La–Mg mixed oxide as a highly basic water resistant catalyst for utilization of CO₂ in the synthesis of quinazoline-2,4(1H,3H)-dione

Abstract

The synthesis of quinazoline-2,4(1H,3H)-dione was done by direct utilization of CO₂ in the cyclization of 2-aminobenzonitrile (2-ABN) using lanthanum magnesia mixed oxide (La–Mg MO) as a strong basic catalyst under mild reaction conditions in water. It gave a conversion of ∼92% with 100% selectivity at 140 °C in 14 h. La–Mg MO was prepared by hydrothermal method using urea as homogeneous precipitating agent. The catalyst was characterized by different analytical techniques like BET, XRD, FT-IR, SEM, and TGA, and the basicity by CO₂-TPD and acidity by NH₃ TPD. Various reaction parameters were studied to predict the reaction mechanism and kinetics. The reaction follows the Langmuir–Hinshelwood–Hougen–Watson (LHHW) type kinetics model with an apparent activation energy of 23.3 kcal mol⁻¹. The catalyst was recycled three times with an insignificant change in activity. The overall process is clean and green.

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Introduction

Global warming is the outcome of anthropogenic emission of greenhouse gases. Recent reports show that the main reason for the increasing CO₂ level is the increasing energy demand (i.e. burning fossil fuel).^1,2 CO₂ is abundantly available, non-toxic, non-flammable and renewable carbon source.^3,4 CO₂ utilization as a C1 source is an elegant approach to synthesize several value-added chemicals like methanol, formic acid, dimethyl carbonate (DMC), syngas, etc.^5–8 It also provides the best alternative to highly toxic carbon monoxide and phosgene.^9,10 CO₂ is utilized for the synthesis of organic carbonates,¹¹ reduction to hydrocarbons,^12,13 carbonylation reactions¹⁴ and methane reforming.¹⁵

Quinazoline-2,4(1H,3H)-dione (quinazolinedione) and its derivatives are conversably used as key building blocks in the pharmaceutical industry for the synthesis of antihypertensive, antidiabetics, heart disease controlling and other important medicines.¹⁶ The green chemistry point of view synthesis of quinazolinedione using CO₂ as a C1 source has achieved great importance. Several heterogeneous catalytic systems were developed to synthesize quinazolinedione using 2-aminobenzonitrile (2-ABN) and CO₂ as common starting materials such as; MgO/ZrO₂,¹⁷ tungsten oxide,¹⁸ mesoporous smectites,¹⁹ amine functionalized MCM-41,²⁰ [Hmim]OH supported on SiO₂,²¹ TBD supported on Fe₃O₄ ²² and amide supported on the polymer.²³ Also ionic liquids and organic bases such as; [Bmim]OH,²⁴ [TBP][Arg],²⁵ [HDBU⁺][TFE⁻],²⁶ reversible IL (DBU based),²⁷ [Bmim]Ac,²⁸ DBU,²⁹ guanidine base,³⁰ DBU in ScCO₂ (solvent and reactant),³¹ N-heterocyclic carbene (NHC) with K₂CO₃ ³² and CsCO₃ ³³ were reported. These methods have common drawbacks like the use of organic solvents, low atom economy and use of costlier reagents.

Recently, Ma et.al.³⁴ developed catalyst-free synthesis of quinazolinedione in water, (160 °C, 14 MPa of CO₂ pressure for 21 h) which works at harsh reaction condition. To overcome these difficulties herein, we report a comparatively simple synthesis of quinazolinedione in water using lanthanum magnesia mixed oxide (La–Mg MO) as catalyst (Scheme 1). La–Mg MO was previously used as strong base catalyst in synthesis of cyclic carbonate,³⁵ Wittig reaction³⁶ and aldol reaction.³⁷

Scheme 1 Synthesis of quinazolinedione by coupling of 2-ABN and CO₂.

In CO₂ utilization pathways, the main challenge is activation of CO₂ due to its low reactivity. In this perspective, we report the use of La–Mg MO as a basic catalyst in the synthesis of quinazolinedione using water as a solvent. The catalyst was previously characterized by different analytical techniques; XRD, FT-IR, CO₂ and NH₃-TPD, BET surface area and TGA before and after use. The reaction mechanism and kinetics of this reaction were studied.

Results and discussion

La–Mg MO

Kantam et al.³⁶ reported that La–Mg MO of 1 : 3 molar ratio (La : Mg) showed the highest basicity. Conventionally, La–Mg MO is synthesized by co-precipitation method at constant pH (9–10) using nitrate precursors and NaOH or ammonia as precipitating agent.^36,38 We prepared La–Mg MO with the hydrothermal method, using urea hydrolysis as ammonia source.

Role of urea in synthesis of La–Mg MO

The urea hydrolysis technique is used to get mono-dispersed nano-sized metal oxides.^39–42 In this method controlled hydrolysis of urea supplies ammonium hydroxide and carbonate at decomposition temperature of urea, which promotes the precipitation of metal ion to hydroxides, carbonates, and oxycarbonates with uniform size.⁴³ The metal nitrate to urea mole ratio was studied in a range of 1 : 0.5 to 1 : 4. It was found that the surface area of La–Mg MO increases significantly from 1 : 0.5 to 1 : 2 and thereafter decreases with increase in urea concentration. So, the concentration of urea has significant effect on the surface properties of the catalyst, which is studied by BET surface area analysis. The 1 : 2 (nitrate : urea) shows highest surface area (78.2 m² g⁻¹) (Table 1). The pH value substantially increases with increase in urea concentration. Therefore, at a lower concentration of urea, there was incomplete precipitation.^44,45 At higher pH, the nucleation process of the metal hydroxide is very high. Due to higher concentration of ammonium hydroxide, the nuclei of metal hydroxide and OH⁻ ions get attracted to each other with hydrogen bonding. The strong hydrogen bonding is responsible for formation of agglomeration in metal hydroxide particles, which results in low surface area of catalyst.⁴⁶

Table 1 Literature review for synthesis of quinazoline-2,4(1H,3H)-diones

Sr. No.	Solvent	Catalyst used	Temp. (°C)	Time (h)	CO2 pressure (MPa)	% conversion	Reference no.
1	Solventless	ReIL	40	15	0.1	98	27
2	Solventless	[Bmim]OH	120	18	3.0	90	24
3	Solventless	[HDBU][TFE]	30	24	0.1	97	26
4	Solventless	[Bmim]Ac	90	10	0.1	92	28
5	DMF	MgO/ZrO2	130	12	3.7	92	17
6	DMF	TBD@Fe3O4	140	16	4.0	64	22
7	DMF	DBU, DBN (3 eq. base)	20	24	0.1	97	29
8	DMF	Mesoporous smectites	130	6	4.0	68	19
9	DMF	[TBP][Arg]	100	12	8.5	92	25
10	DMSO	NHC, K2CO3	120	15	0.1	95	32
11	NMP	TBA2[WO4]	140	24	2.0	91	18
12	ScCO2	DBU (0.05 eq.)	80	4	10	89	31
13	Water	—	160	21	14	92	34
14	Water	Amine functionalize MCM-41	130	18	3.5	91	20
15	Water	La–Mg mixed oxide	140	14	3.5	92	Current work

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Catalyst characterization

XRD. XRD patterns of catalysts were analyzed in the range of 10°–75° 2θ value. La₂O₃, MgO, and La–Mg mixed oxide catalysts show sharp peaks (Fig. 1). The peaks observed at a 2θ value of 25.21°, 30.54°, 46.34° and 55.76° are due to (100), (101), (110) and (201) planes of La₂O₂CO₃ and La₂O₃, respectively (JCPDS card # 49-0981).⁴⁷ The peaks observed at 2θ values of 42.3° and 62.4° confirm the crystalline phase of MgO (JCPDS data card # 45-946).⁴⁸

Fig. 1 X-ray diffraction pattern of (a) La–Mg mixed oxide (b) MgO and (c) La₂O₃.

In case of La–Mg MO, peaks related to the different phases of La₂O₃ and La₂O₂CO₃ are clearly observed and MgO phases nearly disappeared.^49,50 Thus, it shows well dispersion of lanthanum on MgO.

The reused catalyst does not show any significant change in XRD patterns even after third reuse (ESI Fig. S3†). It confirms the structural stability of the catalyst.

Morphology of catalyst (SEM). Pure La₂O₃ shows nanocluster formation due to agglomeration of La₂O₃ nanoparticles. La–Mg mixed oxide forms a dendritic and spherical like structure and MgO shows spherical structure. The reused catalyst shows a change in morphology due partial rehydration (memory effect) in the reaction (Fig. 2).

Fig. 2 SEM images of (a) La₂O₃, (b) La–Mg MO and (c) MgO and (d) reused La–Mg MO.

BET surface area. La–Mg MO shows type IV adsorption isotherm corresponding to the mesoporous structure. La–Mg MO (1 : 2; nitrate : urea ratio) shows high surface area compared to other ratios as described above (Table 1). The surface area of pure La₂O₃ and MgO are 19.3 and 99.2 m² g⁻¹, respectively whereas that of La–Mg MO is 78.2 m² g⁻¹. The increase in surface area of La–Mg MO shows the dispersion of lanthanum on the surface of MgO. La–Mg MO (1 : 2 nitrate : urea; hydrothermal method) was used for further experiments, kinetic studies and characterizations.

Acidity and basicity of catalyst. CO₂ and NH₃-TPD analysis were performed to measure basicity and acidity of catalysts, respectively. La–Mg MO shows 0.79 mmol g⁻¹ and pure La₂O₃ shows 0.28 mmol g⁻¹ basicity. Further, the basicity of La–Mg MO increased significantly due to well the dispersion of La₂O₃ on MgO⁵¹ (Table 1). According to the previously published report,⁵² the CO₂ desorption was divided into three regions, i.e. weakly basic (50 to 150 °C), moderately basic (200 to 400 °C) and strongly basic (more than 400 °C). The CO₂ TPD of catalyst shows two desorption peaks at 198 and 347 °C. The desorption of peak associated with 198 °C shows weak basic sites (0.37 mmol g⁻¹) and peak at 347 °C is attributed to the desorption of CO₂ from moderately basic sites (0.41 mmol g⁻¹). The NH₃-TPD of La–Mg MO shows mild acidity (0.18 mmol g⁻¹). The reused La–Mg MO shows nearly the same basicity (0.73 mmol g⁻¹) as the fresh catalyst which suggests that the fidelity of the catalyst is retained on reuse.

FT-IR. Fig. 3 shows absorbance bands at about 642 and 450 cm⁻¹ which are related to lattice vibration modes due to La–O bond. The band at 1085 cm⁻¹ is weak but sharp and is observed due to symmetric (CO₃)²⁻. Weak bands are seen at 1750 and 1832 cm⁻¹. The peak at 1750 cm⁻¹ may be due to carbonyl group of carbonate and that at 1832 cm⁻¹ due to bridged carbonate containing metal–oxygen bond.⁵⁰ The peaks observed at 642 and 450 cm⁻¹ which are related to lattice vibration due to La–O bond. The individual FT-IR spectra are provided in ESI.†

Fig. 3 FT-IR of fresh and reused La–Mg mixed oxide.

TGA. Thermal stability of calcined, uncalcined and reused La–Mg MO (without calcination) catalysts were studied by TGA (Fig. 4). Fresh La–Mg MO does not show a considerable loss in weight, due to the formation of stable oxide form. The reused and uncalcined La–Mg MO shows nearly the same TGA curve, which indicates that La–Mg MO shows memory effect like LDH.^53,54 Uncalcined and reused La–Mg MO show 1–2% loss in weight due to loss of water (humidity) at 100 °C. 5–6% loss at 320 to 380 °C indicates the decomposition of metal oxycarbonates and evolution of CO₂ to get stable oxide forms. The carbonate content of reused catalyst is 6–7%.⁵⁵ Thereafter there is no significant loss in weight which shows the formation of stable oxide forms.

Fig. 4 TGA of La–Mg MO (a) calcined, (b) uncalcined and (c) reused catalyst.

XPS. Fig. 5 and 6 show the XPS spectra of fresh and reused La–Mg MO. The survey spectra (Fig. 5) show signals for La and Mg. La₂O₃ shows high-resolution La 3d_5/2 and La 3d_3/2 XPS peaks are observed at 835.37 and 852.41 eV respectively with a spin–orbit splitting 17.02 eV and for reused La–Mg MO spin–orbital splitting is 17.56 eV. Splitting of La 3d_5/2 is attributed to the formation of hydroxide and the binding energy is 835.41 eV, which indicates formation of carbonate phase in the catalyst. This is observed due to adsorbed moisture and atmospheric CO₂ on the surface of the catalyst. A sharp O 1s was observed at 530.1 eV for prepared the samples. Due to MgO, the peaks observed for Mg 2s at 89.3 and 88.9 eV are for fresh and reused catalysts, respectively. The peak observed at 1304 eV is due to MgO in both fresh and reused catalysts.⁵⁶

Fig. 5 Survey spectra of La–Mg Mo.

Fig. 6 XPS of La–Mg MO; (a), (b) and (c), (d) XPS spectra of Mg 2s and La 3d_3/2 fresh and reused catalyst, respectively.

Reaction parameter study

Catalyst screening. The prepared catalyst was studied for its activity in utilisation of CO₂ in quinazolinedione synthesis as shown in Table 3. La–Mg MO (1 : 2 nitrate : urea; hydrothermal method) gives the highest conversion as compared to MgO, La₂O₃ and La–Mg MO (co-precipitation method).

Table 2 Surface area, pore size, pore volume and basicity of various synthesized catalysts

Sr. no.	Catalyst	Nitrate to urea mole ratio	CO2 TPD (mmol g^−1)	Surface area (m^2 g^−1)	Pore size (nm)	Pore volume (cm^3 g^−1)	pH of supernatant water
1	La–Mg MO	1 : 0.5	0.29	28.6	7.8	0.21	8.0
2	La–Mg MO	1 : 1	0.45	52.3	8.5	0.25	8.5
3	La–Mg MO	1 : 2	0.79	78.2	8.7	0.24	9.6
4	La–Mg MO	1 : 3	0.56	62.4	9.8	0.11	10.5
5	La–Mg MO	1 : 4	0.36	51.2	11.9	0.10	11.8
6	La2O3	1 : 2	0.29	19.3	24.0	0.12	—
7	MgO	1 : 2	0.39	99.2	6.9	0.17	—
8	La : Mg MO (co-ppt)	—	0.52	37.2	12.1	0.23	—
9	La : Mg MO (reused)	1 : 2	0.73	73.5	7.9	0.21	—

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Table 3 Catalyst screening study

Catalyst	Conversion (%)
La2O3 (commercial)	28.2
La2O3	46.2
MgO	28.4
La–Mg MO (hydrothermal)	92.1
La–Mg MO (co-ppt)	76.8

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CO₂ pressure study. CO₂ pressure was varied in a range of 0.5 to 4.5 MPa to study its influence on the conversion of quinazolinedione. CO₂ pressure increases H₂CO₃ concentration in water and so there is a significant increase in conversion with an increase in pressure up to 3.5 MPa. Thereafter, the conversion is nearly the same with an increase in CO₂ pressure (Fig. 7). It is due to the fact that the system is three-phase i.e. gas (CO₂), liquid (water as solvent) and solid (catalyst). Since the CO₂ pressure was maintained throughout the reaction, the concentration of H₂CO₃ remains constant. There is an equilibrium in between CO₂ and water and so transfer of H₂CO₃ to the catalyst surface for its activation.^57–59 The concentration of H₂CO₃ is much higher than required after 3.5 MPa pressure, which was adsorbed on the surface of the catalyst, and therefore no significant change in conversion was observed at a constant temperature.⁶⁰ Also, at high CO₂ pressure the equilibrium of adsorption and desorption of reactant and product may get disturbed.

Fig. 7 Effect of pressure on synthesis of quinazolinedione; reaction conditions: 2-ABN (5 mmol), La–Mg MO (0.02% (w/v)), reaction time (14 h), reaction temperature (140 °C), 30 mL water.

Effect of speed of agitation. A speed range from 800 to 1000 rpm was studied for evaluation of mass transfer resistance. After 1000 rpm the conversion was practically same. It confirms that there is no mass transfer resistance effect after 1000 rpm. Hence, 1000 rpm was chosen in further experiments to completely overcome the mass transfer resistance (Fig. 8).

Fig. 8 Speed of agitation for synthesis of quinazolinedione; reaction conditions: 2-ABN (5 mmol), La–Mg MO (0.02% (w/v)), CO₂ pressure (3.5 MPa), reaction time (14 h), reaction temperature (140 °C), 30 mL water.

Effect of catalyst loading. Catalyst loading was studied in the range of 0.01 to 0.03 g cm⁻³ reaction volume, keeping all other reaction conditions constant (Fig. 9). Up to 0.02 g cm⁻³ of catalyst loading, there was a significant increase in conversion due to proportional increase in active sites of catalyst available for reaction. However, after 0.02 g cm⁻³, there are no significant increases in conversion. It is due to the availability of more active sites for reaction. Hence, even increase in catalyst loading from 0.02 g cm⁻³ the conversion does not increase with catalyst loading.

Fig. 9 Effect of catalyst loading on synthesis of quinazolinedione; reaction conditions: 2-ABN (5 mmol), reaction time (14 h), CO₂ pressure (3.5 MPa), reaction temperature (140 °C), 30 mL water.

Effect of initial concentration of 2-ABN. The initial concentration of 2-ABN was varied from 0.1 to 0.2 mmol cm⁻³ keeping all other reaction parameters constant (Fig. 10). With increase in concentration the rate of reaction increases linearly up to 0.16 mmol cm⁻³ and thereafter there is no significant increase in the rate of reaction. However, with 0.16 mmol cm⁻³, very high conversion was obtained and hence it was selected for further study.

Fig. 10 Effect of initial moles of 2-ABN on the synthesis of quinazolinedione; reaction conditions: CO₂ pressure (3.5 MPa), La–Mg MO (0.02% (w/v)), reaction temp. (140 °C), reaction time (14 h), 30 mL water.

Effect of temperature. Effect of temperature on the rate of reaction was studied from 120 to 150 °C. The rate of reaction increases substantially with increases in temperature, which shows that the reaction is kinetically controlled (Fig. 11). It is discussed again in kinetic part.

Fig. 11 Plot of ln(1 − X_A) vs. time (h).

Reusability of catalyst. After completion of reaction the reaction mass was filtered and the catalyst washed 3–4 times with methanol to remove adsorbed 2-ABN and quinazolinedione. Then it was dried at 105 °C for 12 h. The reusability of catalyst was studied for three cycles. The rate constants of cyclization of 2-ABN and CO₂ were calculated (Table 4). The rate constant values are comparably the same which indicates that the catalyst is reusable. A first order kinetic plot is shown in Fig. 12. Reused catalyst was well characterized by common analytical techniques. Reused catalyst shows slight changes in surface area, pore size, pore volume and basicity of the catalyst which show the fidelity of catalyst within experimental errors (Table 2).

Table 4 Rate constants for reusability of La–Mg MO

Catalyst La–Mg MO	Pseudo-first order rate constant (h^−1)
Fresh	0.2202
First reuse	0.2183
Sec. reuse	0.2139
Third reuse	0.2076

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Fig. 12 Reusability of the La–Mg mixed oxide for the synthesis of quinazolinedione; reaction conditions: 2-ABN (5 mmol), CO₂ pressure (3.5 MPa), La–Mg MO (0.02% (w/v)), reaction time (14 h), reaction temp. (140 °C), 30 mL water solvent.

For confirming the reusability, the leaching test of catalyst was performed to examine leaching La–Mg MO catalyst. It was observed that 5.8 ppm and 1.8 ppm of MgO and La₂O₃ leached, respectively. In water La and Mg oxides get hydrolysed and thus there is slight leaching during the course of the reaction. The leaching is very insignificant because MgO and La₂O₃ solid phases have very less solubility in water.

Substrates of 2-ABN

The optimized reaction conditions were used to extend the scope of reaction with CO₂ using La–Mg MO as a catalyst. Substrates with different electron-donating and withdrawing groups were studied (Table 5). 2-ABN derivatives with an electron-withdrawing groups (–NO₂, –Cl and –F) give higher conversion as compared to electron-donating groups (–CH₃ and –OMe).

Table 5 Substrate scope of quinazoline-2,4(1H,3H)-dione^a

No.	Substrate	Product	Conversion (%)
a Reaction conditions: 2-ABN (5 mmol), CO2 pressure (3.5 MPa), reaction temperature (140 °C), solvent (30 mL), La–Mg mixed oxide 0.02% (w/v), reaction time (14 h), (a): isolated yield; selectivity 100%.
1			92.3
2			82.8
3			86.8
4			91.5
5			93.3
6			75.9
7			89.2
8			84.6

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NMR data

Quinazoline-2,4(1H,3H)-dione (Table 5 (1)). ¹H NMR: δ = ¹H NMR (500 MHz, DMSO-d₆) δ 11.27 (s, 1H), 11.12 (s, 1H), 7.86 (d, J = 7.4 Hz, 1H), 7.60 (t, J = 7.3 Hz, 1H), 7.15 (t, J = 7.3 Hz, 2H); ¹³C NMR (126 MHz, DMSO d₆) δ 163.27 (s), 150.74 (s), 141.27 (s), 135.37 (s), 127.37 (s), 122.75 (s), 115.74 (s), 114.74 (s).

5-Chloroquinazoline-2,4(1H,3H)-dione (Table 5 (2)). ¹H NMR (500 MHz, DMSO) δ 11.23 (t, J = 20.7 Hz, 2H), 7.52 (dd, J = 11.8, 4.3 Hz, 1H), 7.20–7.15 (m, 1H), 7.10 (dd, J = 8.3, 0.9 Hz, 1H); ¹³C NMR (126 MHz, DMSO) δ 161.24 (s), 150.07 (s), 143.96 (s), 135.11 (s), 134.24 (s), 125.48 (s), 115.18 (s), 111.59 (s).

6-Chloroquinazoline-2,4(1H,3H)-dione (Table 5 (3)). ¹H NMR (500 MHz, DMSO) δ 11.54 (s, 1H), 11.37 (s, 1H), 7.89 (dd, J = 6.7, 2.5 Hz, 1H), 7.80–7.72 (m, 1H), 7.31–7.23 (m, 1H); ¹³C NMR (126 MHz, DMSO) δ 162.25 (s), 150.47 (s), 140.13 (s), 135.23 (s), 126.72 (s), 126.33 (s), 117.94 (s), 116.19 (s).

7-Methylquinazoline-2,4(1H,3H)-dione (Table 5 (4)). ¹H NMR (500 MHz, DMSO) δ 11.18 (s, 1H), 11.06 (s, 1H), 7.74 (dd, J = 7.9, 3.1 Hz, 1H), 7.02–6.84 (m, 2H), 2.32 (s, 3H); ¹³C NMR (100 MHz, DMSO-d₆) δ 163.15, 150.89, 146.06, 141.33, 127.33, 124.11, 115.48, 112.44, 21.86.

6,7-Dimethoxyquinazoline-2,4(1H,3H)-dione (Table 5 (5)). ¹H NMR (400 MHz, DMSO-d₆) δ = 11.09 (s, 1H), 10.94 (s, 1H), 7.21 (s, 1H), 6.62 (s, 1H), 3.89 (s, 3H), 3.65 (s, 3H).

¹³C NMR (100 MHz, DMSO-d₆) δ = 162.86, 155.31, 150.88, 145.44, 136.95, 107.52, 106.6, 98.17, 56.17 (2C).

5-Fluoro-1H-quinazoline-2,4(1H,3H)-dione (Table 5 (6)). ¹H NMR (400 MHz, DMSO) δ = 10.91 (s, 1H), 10.91 (s, 1H), 7.31 (m, 1H), 6.14 (m, 2H); ¹³C NMR (d₆-DMSO) δ 166.28, 151.76, 150.38, 142.95, 135.90, 102.9, 100.72, 97.89.

7-Chloro quinazoline-2,4(1H,3H)-dione (Table 5 (7)). ¹H NMR (400 MHz, DMSO) δ = 11.39 (s, 1H), 11.24 (s, 1H), 7.86 (d, 1H), 7.20 (d, 1H), 7.16 (s, 1H); ¹³C NMR (d₆-DMSO) δ 162.53, 150.61, 142.39, 139.72, 129.47, 122.93, 115.11, 113.76.

6-Nitro quinazoline-2,4(1H,3H)-dione (Table 5 (8)). ¹H NMR (400 MHz, d₆-DMSO) δ = 11.73 (s, 2H), 8.44 (d, 1H), 8.37 (t, 1H), 7.30 (d, 1H); ¹³C NMR (d₆-DMSO) δ 162.13, 150.59, 146.10, 142.29, 129.32, 123.55, 116.30, 115.41.

Possible reaction mechanism

The reaction mechanism for the synthesis of quinazolinedione by condensation of 2-ABN with CO₂ using La–Mg MO is proposed as shown in Fig. 13. Chemical interaction of water and CO₂ forms H₂CO₃; this formation and decomposition of H₂CO₃ mediates through bicarbonate ion as an intermediate.^61–63 CO₂, HCO₃⁻ and H₂CO₃ are in equilibrium by Eigen's triangular scheme. The catalyst La–Mg MO is basic in nature and so there is the possibility of adsorption of H₂CO₃ on the surface of catalyst.

Fig. 13 Possible mechanism for synthesis of quinazolinedione.

Due to the basicity of catalyst, H₂CO₃ (mild acid) gets adsorbed on the basic sites of the catalyst surface and thus, CO₂ molecule gets activated.

Basicity of 2-ABN is too low (pK_a = 0.77) and also H₂CO₃ is weak acid, and therefore direct attack of H₂CO₃ on amine group of 2-ABN is not feasible. Therefore, H₂CO₃ hydrolyses –CN group. This reaction shows a similar type of mechanism pathway as previously reported literature i.e. without using catalyst in synthesis of quinazolinedione in water.^34,64 The major difference is the use of a catalyst to minimize the activation energy of CO₂.

Kinetics

In the absence of mass transfer and intraparticle diffusion resistances, the reaction seems to be kinetically controlled. The reaction was supposed to occur by Langmuir–Hinshelwood–Hougen–Watson (LHHW) with dual site mechanism. To prove this, we have developed a mathematical model to calculate different constants and activation energy for the reaction.^65–68 Considering the reaction between 2-ABN (A) and CO₂ (B) happens on the catalyst surface and A will chemisorb on the S_a (acidic) and B will adsorb on S_b (basic) catalyst site, the following is derived.

For general chemical reaction;

A + B → C

Adsorption of 2-ABN (A) and carbon dioxide (B) on catalyst sites is given by,

Surface reaction of AS_a and BS_b leads to the formation of CS_a with a vacant S_b,

Rate of desorption of quinazolinedione (C) is given by,

The overall rate of reaction is as follows;

rate of reaction of A (−r_A) = k₁C_A·Sa·C_B·Sb − k′₁C_C·Sa·C_Sb (5)

Site balance

C_A·Sa = K_A·C_A·C_Sa (6)

C_B·Sb = K_B·p_B·C_Sb (7)

C_C·Sa = K_C·C_C·C_Sa (8)

The total concentration of the sites is;

For acidic sites,

C_ta = C_A·Sa + C_C·Sa + C_Sa (9)

C_ta = K_A·C_A·C_Sa + K_C·C_C·C_C·Sa + C_Sa

C_ta = (K_A·C_A + K_C·C_C + 1)C_Sa

For basic sites,

C_tb = C_B·Sa + C_Sb

C_tb = (K_B·p_B·C_Sb + C_Sb)

C_tb = (1 + K_B·p_B)·C_Sb

From eqn (10)–(12), we have

For initial rate of reaction, we can write above equation as;

Consider, k₁·C_ta·C_tb = k′w

By solving this equation with solver we have calculated rate constant and adsorption constant for 2-aminiobenzonitrile and carbon dioxide (Table 6).

Table 6 Adsorption and rate constants at different temperatures

Temperature (K)	Rate constant (k) (cm^3 g cat^−1 atm^−1 h^−1)	Adsorption constant (KA, cm^3 mol^−1)	Adsorption constant (KB, atm^−1)
393	0.0284	1.635	0.0045
403	0.0608	1.510	0.0042
413	0.121	1.420	0.0039
423	0.234	1.390	0.0035

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The apparent activation energy was calculated by Arrhenius plot as 23.3 kcal mol⁻¹ (Fig. 14).

Fig. 14 Plot of ln k vs. 1/T (K⁻¹) where k = k′K_AK_B.

Experimental

Chemicals

Urea, La(NO₃)₃·6H₂O and Mg(NO₃)₂·6H₂O were supplied by S. D. fine chemicals, Mumbai, India. 2-ABN (purity > 98%) and all related compounds were supplied by Sigma-Aldrich. All chemicals were used as such without further purification. CO₂ gas (purity 99.99%) was purchased from Rakhangi, Mumbai.

Preparation of La–Mg MO

0.015 mmol Mg(NO₃)₂·6H₂O and 0.005 mmol La(NO₃)₃·6H₂O (mole ratio La/Mg = 1 : 3) were dissolved in 150 mL distilled water. Then, 0.09 mol urea was added (nitrate to urea ratio 1 : 2) into the above solution, which was agitated at 30 °C for 30 min. After that solution was transferred to a Teflon lining bomb reactor and kept at 433 K for 24 h, then cooled naturally at room temperature. Then, the precipitate was filtered and washed multiple times with distilled water to attain neutral pH. Then it was dried at 373 K in oven for 12 h and further calcined at 923 K at a ramp rate of 2 K min⁻¹ in the presence of air for 4 h and stored in air tight container. Similarly, the La₂O₃ and MgO were also prepared by using the same method as La–Mg MO.

Synthesis of quinazolinedione

The reaction of 2-ABN and CO₂ was carried out at 140 °C in autoclave.³⁸ In this experiment, 5 mmol of 2ABN and 30 mL water were added to the autoclave and flushed 2–3 times with CO₂ to remove unwanted air. Then, the reactor was heated at 140 °C and pressurized with CO₂ at 3.5 MPa. The desired CO₂ pressure and the temperature were maintained throughout the experiment. Sampling was done periodically so as to monitor reaction progress by HPLC analysis. At end of reaction, the reactor was cooled to room temperature and CO₂ released. The product was extracted with organic solvent. The product structure was confirmed by GCMS and H NMR.

Conclusion

In summary, we have successfully developed a homogeneous precipitation method (urea method; nitrate to urea mole ratio of 1 : 2) for preparation of La–Mg MO (La to Mg mole ratio of 1 : 3) hydrothermally. This is an efficient catalyst for synthesis quinazoline-2,4(1H,3H)-dione from the coupling of 2-ABN and CO₂. Water was used as a reaction medium; conventionally organic solvents have been used in the synthesis of quinazolinediones. The strong basicity of La–Mg MO is the reason behind the activation of CO₂. The catalyst was recycled for three times without any noticeable loss in its activity. We have developed cost-efficient and eco-friendly La–Mg MO which is highly basic water resistant recyclable catalyst which was used in a number of 2-ABN susbstrates. The overall process fits into the aims and objectives of Green Chemistry.

Acknowledgements

G. D. Yadav received support from R. T. Mody Distinguished Professor endowment and as J. C. Bose National Fellow from Department of Science and Technology, Government of India. K. B. Rasal is greatly thankful to Department of Science and Technology (DST), India for financial support under ‘Indo-Finnish’ project of GDY.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra15802a

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