Mesoporous titania–ceria mixed oxide (MTCMO): a highly efficient and reusable heterogeneous nanocatalyst for one-pot synthesis of β-phosphonomalonates via a cascade Knoevenagel–phospha-Michael addition reaction

Abstract

Mesoporous titania–ceria mixed oxide (MTCMO) was synthesised in a sol–gel method using cerium chloride and titanium isopropoxide (TTIP) as CeO₂ as well as TiO₂ precursors and sodium dodecyl sulphate (SDS) as a structure-directing agent and its application was evaluated in the synthesis of aromatic β-phosphonomalonate derivatives via cascade Knoevenagel–phospha-Michael addition reaction. The catalyst characterised by various techniques like FT-IR spectroscopy, XRD, SEM, EDS, TEM and N₂ adsorption-desorption. Partial substitution of Ti⁴⁺ cations with Ce⁴⁺ improved the BET surface area because TiO₂ crystallites were suppressed to be grown. Furthermore, as a consequence of charge imbalance in the obtained Ti-O-Ce solid solution, some oxygen vacancies have generated that act as Lewis base sites while cations (Ti and Ce) around the oxygen vacancies act as Lewis acid sites. Reusability up to eight consecutive runs without significant decrease in reactivity, Mild reaction conditions, environmentally benign and compatibility with various functional groups are some of the advantages of this work.

Keywords:

• Mesoporous material
• heterogeneous nanocatalysts
• Michael addition
• surfactant
• soft-templating

1. Introduction

The stringent environmental regulations and economic issues compel the industry as well as academic researchers to use sustainable catalysts with high activity, compatibility with the environment, reusability and convenient separation procedure from the product. The exploration of porous materials serves great opportunities in the heterogeneous catalysis thanks to their intrinsic features especially the presence of active sites on their surface. Based on the pore size, International Union of Pure and Applied Chemistry (IUPAC) has classified the porous solid materials into three categories of microporous (<2 nm), mesoporous (2–50 nm) and macroporous (>50 nm) [1]. Although many methods are available for the preparation of the mesoporous materials, soft-templating method is the most important method in which the organic surfactant molecules act as template or structure-directing agent to generate porosity in the synthesis of siliceous (eg MCM-41, MCM-48, SBA-15, etc.) and non-siliceous mesoporous material (eg metal oxides, mixed metal oxides, etc.) [2].

Mesoporous materials with high BET surface area have been investigated in various applications such as adsorption [3], removal of pollutant ions [4], electronics [5], photocatalysis [6], energy storage [7] and energy conversion [8]. Furthermore, mesoporous materials are used in a broad spectrum of organic transformations such as oxidation reactions [9–12], hydrogenation reactions [13], ring-opening [14], coupling reactions [15–18], acid–base reactions such as Friedel–Crafts reaction [19], esterification [20] and transesterification reactions [21]. Mesoporous materials are also used as catalyst in the other reactions [22, 23].

Multi-component reactions (MCRs) are employed to construct target compounds with developed efficiency and atom economy from simple precursors in a single step pathway [24–26]. Among the organophosphorus compounds, phosphonates are highly valuable because of their broad range of applications in material chemistry [27], catalysis [28, 29], enzyme inhibitors [30], peptide mimics [31], metabolic probes [32], antibiotics [33] and pharmacologic agents [34]. Several synthetic methods have been developed for the preparation of phosphonates. Of these, the phosphorus-carbon (P-C), bond formation by direct phospha-Michael addition is one of the prevalent methods [35–37]. Two-step procedures are common to obtain the β-phosphonomalonates during a nucleophilic attack of phosphorus nucleophiles to α,β-unsaturated malonates [38, 39]. Step-by-step isolations and purifications of product and catalysts restrict overall efficiency of this method. To reduce limitations and improve overall efficiency, one-pot reaction found to be an ideal substitute. One-pot synthesis of β-phosphonomalonates via tandem Knoevenagel–phospha-Michael reaction is precious in this field [40–42]. Despite the efficiency of the reported methods, however, some of the disadvantages such as low yields, long reaction time, special reaction conditions, unrecyclable catalysts and use of toxic solvents are still remained. To exploit the valuable advantages of mesoporous materials, herein, we develop a mesoporous titania–ceria mixed oxide (MTCMO). The material was synthesised through soft-templating method using sodium dodecyl sulphate (SDS) a structure-directing agent. After the characterisation of material, its application was examined in the synthesis of β-phosphonomalonates through a one-pot procedure (Scheme 1).

Scheme 1. Synthesis of β-phosphonomalonates using MTCMO.

2. Results and discussion

2.1. Characterisation of MTCMO

FT-IR spectra MTCMO, CeO₂ and TiO₂ nanoparticles are shown in Figure 1(a–c), respectively. FTIR of MTCMO (Figure 1(a)) shows two bands at 3421 and 1636 cm⁻¹ owing to OH banding vibration and physical adsorbed water during gelation process [43]. The band at 2978 cm⁻¹ is attributed to remaining alkyl group of surfactant. The vibration band at 615 cm⁻¹ can be attributed to the vibration absorption peak of Ti–O–Ti and Ce–O–Ce with an overlapping. The weak band at 1057 cm⁻¹ is assigned to symmetrical stretching adsorption of S═O [44]. As shown in the FTIR spectra of samples, a shift to higher wave numbers in the bands position of samples from TiO₂ to MTCMO was observed. This change may be attributed to the change in the particle size or aggregation by the addition of CeO₂ NPs [45] which also supported by XRD data.

Figure 1. FT-IR spectra of MTCMO (a), CeO₂ (b) and TiO₂ (c).

XRD spectra MTCMO, CeO₂ and TiO₂ nanoparticles are shown in Figure 2(a–c), respectively. In the XRD pattern of MTCMO (Figure 2(a)), the anatase phase of TiO₂ is clearly obvious. Rutile lines such as 110 are not observed. Diffraction peaks in the XRD pattern of TiO₂ at 2θ = 26.1^ο, 39.8^ο and 62.3^ο are indexed to (101), (004) and (204 planes, respectively. These peaks are attributed to body-centred tetragonal anatase TiO₂ (JCPDS card no. 21-1272). The diffraction peaks appeared at 2θ = 28.3^ο, 33.8^ο, 47.3^ο, 73.8^ο and 77.8^ο are attributed to (110), (200), (220), (400) and (331) planes of the face centred cubic CeO₂ (JCPDS card no. 81-0792), respectively. Using the Scherrer’s equation (Equation (1)(1) $$D={\rm K}\lambda /\beta \hspace{0.17em}\text{cos}\hspace{0.17em}\theta$$(1) ), the crystallite size (D) of samples was calculated. (1) $$D={\rm K}\lambda /\beta \hspace{0.17em}\text{cos}\hspace{0.17em}\theta$$(1)

Figure 2. XRD pattern of MTCMO (a), CeO₂ (b) and TiO₂ (c).

Where θ is Bragg’s angle, K is Scherrer constant which is 0.9, λ = 0.15406 nm and β is diffraction peak full width at half maximum (FWHM). Using Scherrer’s equation, the crystallite size (D) of TiO₂ and CeO₂ nanoparticles was found to be 11 and 7 nm, respectively. Additionally, the crystallite size of MTCMO was found to be 9 nm. The decrease in crystal size can be attributed to the presence of Ti–O–Ce in the Ce-doped TiO₂ [46].

The morphology and structure of MTCMO were studied by SEM (Figure 3) and TEM (Figure 4). SEM image shows the ordered morphology of MTCMO with acceptable manometer particle size in the range of 60–80 nm. TEM image shows the existence of discrete pores in the structure of MTCMO.

Figure 3. SEM micrograph of the calcined MTCMO.

Figure 4. TEM micrograph of the calcined MTCMO.

In order for further characterisation of MTCMO, energy-dispersive X-ray spectroscopy (EDX) was carried out. The spectrum (Figure 5) shows the characteristic peaks for the Ti, Ce and O, confirming the formation of TiO₂–CeO₂ structure. The results of EDX analysis are depicted in Table 1.

Figure 5. The EDX pattern of MTCMO.

Table 1. The EDX data of MTCMO.

Element	Weight%	Atomic%
O	56.40	76.59
Ti	28.48	12.97
Ce	15.12	10.44
Totals	100.00	100.00

Textural properties of MTCMO were explored by N₂ adsorption–desorption analysis and pore size distribution was calculated by Barrett–Joyner–Halenda (BJH) method. The N₂ adsorption–desorption isotherm of MTCMO (Figure 6) is classified as a type IV isotherm, representative of mesoporous materials with an H2 hysteresis loop which indicates a narrow pore-size distribution. BET surface area of MTCMO is great value of 445.73 m².g⁻¹ and by our knowledge no sample of TiO₂–CeO₂ with this surface area is reported by now. Incorporation of Ce (Ce³⁺ and Ce⁴⁺) into the titania lattice prevents the growth of TiO₂ crystallites [47], and correspondingly leads to a decrease of crystallite size and thus increasing the BET surface area [48]. Pore size distribution curve shows a maximum pore width at 1.29 nm. Mean pore diameter was 8.3506 nm which clearly indicates the mesoporosity of MTCMO. The higher BET surface area and larger pore volume result in more efficient distribution and interaction of substrates and intermediates with the surface of the catalyst [49]. To show the effect of surfactant as an important factor for improving the surface area and total pore volume we synthesised a sample of TiO₂–CeO₂ mixed oxide (TCMO) but in this case without using surfactant. BET surface area of TCMO was 88.7465 m².g⁻¹ that showed a decrease as compared to the BET surface area of MTCMO. In addition, total pore volume of TCMO at P/P₀ =0.99 was 0.169425 cm³/g which showed a decrease as compared to total pore volume of MTCMO which was 0.9305 cm³/g at P/P0 = 0.99. Higher surface area provides more available active sites in the catalyst and therefore we expect that MTCMO should be more active relative to TCMO.

Figure 6. N₂ adsorption–desorption isotherms of MTCMO at 77 K. The pore size distribution curve using BJH method is shown in the inset.

2.2. Catalytic studies

In the next step, to find the effect of nature of catalyst on the promotion of β-phosphonomalonate synthesis, we carried out a set of investigations on a model reaction between benzaldehyde, malonitrile and triethylphosphite to find the most effective catalyst. The reactions were performed with TiO₂, CeO₂, TCMO and MTCMO. For the synthesis of TCMO no surfactant was used, while the other samples synthesised using SDS as a structure-directing and pore-forming agent. All the reactions were performed using 2 mol% of each catalyst under the solvent-free conditions at 50 ^οC for 2 h. The reaction under the catalyst-free condition afforded the desired product in the low yield (Table 2, entry 1). When TiO₂ or CeO₂ nanoparticles or TiO₂₊CeO₂ mixture (Table 2, entries 2–4) were used, the desired product obtained with moderate yields. The synergic effect of CeO₂ and TiO₂ improved the yield of product (Table 2, entry 4). From the obtained results (Table 2, entry 6), MTCMO was found to be the most effective catalyst. Interestingly, with TCMO as catalyst (Table 2, entry 5), the desired product obtained only with 35% yield that highlights the effect of surface area provided by availability of more active sites and pores that generated when SDS was used. Further studies showed that, in addition to surface area, MTCMO is superior catalyst over TCMO in the case of catalyst loading as the desired product obtained with 75% yield when 4 mol% of TCMO was used in the ethanol as solvent and 80 ^οC after 2 h. CeO₂ nanoparticles mainly contain a mixture of Ce³⁺ and Ce⁴⁺ ions on their surface [50]. Ce³⁺ ions are larger than Ce⁴⁺ ions and a result of oxygen vacancies. The higher ratio of Ce³⁺/Ce⁴⁺ improves the catalytic activity of CeO₂ owing to the fact that Ce³⁺ ions generate more strain on the lattice of CeO₂ [51]. The ionic radius of Ce⁴⁺ (1.01 Å) is larger than Ti⁴⁺ (0.68 Å). Therefore, the incorporation of Ce⁴⁺ cations into the crystalline structure of TiO₂ leads to the substitution of some of the Ti⁴⁺ cations and expands the TiO₂ lattice that consequently decreases the symmetry and makes disorder in the titanium sites [52].

Table 2. Screening of different catalysts for a model reaction between benzaldehyde, malonitrile and triethylphosphite.^a

Entry	Catalyst	Conversion (%)	Yield (%)^b
1	–	20	15
2	TiO2 NPs	55	40
3	CeO2 NPs	66	55
4^c	TiO2 +CeO2	70	65
5	TCMO	40	35
6	MTCMO	80	75

^a Reaction conditions: Benzaldehyde (1 mmol), malononitrile (1 mmol), triethylphosphite (1 mmol), solvent-free, 50 °C, 2 h.

^b Isolated yield.

^c The reaction was carried out with 2 mol% of TiO₂ and 2 mol% of CeO₂.

Additionally, the physicochemical properties and catalytic behaviour of the obtained solid solution Ti–O–Ce are different from their individual components [53, 54]. The larger electronegativity of Ti as compared to the electronegativity of Ce results in a charge imbalance at Ti–O–Ce bond and induces more defects like unsaturated chemical bonds [55]. Furthermore, the electron charge density of Ti⁴⁺ is increased in the Ti–O–Ce bond, making Ti⁴⁺ the electronegative center and Ce⁴⁺ as the electropositive center because Ti⁴⁺ strongly pulls the electrons of oxygen [56]. The increased electron charge density around the Ti⁴⁺ gives rise to a bond shortening as reported by Romero-Galarza et al. [57] and as a result of bond shortening, some structural defects such as oxygen vacancies and reduction of Ti⁴⁺ ions are observed [48, 58]. By removing oxygen from the surface of a metal oxide an oxygen vacancy is generated in which two electrons are remained. The unpaired electrons left behind after removing the oxygen atom act as Lewis base sites and cations around the oxygen vacancies act as Lewis acid sites [59, 62].The reactions catalyzed by defective surface sites profit from lower energy barriers relative to the similar reactions catalyzed by non-defective surfaces [63–65]. Therefore, by incorporation of Ce⁴⁺ ions into the structure of TiO₂ the BET surface area improved, structural defects generated, and consequently a better interaction between the surface of the catalyst and substrates lowered the energy barrier of reaction.

In the next step, the effect of solvent, temperature and amount of catalyst was studied (Table 3). First, equivalent reactions were performed with 2 mol% of the catalyst in the various solvents as well as solvent-free conditions at 50 °C. As shown, the reaction proceeded with better yield in ethanol as compared to the other solvents (Table 3, entries 1–6). By increasing the amount of the catalyst to 3 mol% an improvement in the yield of product was observed (Table 3, entry 7), and by increasing the temperature to 60 °C the best result was obtained (Table 3, entry 8). Therefore, the optimum condition was determined as a reaction with 3 mol% of MTCMO, 1 mmol of aldehyde, 1 mmol of malonitrile and 1 mmol of triethylphosphite in the 5 mL of ethanol at 60 °C.

Table 3. Optimisation of the reaction conditions.^a

Entry	MTCMO (mol%)	Solvent	Temperature (^οC)	Yield (%)^b
1	2	–	50	75
2	2	MeOH	50	80
3	2	Toluene	50	50
4	2	CH2Cl2	50	60
5	2	CH3CN	50	72
6	2	EtOH	50	85
7	3	EtOH	50	90
8	3	EtOH	60	95

^a Reaction conditions: Benzaldehyde (1 mmol), malononitrile (1 mmol), triethylphosphite (1 mmol) solvent (5 mL), 2 h.

^b Isolated yields.

Under the optimised reaction conditions, we explored the scope and efficiency of MTCMO for the preparation of β-phosphonomalonates from aromatic and heterocyclic aldehydes bearing electron-donating and electron-withdrawing groups (Table 4). Aromatic aldehydes with electron-withdrawing groups such as NO₂ and –Cl, on the benzene ring gave shorter reaction times and yields as compared with the aldehydes with electron-donating groups like –Me, –OMe and –OH. Heteroaromatic aldehydes, such as thiophene-2-carbaldehyde and furfuraldehyde were also effective substrates to accomplish the tandem Knoevenagel–phospha-Michael (Table 4, entries 9 and 10). Electron-withdrawing groups showed more reactivity and afforded higher yields in position 4 of benzene.

Table 4. One-pot synthesis of β-phosphonomalonates catalysed by MTCMO.^a

Entry	R-	Time (h)	Yield (%)^b	Mp (Lit.)^Ref./°C
1		2	95	54 (53–54)^[^66^]
2		1	92	94–96 (93–95)^[^66^]
3		1.5	90	94 (95–96)^[^66^]
4		0.5	95	120 (119–120)^[^67^]
5		0.5	96	104 (104–105)^[^66^]
6		4	96	61–63 (60–62)^[^66^]
7		3.5	95	94–95 (93–95)^[^66^]
8		3	90	131 (129–131)^[^40^]
9		2.5	96	Oil (oil)^[^66^]
10		3	95	Oil (oil)^[^66^]

^a Reaction conditions: Aldehyde (1 mmol), malononitrile (1 mmol, 0.06 g), triethylphosphite (1.5 mmol) and MTCMO (3 mol%) at 60 ^οC in ethanol (5 mL).

^b Isolated yields.

A comparison of the efficiency and applicability of our method with some of the recently reported methods is collected in Table 5. As it can be seen, our catalyst system shows better catalytic efficiency in terms of time, yield and catalyst loading. The results are collected for diethyl [2,2-dicyano-1-(4-methoxyphenyl)-ethyl]phosphonate as a model substrate (Table 5, entry 6).

Table 5. Comparison of catalytic activity of the MTCMO with some of the recently reported systems.

Entry	Reaction conditions	Catalyst loading (mol%)	Time	Yield (%)^Ref.
1	DMAP, H2O, MW/Reflux	20	12 min/8 h	95/90^[^68^]
2	HClO4-SiO2, Solvent-free	3	3 h	83^[^38^]
3	Silica-bonded 2-HEEA, r.t.	1	15 min	88^[^69^]
4	γ-Fe2O3@SiO2–La(OTf)2, Solvent-free, r.t.	1	3 h	83^[^70^]
5	This work	3	4.5 h	96

Furthermore, a comparison between the BET surface area of MTCMO with some similar oxides and mixed metal oxides is collected in Table 6. Obviously, the MTCMO (TiO₂–CeO₂) shows higher BET surface area relative to TiO₂ and CeO₂. The MTCMO also describes higher BET surface area as compared to TiO₂–Fe₂O₃, TiO₂–ZrO₂ (95:5 mol%), CeO₂–ZrO₂ (50:50 mol%), ZrO₂ and TiO₂.

Table 6. Comparison of the BET surface area between MTCMO and some of similar oxides and mixed oxides.

Entry	Oxide/mixed oxide^Ref.	Surface area (m^2/g)
1	TiO2–Fe2O3^[^71^]	367
2	TiO2–ZrO2 (95:5 mol%)^[^72^]	109
3	CeO2–ZrO2 (50:50 mol%)^[^73^]	309
4	TiO2^[^74^]	58.9
5	ZrO2^[^74^]	31.3
6	Nano ceria^[^75^]	214
7	This work (TiO2–CeO2)	445.73

2.3. Recyclability study of MTCMO

A simple separation procedure from the reaction mixture and efficiency after several cycles are two important factors needed for a heterogeneous catalyst. Recyclability of MTCMO was investigated in a model reaction using benzaldehyde, malonitrile and triethylposphite. After completion of the reaction, the reaction mixture was diluted with ethanol and heated at 80 °C for 10 min. Then, the catalyst was separated by filtration and washed three times with acetone (3 × 10 mL) and EtOAc. Finally, dried in an oven at 80 °C for 3 h. The catalyst was reused up to eight consecutive runs with slight decrease in the efficiency (Figure 7).

Figure 7. Reusability of MTCMO in a model reaction between malonitrile, triethylphosphite and benzaldehyde.

2.4. Possible reaction mechanism over MTCMO

Plausible reaction mechanism for the synthesis of β-phosphonomalonates is depicted in Scheme 2. The reaction profits from the presence of both Lewis acid and Lewis base sites and high BET surface area in the MTCMO catalyst. The aromatic aldehyde is first activated by the coordination of oxygen atom on the Lewis acid sites of MTCMO, followed by nucleophilic attack of activated malonitrile. A Knoevenagel condensation between the aldehyde and malonitrile produces α,β-unsaturated malonates 1. Next, the cyano group of 1 is activated by MTCMO to give the corresponding β-phosphonomalonates through a phospha-Michael addition reaction.

Scheme 2. Possible reaction mechanism for the preparation of β-phosphonomalonates over MTCMO.

3. Experimental

3.1. General

All commercially available chemicals were purchased from Merck (Kenilworth, NJ) or Aldrich (St. Louis, MO) company and used without further purifications. The products were characterised by comparison of their spectral data and physical properties with those reported in the literature. The progress of the reaction was followed by TLC using Merck 0.2 mm silica gel 60 F-254 Al-plates. Melting points were determined in open capillaries with a stuart scientific SMP2 melting point apparatus and are uncorrected. FT-IR spectra were determined as KBr pellets on a Perkin-Elmer 683 instrument (Perkin-Elmer, Waltham, MA). The crystal structure of the catalyst was analysed on a X’Pert MPD Philips diffractometer with Cu radiation source (λ = 1.54050 Å) at 40 kV voltage and 30 mA current. Field emission scanning electron microscopy measurements (FESEM) were performed on a VEGA TESCAN microscope and TEM micrographs were recorded on a CM120 1998. BET analysis was carried out on a BELOPSOR mini Π Czech 1999. The sample was degassed for 4 h at 423 K before the measurements. EDX was performed on a Philips XL‐300 instrument.

3.2. Preparation of MTCMO

MTCMO was synthesised by following procedure. First, SDS (0.82 g) was dissolved in 5 mL water, then, stirred for 30 min at room temperature to obtain a clear micellar solution. Afterwards, a solution of CeCl₃ (1.23 g) dissolved in distilled water (5 mL) was added slowly to the first solution. The resulting mixture was stirred for 30 min. Titanium (iv) isopropoxide (1.42 g) dissolved in 2.5 mL isopropyl alcohol was then added to the premixed solution. The pH of the solution was maintained about 11 by adding 35% NH₃.H₂O. The resulting mixture was stirred for 3 h and kept on freezing conditions (4 °C) for 48 h. After that, the solid was collected by centrifugation and dried in an oven at 75 °C. The synthesised solid was extracted with ethanol and 2 M HCl. Finally, the solid washed with water and dried at 200 °C to produce mesoporous TiO₂–CeO₂ mixed oxide.

3.3. Synthesis of TCMO

A solution of CeCl₃ (1.23 g) dissolved in distilled water (5 mL) was first prepared. To this solution, a solution of titanium (iv) isopropoxide (1.42 g) dissolved in 2.5 mL isopropyl alcohol was then added and the resulting mixture was stirred for 30 min. Then, the pH of solution was adjusted about 11 by adding 35% NH₃.H₂O. After stirring for 3 h, the mixture was kept on freezing conditions (4 °C) for 48 h. The solid was then collected by centrifugation and dried in an oven at 75 °C. Finally, the solid washed with water and dried at 200 °C to produce TCMO.

3.4. Synthesis of CeO₂ nanoparticles

To synthesise CeO₂ nanoparticles, following procedure was used: SDS (0.82 g) was dissolved in 5 mL water and stirred for 30 min at room temperature to obtain a clear micellar solution. To this solution, a solution of CeCl₃ (1.23 g) dissolved in distilled water (5 mL) was added slowly and the resulting mixture was stirred for 30 min. By dropwise addition of 35% NH₃.H₂O, the pH of solution was maintained about 11. The resulting mixture was stirred for 3 h and then the resulted solid was collected by centrifugation. After drying in an oven at 75 °C, the synthesised solid was extracted with ethanol and 2 M HCl. Finally, washed with water and ethanol and dried at 200 °C to produce CeO₂ nanoparticles.

3.5. Synthesis of TiO₂ nanoparticles

TiO₂ nanoparticles were synthesised using this procedure: SDS (0.82 g) was dissolved in 5 mL water and stirred for 30 min at room temperature to obtain a clear micellar solution. To this solution, a solution of titanium (iv) isopropoxide (1.42 g) dissolved in 2.5 mL isopropyl alcohol was then added and the resulting mixture was stirred for 30 min. By dropwise addition of 35% NH₃.H₂O, the pH of solution was maintained about 11. The resulting mixture was stirred for 3 h and then the solid was collected by centrifugation. After the solid was dried in an oven at 75 °C, it was extracted with ethanol and 2 M HCl. Finally, washed with water and ethanol and dried at 200 °C to produce TiO₂ nanoparticles.

3.6. General procedure for the synthesis of β-phosphonomalonates

In a round-bottomed flask, aryl aldehyde (1 mmol), malononitrile (1 mmol, 0.066 g) and triethylphosphite (1 mmol, 0.277 mL) were mixed with MTCMO (0.015 g, 3 mol%) in ethanol (6 mL) and stirred at 60 °C for the appropriate time as shown in Table 4. The progress of the reaction was monitored by TLC (n-hexane/ethyl acetate 7:3). After completion of the reaction, ethanol (10 mL) was added to the reaction mixture, and the catalyst was filtered. Then, ethanol was evaporated under reduced pressure, and the crude product was recrystallised from n-hexane and ethyl acetate to obtain the pure product in the excellent yield.

4. Conclusion

In summary, MTCMO was prepared using SDS as a structure-directing agent, cerium chloride as the precursor of CeO₂ and titanium isopropoxide (TTIP) as the precursor of TiO₂. During the substitution of Ti⁴⁺ cations with Ce⁴⁺, the BET surface area increased because the TiO₂ crystallites were suppressed to be grown. In addition, high surface area and presence of defective sites (reduced cations and oxygen vacancies) improved the reactivity of MTCMO. Furthermore, the catalyst demonstrated both Lewis acid and Lewis base character. The described catalyst was highly efficient for the synthesis of β-phosphonomalonates via cascade Knoevenagel–phospha-Michael addition reaction from triethylphosphite, malonitrile, and various aromatic aldehydes under the mild reaction conditions.

Acknowledgements

The authors acknowledge the Razi University Research Council for support of this work.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was supported by the Razi University Research Council.

Notes on contributors

Mohammed Salim Mohammed

Mohammed Salim Mohammed and Mohsen Bakhtiarian are M.Sc. students of organic chemistry under the guidance of Dr. Kiumars Bahrami at faculty of chemistry, Razi university, Kermanshah, Iran. Their interests cover the application of nanocatalysts in organic reactions.

Mohsen Bakhtiarian

Mohammed Salim Mohammed and Mohsen Bakhtiarian are M.Sc. students of organic chemistry under the guidance of Dr. Kiumars Bahrami at faculty of chemistry, Razi university, Kermanshah, Iran. Their interests cover the application of nanocatalysts in organic reactions.

Kiumars Bahrami

Dr. Kiumars Bahrami is working as a Professor of organic chemistry in faculty of chemistry, Razi university, Kermanshah, Iran. He has published many papers in national and international journals. He has also participated and presented several papers at national and international conferences. To date, he has supervised 33 M.Sc. and 12 Ph.D. students. He is developing research programs in the areas of nanostructured materials in organic syntheses.