanoacetate over BZ-1.^[a]

Entry	Substitution on	Malononitrile		Ethyl cyanoacetate
	benzaldehyde	t [h]	Yield [%]	t [h]	Yield [%]
[a] Reaction conditions: substitute benzaldehyde (10 mmol), malononitrile or ethyl cyanoacetate (12 mmol), 5 wt. % of catalyst with respect to aldehyde, RT.
1	H	30	98	15	99
2	p-Cl	10	96	15	98
3	o-Cl	20	92	20	93
4	p-NO2	15	95	15	94
5	o-NO2	15	91	15	92
6	p-Me	15	96	15	97
7	o-Me	25	92	20	92
8	p-OH	20	98	15	96
9	o-OH	30	89	30	91
10	p-OMe	45	91	40	90

According to the literature,37, 38 considering only the enhanced acidic properties of the BZ-1 catalyst, it is very difficult to explain the high catalytic activity for the Knoevenagel condensation reaction. The superior activity of the prepared catalyst can be explained by considering the acid–base bifunctional behavior of the catalyst. The Lewis acid sites interact with the carbonyl group of benzaldehyde, increases the positive charge on carbonyl carbon, and facilitates the nucleophilic attack. At the same time, the Lewis acid sites increase the positive charge on CO/C≡N carbon of ethyl cyanoacetate and malononitrile and increase the acidity of the hydrogen at corresponding active methylene group; and help the deprotonation by the weak base. Based on the above discussion a probable pathway of the reaction is proposed (Scheme 3) considering the acid–base bifunctionality of the synthesized BZ-1.

Scheme 3. Probable reaction pathway of the Knoevenagel condensation reaction over BZ.

The synthesized BZ-1 and adopted methodology for the Knoevenagel reaction is superior to that of other reported catalysts and adopted methodology with respect to yield, reaction temperature (conservation of energy) and use of solvent (greener) (Table S3, Supporting Information).

The BZ-1 catalyst was reused three times to examine its reusability. The reactivity of regenerated catalysts was almost identical to the fresh catalyst. In the third cycle, the yield was 94 % (Figure S3, Supporting Information). The concentration of boron in the used catalyst after the 3^rd cycle was almost identical to the fresh catalyst (determined by ICP OES analysis). Moreover, ¹¹B NMR experiment of reaction mixture indicated no distinguishable peak for a borate species (Figure S4, Supporting Information). This clearly indicates that no borate species was leached during reaction.

The catalytic activity as well as the presence of acid–base bifunctionality of the synthesized BZ-1 catalyst was further confirmed by performing a Claisen–Schmidt condensation reaction between benzaldehyde and acetophenone in solvent-free condition at 140 °C (Scheme 4), in which active methylene groups are not present. The reaction was completed within 2 h with 92 % yield of the chalcone. From the results, it is evident that the activity of synthesized BZ-1 as a catalyst is quite high even for a Claisen–Schmidt condensation reaction. The high catalytic activity can be explained by considering the acid–base bifunctionality in the catalyst. The acid site of the catalyst assists the tautomerization of the acetophenone and the weak base site of the catalyst facilitates the nucleophilic attack of the tautomer on the benzaldehyde that is chemically adsorbed on the acid site. Finally, chalcone was formed on removal of water in the acid and base sites.

Scheme 4. Claisen–Schmidt condensation reaction of benzaldehyde and acetophenone over BZ.

Inspired from the above results, the BZ-1 catalyst was used for the synthesis of some targeted molecules such as cinnamyl methyl ester (Scheme 5), coumarin and their ethyl esters (Scheme 6) by Knoevenagel condensation. The reactions were performed at 140 °C after optimization under solvent-free condition for 3 h. The cinnamyl ethyl ester (44 %) was formed from the condensation of the benzaldehyde and diethyl malonate via the intermediate Knoevenagel condensation product benzylidene diethylmalonate (48 %). A negligible amount of cinnamic acid (≈2 %) was produced. However, coumarin was synthesized through condensation of salisaldehyde and diethylmalonate at 140 °C in the presence of BZ-1. During the reaction, initially ortho-hydroxybenzylidene diethylmalonate was formed using Knoevenagel condensation reaction. In the presence of the catalyst, coumarin (15 %) was formed through two probable pathways. In one pathway, the ortho-hydroxybenzylidene diethylmalonate converts to unstable coumaric acid ethyl ester as an intermediate, which easily undergoes cyclisation to coumarin. On the other hand, in another parallel reaction, an ester group from ortho-hydroxybenzylidene diethylmalonate (10 %) is eliminated and then followed by cyclisation, which leads to the coumarin-3-carboxylic acid ethyl ester (70 %), a precursor of coumarin. The obtained results are better than other reported results.39, 40 The enhanced catalytic activity of the BZ catalyst can be explained by considering acid–base bifunctionality.39, 40

Scheme 5. Knoevenagel condensation reaction of benzaldehyde and diethyl malonate over BZ.

Scheme 6. Knoevenagel condensation reaction of salisaldehyde and diethyl malonate over BZ.

Conclusions

Borated zirconia (BZ), with high surface area was synthesized through a simple, versatile, and economical synthetic protocol using aqueous zirconium ammonium carbonate complex solution as a precursor. The characterization results (NH₃- and CO₂-TPD) show that the synthesized BZ catalyst contains both acidic and basic properties. The synthesized BZ catalyst shows excellent catalytic activity (>90 % yield) for Knoevenagel condensation reaction between benzaldehyde/substituted benzaldehyde and malononitrile/ethyl cyanoacetate at room temperature within 15–30 min of reaction. The high catalytic activity is explained by considering the acid–base bifunctional properties of the BZ catalyst and a probable reaction mechanism can be proposed. No side products through self-condensation, dimerization or rearrangement were observed. The acid–base bifunctional nature of the BZ catalyst was further confirmed by solvent-free Claisen–Schmidt condensation reaction of benzaldehyde and acetophenone to chalcone and shows excellent yields of chalcone in a short period of time. Furthermore, the catalyst was successfully used for targeted synthesis of cinnamyl ethyl ester and coumarin or coumarin ester and results in a good yield. The catalyst was easily recoverable from the reaction system and reused without a reasonable change in catalytic activity. The developed BZ catalyst can be a potential alternative for other CC bond formation reactions.

Experimental Section

Materials

Analytical grade zirconium oxychloride (ZrOCl₂⋅8 H₂O, 96 %), ammonium carbonate (NH₄HCO₃& NH₂CO₂NH₄, 95.3 %) and borax (Na₂B₄O₇⋅10 H₂O) was procured from SDFCL (s.d. fine-chem limited) India. Benzaldehyde (99.5 %), malononitrile (99 %), ethyl cyanoacetate (98 %) and other organic compounds were purchased from Sigma Aldrich, USA. All the chemicals were used as received without further purification. Water (resistivity, 18 MΩ cm) was obtained from a Millipore water purifier.

Catalyst synthesis

In a typical synthetic procedure, dilute ammonium carbonate solution was slowly added with to an aqueous solution of 15 g, ZrOCl₂⋅8 H₂O as it was stirred. A white precipitate of zirconium carbonate formed. After precipitation had completed, it was filtered, washed with a large amount of distilled water to remove chloride ions (AgNO₃ test). The precipitates of zirconium carbonate were re-dissolved in an aqueous solution of ammonium carbonate (8 g in 100 mL) by controlled addition with constant stirring and a clear solution of zirconium carbonate complex was obtained. The volume of the solution was made up to 150 mL. An aqueous solution of borax (Na₂B₄O₇, 13.3 g, 268.5 mL) was then added into the reaction mixture with constant stirring. Then, the clear solution was added to an aqueous solution of CTAB (5.1 g in 418.5 mL) with constant stirring, a white precipitate formed. After 12 h stirring, the mixture was aged at 60 °C for 2 days, at 75 °C for 1 day and another 2 days at 90 °C in a closed glass bottle. After cooling, the white material was filtered, washed thoroughly with distilled water and dried. The dry sample was calcined at 550 °C for 6 h.

The synthesis was performed using a molar ratio of Zr⁴⁺/Na₂B₄O₇/CTAB/H₂O:1:(0.5–1.5): 0.3: 1000. The amount of borax was varied to obtain the best catalyst for the solvent-free Knoevenagel reaction. For the comparison, mesoporous zirconia (BZ-0) was synthesized using the same protocol in the absence of borax.

Catalyst characterization

The nitrogen adsorption–desorption measurements at −196 °C were performed by using a ASAP 2010 Micromeritics, USA, after the degassing of samples under vacuum (10⁻² Torr) at 250 °C for 3 h. The surface area was determined by BET equation. Pore size distributions were determined using the BJH model of cylindrical pore approximation.

Powder X-ray diffraction patterns were collected in two different 2 θ ranges of 1–7° and 20–80° by using a Rigaku X-ray powder diffractometer using Ni filtered Cu_Kα (λ=1.54178 Å) radiation with a scan speed of 0.2 s⁻¹.

A scanning electron microscope (Leo series 1430 VP) equipped with INCA was used to determine the morphology of samples. The sample powder was supported on aluminum stubs and then coated with gold by plasma prior to taking the image.

TEM images were collected by using a JEOL JEM 2100 microscope and samples were prepared by mounting an ethanol dispersed sample on a lacey carbon coated Cu grid.

The FTIR spectroscopic measurements were performed by using a Perkin-Elmer GX spectrophotometer. The spectra were recorded in the range 400–4000 cm⁻¹ using a KBr technique.

TPD measurements were conducted by using a Micromeritics Autochem-II Chemisorption analyzer instrument; and for this 100 mg of calcined sample was placed in a U shaped sample tube. The sample was flashed with helium for 1 h at 150 °C and then cooled to 50 °C. NH₃ and CO₂ were used to study acidic and basic strength respectively. The corresponding gas was adsorbed on the sample for 30 min. the loosely adsorbed species was flushed out with helium for 30 min and the temperature was increased to 800 °C with ramp rate 5 °C min⁻¹. The graph was recorded using a TCD detector.

An inductively coupled plasma-optical emission (ICP-OES) spectrophotometer (Optima 2000 DV, Perkin–Elmer, Eden Prarie, MN) was used to determine the percentage of the zirconium and boron (borate ion) present in the synthesized materials. The material was digested in concentrated HF and diluted with water.

A Bruker Advance II-500 spectrometer equipped with a magic angle spin probe was used for the solid state ¹¹B NMR study of the materials at room temperature. The sample may have hydrolyze partially as it was exposed to the atmosphere after calcination and prior to analysis. B(OCH₃)₃ in chloroform (18.1 ppm) was use as the external reference. The samples were spun at 8 kHz and the spectra were resolved from an average of 4000 scans.

Catalytic activity

Typical procedure for Knoevenagel reaction benzaldehyde and malononitrile or ethyl cyanoacetate

A mixture of benzaldehyde or substituted benzaldehyde (10 mmol) and malononitrile or ethyl cyanoacetate (12 mmol) was taken in a two necked round bottom flask containing 5 wt. % of catalyst with respect to the aldehyde. The mixture was vigorously stirred at room temperature under a nitrogen atmosphere for the duration. Next, ethanol (5 mL) was added to the mixture and it was stirred for 5 min. Workup included filtration, evaporation and re-crystallization. The characterization of the product was performed with GC–MS (Shimadzu, GCMS-QP 2010) techniques. The yield was calculated from gas chromatography (Varian-450 GC) results using n-tetradecane as internal standard.

Typical procedure for Claisen–Schmidt condensation reaction benzaldehyde and acetophenone

A mixture of acetophenone (5 mmol) and benzaldehyde (5 mmol) (Scheme 3) was taken in a two necked round bottom flask containing 10 wt. % of catalyst with respect to acetophenone. The mixture was vigorously stirred at 140 °C under nitrogen atmosphere for the desired duration of time. After that, acetone (5 mL) was added to the mixture and it was stirred for 5 min. The product was analyzed using GC–MS (Shimadzu, GCMS-QP 2010) techniques. The yield was calculated from gas chromatography (Varian-450 GC) results using n-tetradecane as internal standard.

Typical procedure for targeted synthesis of cinnamyl ethyl ester and coumarin or coumarin ester

A mixture of a benzaldehyde or salicaldehyde (10 mmol) diethylmalonate (12 mmol) was taken into a two necked round bottom flask containing 5 wt. % of catalyst with respect to aldehyde. The mixture was vigorously stirred at 140 °C under nitrogen atmosphere for the desired duration of time. After that, ethylacetate (5 mL) was added to the mixture and it was stirred for 5 min. The product was analyzed using GC–MS (Shimadzu, GCMS-QP 2010) techniques. The yield was calculated from gas chromatography (Varian-450 GC) results using n-tetradecane as internal standard.

Acknowledgements

This work was supported by Department of Science and Technology (DST), India [SR/S1/IC-33/2011 and SR/S1/PC-1/2010]. Authors also acknowledge analytical discipline of CSMCRI for materials characterization. P.P. acknowledges CSIR, India, for a research fellowship.