Comparative Study of Potassium Salt-Loaded MgAl Hydrotalcites for the Knoevenagel Condensation Reaction.

A series of potassium salt-loaded MgAl hydrotalcites were synthesized by wet impregnation of KNO3, KF, KOH, K2CO3, and KHCO3 salts over calcined MgAl hydrotalcite (Mg-Al = 3:1). The samples were characterized by X-ray diffraction, Fourier transform infrared, thermogravimetry-differential thermal analysis, scanning electron microscopy, and N2 absorption-desorption techniques to investigate their structural properties. The results showed formation of well-developed hydrotalcite phase and reconstruction of layered structure after impregnation. The prepared hydrotalcites possess mesopores and micropores having pore diameters in the range of 3.3-4.0 nm and Brunauer-Emmett-Teller surface area 90-207 m2 g-1. Base strengths calculated from Hammett indicator method were found increasing after loading salts, where KOH-loaded hydrotalcite showed base strength in the range of 12.7 < H- < 15, which was found to be the preferred catalyst. Subsequently, KOH loading was increased from 10 to 40% (w/w) and catalytic activity was evaluated for the Knoevenagel condensation reaction at room temperature. Density functional theory calculations show that among all of the oxygen atoms present in the hydrotalcite, the O atom attached to the K atom has the highest basic character. In this study, 10% KOH-loaded hydrotalcite showing 99% conversion and 100% selectivity was selected as the preferred catalyst in terms of base strength, stability, and catalytic efficiency.

Introduction

Design, synthesis, and application of solid base catalysts have been of major concern in recent years due to their tunable basicity, environmentally acceptable nature, and capacity to catalyze diverse reactions.[1,2] Although a large number of basic materials have been reported in the literature, the study of structure–property relationship of supported solid bases and their basic properties has aroused renewed interest recently.[3] Host materials like zeolites (e.g., NaY, KL, NaX), porous metal oxides (e.g., Al2O3, ZrO2), mesoporous silica (e.g., SBA-15, MCM-41), mixed metal oxides, montmorillonite clay, ALPO series, etc. are employed to prepare solid strong bases and super bases.[4,5] Among various guest materials and alkali and alkaline earth metal salts, transition metals like Ag, Cu, Co, etc. are the most common.[6,7] Among various alkali metal salts, potassium salt-loaded catalysts, e.g., KNO3/Al2O3, KNO3/ZrO2, KF/NaY, K2CO3/Y zeolite, KOH/NaX, KOH/Al2O3, KOH/ZrO2, KOH/MgO, etc., have been reported as excellent basic materials for a good number of organic reactions.[8−10] On the other hand, oxide hosts due to their low surface area and silicon-containing hosts due to reaction with some basic guests have limited their use for many reactions.[11] Hydrotalcites (HTs) and hydrotalcite-like layered materials are another class of solid hosts that are used to generate strong bases.[12] This is a class of solid superbase that can catalyze a large number of organic reactions, such as transesterification, aldol condensation, Knoevenagel condensation, isomerization, and epoxidation reaction.[13,14] However, selection of metal salts to suit the host material and their effect over structure and basicity of the host are yet to be explored for hydrotalcite-like compounds. Besides, conditions for a suitable catalyst in terms of its base strength, catalytic activity, stability of host material, recovery, etc. are not easily met.[15] Recently, Zhao et al. have reported that potassium-loaded MgAl mixed oxides using KOH has base strength (H–) above 26.5 and shows high catalytic activity for the Knoevenagel condensation reaction at room temperature.[16] On the basis of the aforementioned ideas, we aimed to investigate the structure and basicity of Mg–Al hydrotalcite after modification with a series of potassium salts.

The Knoevenagel condensation is a versatile C–C bond-forming reaction between a carbonyl compound and an active hydrogen compound to form an unsaturated compound, which can lead to the formation of various chemically and biologically important intermediates, such as α-cyanocinnamates, α,β-unsaturated esters, α,β-unsaturated nitriles, cinnamic acid, etc. These intermediates are commonly used for the production of perfumes, cosmetics, drugs, antihypertensives, calcium antagonists, polymers, fine chemicals, pharmaceuticals, etc. Kantam et al. reported a modified method for activation of MgAl hydrotalcite and applied them for the quantitative formation of the Knoevenagel condensation product in liquid phase.[17] In another report, Ebitani et al. showed that reconstructed hydrotalcite is more active than untreated hydrotalcite and provides a unique acid–base bifunctional surface capable of promoting the Knoevenagel and Michael reactions.[18] They have also found that reconstructed hydrotalcite gives three times more yield than untreated hydrotalcites for aldol and Knoevenagel condensation reactions. Similarly, modified Mg–Al hydrotalcite with tert-butoxide anion, layered double hydroxide fluoride, and hydrotalcite in ionic liquid medium are reported as efficient catalysts for this reaction.[19] Garcia et al. and Čejka et al. have recently reviewed varieties of effective catalysts, such as cation-exchanged zeolites,[20] metal-organic frameworks,[21−24] mesoporous molecular seives,[25] etc., for the Knoevenagel condensation reaction. Other types of solids like ZnO, MgO, alumina, potassium carbonates, zeolites, modified zeolites, natural phosphates, etc. are reported as potential catalysts for this reaction.[26] These are considered as more benign in comparison to traditionally used alkaline hydroxides.[27]

In this work, we have impregnated a series of potassium salts, i.e., KNO3, KF, KOH, K2CO3, and KHCO3, over calcined Mg–Al hydrotalcite (3:1 ratio), studied the effect of base precursors on the structure and basicity of the host, and finally employed for the Knoevenagel condensation reaction.

Results and Discussion

Powder X-ray diffraction (XRD) patterns of parent hydrotalcite and potassium salt-loaded hydrotalcites are presented in Figure . Reflections at 2θ values of 11.5, 23.05, 34.7, 38.35, 45.95, 60.5, and 61.75° corresponding to (003), (006), (102), (105), (108), (110), and (113) planes indicate the formation of highly crystalline layered structure of hydrotalcite, which corroborates well with literature reports.[28] Again all potassium salt-loaded samples show strong peaks for (003) and (006) planes, which clearly shows the presence of hydrotalcite phase; this confirms rehydration and reconstruction of hydrotalcite phase after loading. However, shifting of 2θ positions to slightly higher or lower values and diminished intensity of loaded samples can be attributed to the presence of potassium salts on hydrotalcite structure. It has been observed from Figure that the salts KOH, KHCO3, and K2CO3 are well dispersed over the support, whereas KF- and KNO3-loaded samples showed some additional diffraction lines in the XRD pattern. Hence, preferred catalyst can be chosen among KOH, KHCO3, and K2CO3 in terms of phase stability. This observation was further investigated through crystallinity study, which did not follow the similar trend. The crystallinity study revealed that the highest crystallinity after loading was achieved for KOH/HT and the lowest was for KHCO3/HT (Figure S1, Supporting Information). It shows that although KHCO3 disperses better than KF and KNO3 onto the host, it decreases the crystallinity of the overall catalyst. Thus, crystallinity loss is dependent on the nature of the salt but not on dispersion. Thus, observing both crystallinity and phase stability, we can observe that KOH/HT is a stable and highly crystalline catalyst among KOH, KHCO3, and K2CO3. Therefore, KOH/HT is chosen as the preferred salt from XRD. We have also calculated crystallite sizes, unit cell parameters (a), and basal spacing between the layers (d), which are summarized in Table . Peaks for (003) and (006) reflections were considered to calculate basal spacings between the layers. Peak for (110) reflection was used to calculate unit cell parameter “a” according to the formula a = 2d, whereas the (003) plane was used to calculate “c” according to the formula c = 3d.[29] Comparison of the original hydrotalcite to the loaded hydrotalcites shows that the basal spacings and unit cell parameters increase after loading. Increase of basal spacings and unit cell parameters further supports increase of Mg (+2) ions gradually in the LDH, indicating interaction of some Al (+3) ions with potassium salts and decrease of Coulombic interaction between interlayer anions and brucite-like layers.[30,31] The increases of both “d” and a are larger for KOH/HT, K2CO3/HT, and KNO3/HT (d003 = 7.80–7.93; d006 = 3.88–3.91; a = 3.06–3.07) than for KF/HT and KHCO3/HT (d003 = 7.70–7.73; d006 = 3.85–3.86; a = 3.05–3.06).

Figure 1

Powder X-ray diffraction pattern of potassium-loaded hydrotalcites.

Table 1

Calculation of Lattice Parameter and Basal Spacings for Potassium Salt-Loaded Hydrotalcites

sample	(003) reflection, 2θ (deg)	d003 (Å)	(006) reflection, 2θ (deg)	d006 (Å)	(110) reflection, 2θ (deg)	d110 (Å)	a (Å)	c (Å)	crystallite size (003)
HT	11.50	7.70	23.05	3.85	60.50	1.53	3.06	23.10	128.09
KF/HT	11.50	7.70	23.00	3.86	60.60	1.52	3.05	23.10	135.56
KHCO3/HT	11.45	7.73	23.10	3.85	60.55	1.52	3.05	23.19	153.65
K2CO3/HT	11.35	7.80	22.70	3.91	60.35	1.53	3.06	23.40	135.70
KNO3/HT	11.35	7.80	22.90	3.88	60.20	1.53	3.07	23.40	164.93
KOH/HT	11.15	7.93	22.85	3.89	60.35	1.53	3.06	23.79	82.31

Thus, a stability order of the salts over the support can be understood from overall results of X-ray diffraction study, which follows the trend KOH/HT ∼ K2CO3/HT > KHCO3/HT > KF/HT ∼ KNO3/HT. Hence, among the studied salts over hydrotalcite support, KOH is chosen as the preferred salt. Smaller crystallite size of KOH/HT in comparison to the others is again in favor of its selection as preferred catalyst for base-catalyzed reaction.[32] Following these results, we have loaded KOH amount of 15–40% (w/w) by identical impregnation method to gain better understanding of the effect of the salt over the support.

Figure shows that hydrotalcite layered structure preserves up to 20% loading of KOH and collapses above it, where the reflection for the (003) plane was completely destroyed for 25–40% loaded samples. However, crystallinity loss was quite higher for 15–40% loaded samples. When 15% KOH was loaded, crystallinity loss was increased 7 times more in comparison to 10% loaded sample. Therefore, KOH loading beyond 10% is believed to be not effective over hydrotalcite support. Hence, 10% KOH/HT has been conceded as the best catalyst in this study.

The thermogravimetric analysis (TGA) and differential thermal analysis (DTA) results obtained from thermal analysis of uncalcined samples in the temperature range of 20–500 °C shows four decomposition steps, which are typical for hydrotalcite-like compounds (Figure S2, Supporting Information). The first weight loss step in the temperature range 20–90 °C corresponds to the loss of physically adsorbed water on the surface, which is found to be in the range of 1–12% of the total weight loss (Table S1, Supporting Information). In the second step, 12–15% weight loss takes place in the range of 80–250 °C due to the loss of interlayer water molecules. We have observed that the weight loss steps of all loaded samples are similar to the parent hydrotalcite, except the step for interlayer water loss. For HT, KF/HT, and KNO3/HT, interlayer water loss takes place only in a single step, whereas it takes place in several steps for KOH/HT, KHCO3/HT, and K2CO3/HT. This shows that the water molecules may be linked in different environments in the presence of different guest molecules. In the third step i.e., 250–415 °C, dehydroxylation and partial loss of carbon dioxide take place, showing 13–22% weight loss. It is noteworthy that dehydroxylation becomes slower after loading of the salts and collapse of the brucite layers starts at low temperature in comparison to the parent hydrotalcite. Besides, dehydroxylation is the fastest in case of KNO3/HT and the slowest in case of KOH/HT, which in turn confirms that the HT phase is thermally less stable in the presence of KNO3 salt and more stable in the presence of KOH. In the fourth weight loss step, loss of carbon dioxide from the samples takes place in the temperature range of 395–518 °C, showing a weight loss of 2–4%. Thus, TGA analysis reveals that the hydrotalcite layer collapses earlier for KNO3/HT and KF/HT in comparison to the other three, i.e., KOH/HT, KHCO3/HT, and K2CO3/HT, which gives thermal stability order of the loaded salts as KNO3/HT < KF/HT < KHCO3/HT < K2CO3/HT < KOH/HT. Thus, TGA analysis also reveals that KOH/HT is the most thermally stable catalyst and the preferred catalyst in our study.

The Fourier transform infrared (FTIR) spectra of prepared hydrotalcites are presented in Figure . Characteristic bands for hydrotalcite at around 3450, 1640, 1380, 860, 661, and 510 cm–1 clearly indicate the presence of interlayer water molecules, hydroxyl groups in the brucite-like layers, carbonate ions in the interlayer galleries, and Mg–O and Al–O bonds in all samples. Again, bands at around 3600–2800 cm–1 for KOH/HT, K2CO3/HT, and KHCO3/HT samples are broader compared to KNO3/HT and KF/HT, which may be due to the high water content of these samples in the surface as well as in the interlayer spaces. This again correlates well with thermogravimetric analysis of the samples, confirming high water content of these samples in the interlayer spaces.

Figure 2

FTIR patterns of hydrotalcites loaded with potassium salts.

Nitrogen adsorption–desorption measurements were carried out to investigate the surface areas, pore sizes, and pore volumes of the materials. Table shows textural properties of the parent hydrotalcite and potassium salt-loaded hydrotalcites. The parent hydrotalcite exhibits a Brunauer–Emmett–Teller (BET) surface area of 207 m2 g–1. Introduction of potassium species decreases the BET surface area of all of the samples. The surface area of hydrotalcite loaded with K2CO3 is lost to a greater extent compared to the other four salts and decreases up to 90 m2 g–1. This is due to the blockage of pores and interlayer spaces of hydrotalcite by CO32– anions of K2CO3 salt, thus preventing N2 molecules to adsorb onto the surface. This observation has been further confirmed by the pore volume calculation of the samples, which shows that the pore volume of K2CO3/HT is the smallest and decreases from 0.23 cm3 g–1 of parent hydrotalcite to 0.19 cm3 g–1 of the loaded one. On the other hand, surface areas of KNO3/HT and KF/HT are 201 and 197 m2 g–1, respectively. This can be expected for low water content of these materials, as described in TGA and DTA analysis. We have found intermediate values of surface areas for the samples KOH/HT and KHCO3/HT. This can be attributed to the high water content in these samples. On the other hand, the KOH/HT sample shows the largest pore volume and pore diameter with comparable surface area to the parent hydrotalcite. Thus, surface areas and pore sizes of loaded hydrotalcites other than K2CO3 salt are not abruptly affected after loading the salts. The N2 adsorption–desorption measurements showed that all hydrotalcites exhibit type II isotherm and H3 hysteresis loop characteristics of both monolayer and multilayer adsorption typical for aggregated powders like clays or cements having no uniform pore structures (Figure S3, Supporting Information).[33,34] Type H3 hysteresis loops are generally found for nonrigid aggregates of platelike particles or assemblages of slit-shaped pores in the sample. It has again been confirmed from the pore size distribution curves of the samples, which show broad range of pore sizes in the samples. However, narrow pore size distribution centered at a pore radius of 20 Å and large pore volume of KOH/HT has led us to select it as the optimum sample in this study.

Table 2

Textural Properties of Potassium Salt-Loaded Hydrotalcites

entry	sample	BET area (m2 g–1)	pore volume (cm3 g–1)	pore diameter (nm)	base strengths (H–)	total basicity (mmol g–1)	soluble basicity (mmol g–1)
1	HT	207	0.23	3.30	9.6 < H– < 11.1	0.12	0.00a
							0.00b
2	KF/HT	197	0.32	3.38	12.7 < H– < 15	0.18	0.02
3	KHCO3/HT	184	0.25	3.95	9.6 < H– < 11.1	0.18	0.03
4	K2CO3/HT	90	0.19	3.99	12.7 < H– < 15	0.23	0.05
5	KNO3/HT	201	0.32	3.66	12.7 < H– < 15	0.19	0.00
6	KOH/HT	185	0.36	4.00	12.7 < H– < 15	0.22	0.03

Hydrotalcite.

Reconstructed hydrotalcite obtained by immersion of MgAl mixed oxide in water for 24 h.

The scanning electron microscopy (SEM) images of parent hydrotalcite and KOH/HT in magnification of X5500 and X4000 are shown in Figure . The parent hydrotalcite has crystal sizes in nanometer ranges with homogeneous shapes of the crystals. On loading KOH onto it, particle size decreases, showing layers of agglomerated sheets in the range of 1–2 μm.

Figure 3

Scanning electron micrographs of (a1, a2) HT and (b1, b2) KOH/HT at two different resolutions.

The results of base strengths of all hydrotalcites, which were determined by the Hammett indicator method, are summarized in Table . Hydrotalcites loaded with KF, K2CO3, KNO3, and KOH after drying at 80 °C for 15 h showed color change in the presence of Tropaeolin O (H = 11.1–12.7) indicator and could not change the color of 2,4-dinitroaniline (H = 15). Therefore, they showed similar base strength in the range of 12.7 < H < 15. On the other hand, the unloaded parent hydrotalcite and KHCO3/HT showed color change with phenolphthalein (H = 8.0–9.6), whereas no change was observed with Tropaeolin O (H = 11.1–12.7). Thus, their base strengths lie in the range of 9.6 < H < 11.1. Total basicity measurement of the samples shows that the number of total basic sites for K2CO3/HT and KOH/HT are higher than the other samples. This is reflected in the catalytic activities shown in Table that the conversions for KOH/HT and K2CO3/HT are higher than the other catalysts. On the other hand, the total number of basic sites of all of the catalysts is relatively low compared to calcined hydrotalcites reported in the literature. However, it is quite obvious from the results that although the number of total basic sites is not quite high, uncalcined hydrotalcites also can fairly catalyze the Knoevenagel condensation reaction with its moderate base strengths. Thus, loading of alkali metal compounds over hydrotalcites would be acknowledged to catalyze some mild base-catalyzed organic reactions, such as the Knoevenagel condensation reaction, nitroaldol condensation reaction, aldol condensation, etc. Soluble basicities of all of the samples are low, where no soluble base was found at all for KNO3/HT. This again indicates more interaction of KNO3 with host hydrotalcite structure, thus preventing their loss as soluble base.

Table 4

Knoevenagel Condensation Reaction with Different Potassium Salt-Loaded Hydrotalcites at Room Temperaturea

entry	sample	time (min)	% conversionb	% selectivityb
1	no catalyst	30	0	0
2	HTc	30	41	90
3	KNO3/HT	30	57	91
4	KF/HT	30	42	90
5	K2CO3/HT	30	63	69
6	KOH/HT	30	66	81
7	KHCO3/HT	30	61	76
8	KOHd	30	0	0

Conditions: p-nitrobenzaldehyde (1 mmol), malononitrile (1 mmol), methanol (3 mL), catalyst amount: (25 mg, 8 w % of potassium ion), reaction temperature: room temperature.

Obtained from 1H NMR analysis of the crude reaction mixture.

Reconstructed hydrotalcite obtained by immersion of MgAl mixed oxide in water for 24 h.

Reaction with KOH salt (8 wt % of 25 mg).

We again calculated the reactivity of the O atoms using density functional-based reactivity descriptor, the Fukui function. Fukui functions, fO+ and fO–, are evaluated using Hirshfeld population analysis (HPA) and Mulliken population analysis (MPA) schemes to locate the nucleophilic and electrophilic sites, respectively. Although an analytical expression for the Fukui function is not available, it is usually calculated by finite difference approximation, which is called the condensed Fukui function. The condensed Fukui function of an atom “O” in a molecule with N electrons at constant external potential v(r⃗) can be expressed aswhere ρO(No), ρO(No + ΔN), and ρO(No + ΔN) are charge densities on atom O of the system with No, No + ΔN, and No – ΔN electron systems, respectively. In conventional Fukui function computations, a value of 1.0 is used for ΔN. In the present calculation, we have used a value of 0.1 for ΔN. The values of Fukui functions are given in Table . We have calculated the Fukui functions (fO+ and fO–), relative electrophilicity (fO+/fO–), and relative nucleophilicity (fO–/fO+) for those oxygen atoms in the hydrotalcite system having higher values of these reactivity parameters. In general, it is observed that for a particular atom in a molecule, the increase in fO– values is the indication of high basicity. From Table , it has been seen that the highest fO– value is obtained for the oxygen atom number 38, which is the one attached to the potassium atom. Relative electrophilicity (fO+/fO–) and relative nucleophilicity (fO–/fO+) are better reactivity parameters to locate the preferable site for nucleophilic and electrophilic attacks, respectively, in a chemical system.[35,36] The basicity of a system increases with the increase of the fO–/fO+ ratio. Thus, the oxygen attached to the potassium having the highest value of relative nucleophilicity (fO–/fO+) will be the most basic site. From this calculation, we have inferred that the O atom attached to the K atom has highest values of fO– as well as fO–/fO+. Therefore, we can conclude that O 38 is the most basic site in the hydrotalcite system. The optimized structure of the hydrotalcite is shown in Figure . The oxygen atoms for which the reactivity parameters have been evaluated are marked, and these are the atoms having higher basic character in the system.

Table 3

Values of Fukui Functions with Respect to Mulliken and Hirshfeld Charges of the Basic Oxygen Atoms of the Metal Oxide

	Fukui function (fO+)		Fukui function (fO–)		relative electrophilicity (fO+/fO–)		relative nucleophilicity (fO–/fO+)
atom	MPA	HPA	MPA	HPA	MPA	HPA	MPA	HPA
O 38	0.026	0.025	0.019	0.027	1.37	0.93	0.73	1.08
O 65	–0.001	0.006	0.002	0.005	–0.50	1.20	–2.00	0.83
O 69	0.021	0.022	0.021	0.022	1.00	1.00	1.00	1.00
O 80	0.004	0.006	0.002	0.006	2.00	1.00	0.50	1.00
O 83	0.017	0.019	0.017	0.020	1.00	0.95	1.00	1.05

Figure 4

Optimized structure for the most stable geometry of Mg3Al(OH)8KOH. The green balls represent magnesium, pink balls represent aluminum, purple balls represent potassium, red balls represent oxygen, and gray balls represent hydrogen atoms in the optimized geometry. The oxygen atoms having larger values of f(−) (higher basicity) are numbered, and the bond lengths are in angstrom.

The catalytic activities of the parent and potassium salt-loaded hydrotalcites were evaluated for liquid-phase Knoevenagel condensation reaction (Scheme ) at room temperature with a variety of aldehydes. The conversion of reactants to the products in an organic reaction is mainly influenced by parameters such as solvent, temperature, effect of substituted groups in the substrates, amount of catalysts, etc. Taking these points into consideration, we have optimized the reaction conditions by varying the conditions. First, the reaction was performed at room temperature without any catalyst by taking malononitrile and p-nitrobenzaldehyde as model reactants and methanol as solvent. To our expectation, no formation of the product was observed. Therefore, we have performed all other reactions in the presence of catalysts, i.e., with modified hydrotalcites and rehydrated hydrotalcite under the same reaction conditions. The results are summarized in Table . Reaction with rehydrated hydrotalcite is comparatively slower than with the loaded hydrotalcite (Table , entry 2). It has been observed that the reaction in methanol is not selective with all of the catalysts and conversions are lower than reported methods. However, KNO3/HT and KF/HT (Table , entries 3 and 4) catalysts show better results than the other catalysts in terms of selectivity and it can be correlated to their larger surface areas than the other loaded catalysts. This can be correlated again from TGA results described above that due to low water content in KNO3/HT and KF/HT, aprotic environment of the catalyst makes the aldol-type intermediate dehydrate easily, thus showing better selectivity of the Knoevenagel product compared to KOH/HT, KHCO3/HT, and K2CO3/HT (Table , entries 5–7), where more interlayer water content of these catalysts offers protic environment in the catalysts, thus stabilizing the intermediate aldol-type product before the dehydration step. However, low conversions of the product with KNO3/HT and KF/HT catalysts can be attributed to the interaction of the salts with the support forming new phases and thus reducing active sites of the catalysts (XRD). On the other hand, the preferred catalyst of this study, i.e., KOH/HT (Table , entry 7), showed the highest conversion, largest pore volume, longest pore diameter, and good selectivity within 30 min and therefore the reaction was further carried out with this catalyst to optimize the reaction conditions. High conversion of KOH/HT is also supported by its small crystallite sizes (Table ), which favors the active hydroxyl groups to take part in the reaction.[37]

Scheme 1

Following this study, we have investigated the effect of solvents with 10% KOH/HT at room temperature by taking six different solvents of different polarities. It is observed that solvents play a significant role on both conversions and selectivities. When aprotic polar solvents were used (Table , entries 2, 4, and 5), 81–99% conversion was observed giving 100% selectivity of the product within 15 min. On the other hand, when protic polar solvent methanol was used, the reaction was slow and both conversion and selectivity was poor (Table , entry 3). Nonpolar solvents like toluene and diethyl ether (entries 1 and 6) take longer reaction time than polar solvents, giving 61–99% conversion and 100% selectivities within hours. It is noteworthy to mention that in this study dimethylformamide (DMF) is superior to the most commonly used solvent toluene for the Knoevenagel condensation reaction in the presence of hydrotalcite catalysts. It can be attributed to the fact that the reactants are miscible well in polar environment and the catalyst mixes homogeneously in the reaction mixture during vigorous stirring condition. Thus, interaction of the catalyst with reactants becomes feasible in DMF compared to nonpolar solvents toluene or diethyl ether. Therefore, it is clear from Table that DMF is the best choice in terms of conversion, selectivity, and reaction time, and 10% KOH/HT can be chosen as the optimum catalyst with DMF.

Table 5

Effect of Various Solvents on the Knoevenagel Condensation Reaction at Room Temperaturea

entry	solvent	time	% conversionb	% selectivityb
1	toluene	40 min	99	100
2	DMF	15 min	99	100
3	MeOH	30 min	66	81
4	acetonitrile	15 min	81	100
5	DCM	15 min	99	100
6	diethyl ether	4 h	61	100

Conditions: p-nitrobenzaldehyde (1 mmol), malononitrile (1 mmol), solvent (3 mL), catalyst (25 mg); catalyst: 10% KOH/HT.

Obtained from 1H NMR yield of the crude reaction mixture.

We next used 10% KOH/HT for the Knoevenagel condensation reaction with various aldehydes bearing electron-donating and electron-withdrawing groups, active methylene compounds (malononitrile and diethyl malonate), and DMF at room temperature. The results are summarized in Table . The Knoevenagel condensation reaction has been reported previously with hydrotalcite-like catalysts, such as rehydrated Mg–Al hydrotalcite and metal-loaded Mg–Al hydrotalcite in different reaction media and optimized reaction conditions.[38] At this time, rehydration and loading have been done in the same step and found an efficient catalyst for this reaction. Aldehydes with both electron-donating and electron-withdrawing groups reacted efficiently with malononitrile under similar reaction condition to give 99% conversions and 100% selectivity of the corresponding olefins (entries 5 and 7). Aldehydes containing electron-withdrawing groups in ortho and para positions do not have much effect over the reaction time (Table , entries 2 and 5) and reacted faster than aldehydes bearing electron-donating groups (entries 7 and 8). On the other hand, unloaded catalyst takes comparatively longer reaction time than loaded catalysts and gives 99% conversion and 100% selectivity within 1.5 h (Table , entry 3). It may be due to the presence of potassium salts and improved basicity of the loaded catalysts, thus completing the reaction within short time. Heterocyclic aldehyde, i.e., 2-furaldehyde, also reacted faster, giving 99% conversion within 15 min. However, reactions with aliphatic aldehydes and aldehydes with big molecular size are comparatively slower than simple aldehydes showing moderate yield (52–65%, entries 12–13) within 5 h. With diethyl malonate as active methylene compound, the acidity of the acidic protons decreases due to two ester groups, thereby increasing the reaction time for all substrates and giving 70–90% conversion and 100% selectivity within 4–8 h. Polycyclic aromatic aldehydes, such as 1-naphthaldehyde, also reacted efficiently, giving high conversion to the product. It is observed that uncalcined KOH/HT with base strength in the range of 12.7 < H < 15 shows comparable results to calcined mixed oxide catalyst, i.e., 21 wt % MgO–ZrO2[39] and 10.3 wt % K–MgAl(O),[40] having base strength 26.5 ≤ H < 33.0 (Table , footnotes c and d). This can be understood as high BET surface area of KOH/HT compared to other two made the reaction comparable to each other. We have also evaluated the efficiency of the catalyst by performing the reaction between malononitrile and 4-chlorobenzaldehyde for four repetitive cycles. It is observed that the catalyst is active even at the fourth cycle of the reaction, which gives 99% conversion and 100% selectivity within 15 min (Table , entry 20). On the whole, potassium hydroxide-loaded MgAl hydrotalcite acts as an efficient solid base catalyst for the Knoevenagel condensation reaction at room temperature.

Table 6

Knoevenagel Condensation Reaction of Different Aldehydes and Active Methylene Compounds with 10% KOH/HTa

entry	R	X	Y	time	% conversionb	% selectivityb
1	Ph	CN	CN	30 min	99	100
2	4-NO2C6H4	CN	CN	15 min	99	100
				15 min	97c
				10 min	99d
3	4-NO2C6H4	CN	CN	40 min	99e	100
4	4-NO2C6H4	CN	CN	90 min	99f	100
5	2-NO2C6H4	CN	CN	15 min	99	100
6	4-ClC6H4	CN	CN	10 min	99	100
7	4-CH3C6H4	CN	CN	60 min	99	100
8	4-OHC6H4	CN	CN	60 min	99	100
9	1-naphthyl	CN	CN	60 min	99	100
10	2-furyl	CN	CN	15 min	99	100
11	propionyl	CN	CN	90 min	99	100
12	isobutyl	CN	CN	5 h	52	100
13	cinnamic	CN	CN	5 h	65	100
14	4-NO2C6H4	COOEt	COOEt	4 h	99	100
15	2-NO2C6H4	COOEt	COOEt	4 h	99	100
16	4-ClC6H4	COOEt	COOEt	4 h	99	100
17	4-CH3C6H4	COOEt	COOEt	8 h	82	100
18	4-OHC6H4	COOEt	COOEt	8 h	77	100
19	1-naphthyl	COOEt	COOEt	8 h	68	100
20	4-ClC6H4	CN	CN	15 min	99g	100

Conditions: aldehyde (1 mmol), active methyl compound (1 mmol), DMF (3 mL), 10% KOH/HT (25 mg).

Obtained from 1H NMR yield of the crude reaction mixture.

Aldehyde (2 mmol), active methylene compound (2 mmol), 21 wt % MgO–ZrO2 (20 mg), DMF (1 mL).

Aldehyde (2 mmol), active methylene compound (2 mmol), 10.3 wt % K–MgAl(O) (20 mg), DMF (1 mL).

Reaction with rehydrated hydrotalcite.

Reaction with as-prepared hydrotalcite after drying at 80 °C for 15 h.

Fourth run with recovered catalyst.

Following this, the leaching of potassium species into solution was tested by flame photometry study. For this purpose, the reaction was performed with 2 mmol p-nitobenzaldehyde, 2 mmol malononitrile, and 100 mg of 10% KOH/HT in DCM solvent. After completion of the reaction, 10 mL of water was added to dissolve the leached out potassium and to separate organic portion. We observed that 0.355 mg of K was leached out from 100 mg of catalyst, which is quite low compared to the loading amount. Thus, leaching of a small amount of potassium does not affect the overall reaction speed and conversion.

Conclusions

In conclusion, we have found that KOH-loaded MgAl hydrotalcite prepared through wet impregnation of potassium salts over calcined MgAl hydrotalcite is the most stable while KNO3-loaded hydrotalcite is the least stable among a variety of potassium salt-loaded MgAl hydrotalcites, i.e., KF, KOH, KNO3, KHCO3, and K2CO3. It was found that 10% (w/w) KOH/HT is the best catalyst and DMF is the best solvent in the study and catalyzes the reaction efficiently giving 99% conversion and 100% selectivity within 10 min. It is also concluded that polar aprotic solvents like DMF and acetonitrile speed up the reaction, whereas nonpolar solvents slower the reaction.

Experimental Section

Preparation of MgAl Hydrotalcite and MgAl(O) Mixed Oxide

Magnesium aluminum carbonate (MgAl-CO3) hydrotalcite was prepared according to the procedure reported by Nyambo et al.[41] In this method, solution A containing 38.46 g of Mg(NO3)2·6H2O (0.15 mol) and 18.75 g of Al(NO3)2·9H2O (0.05 mol) in 125 mL of deionized water was added dropwise over 1 h to solution B containing 14 g of NaOH (0.35 mol) and 15.9 g of Na2CO3 (0.1 mol) in 145 mL of deionized water at pH 10–12 with vigorous stirring. The solution was kept stirring vigorously for another 1 h and the white precipitate formed was aged without stirring for 24 h at 65 °C, cooled to room temperature, filtered, and washed several times with deionized water until the filtrate becomes neutral. Finally, the precipitate was dried at 80 °C for 15 h and then calcined at 450 °C for 6 h to obtain MgAl(O) support.

Preparation of Potassium Salt-Loaded MgAl Hydrotalcites

The potassium salt-modified hydrotalcites (HTs) containing same amount of potassium ions (8% w/w) were prepared by a wet impregnation method reported earlier. In this method, a solution of the metal salt containing 1 mmol of the salt (except for K2CO3, where 0.5 mmol was taken) in 8 mL of deionized water was stirred with 500 mg of calcined hydrotalcite for 24 h and the slurries were dried at 80 °C for 15 h to obtain the loaded catalysts. The samples were denoted as KNO3/HT, KOH/HT, K2CO3/HT, KHCO3/HT, and KF/HT. Following the same procedure, 10–40% KOH/HT was prepared by taking appropriate quantity of KOH and the support.

Theoretical Calculations

For the theoretical part, density functional calculations were performed using DMol3 program package,[42] as implemented in the Materials Studio program system.[43] Geometry optimization was done by treating the exchange–correlation interaction with generalized gradient approximation (GGA) using the Perdew, Burke, and Ernzerhof (PBE) functional.[44] We used the double numerical with polarization[1] basis set for our calculations. The PBE functional was demonstrated to be reliable for predicting structures of inorganic oxides and it is one of the most universally applied GGA functionals.[45]

Characterization

Powder X-ray diffraction patterns were recorded on a Rigaku (MiniFlex, U.K.) X-ray diffractometer with Cu Kα radiation (1.5418 Å) at a scan speed of 7° min–1 and 2θ range of 5–70° at 30 kV and 15 mA. The percentage crystallinities of the samples were determined by integrating XRD peaks and using the formula, % crystallinity = (AS × 100)/AR, where AR is the integrated area of the reference material under the peaks between a set of 2θ limits and AS is the integrated area of the sample under the peaks between the same set of 2θ limits as that of the reference. The crystallite sizes were determined by X-ray line broadening method considering 003 reflection plane and using the Debye–Scherrer equation (t = 0.89/β cos θ, where “t” is the crystallite size, “β” is the full width at half-maximum, and “θ” is the angle of diffraction).[37] FTIR spectra of various catalysts were recorded on a Nicolet Impact model-410 spectrometer with 1 cm–1 resolution and 32 scans in the mid-IR (400–4000 cm–1) region using the KBr pellet technique. The SEM measurements were carried out using a JEOL JSM-6390LV scanning electron microscope. Thermogravimetric analysis was performed using a Shimadzu thermogravimetric analyzer (TGA-50) in the temperature range of 25–600 °C at a heating rate of 10 °C min–1, and differential scanning calorimetric analysis was performed using a Shimadzu differential scanning calorimeter (DSC-60) in the temperature range of 25–300 °C at a heating rate of 10 °C min–1. Both analyses were performed under nitrogen atmosphere using 10–15 mg of sample. The specific surface areas were determined from N2 adsorption–desorption isotherms obtained by the Brunauer–Emmett–Teller method using a Quantachrome NOVA 1000e surface area and pore size analyzer at −196 °C. Pore size distributions were calculated using the Barrett–Joyner–Halenda equation from the amount desorbed at a relative pressure of about 0.05–1 using N2 desorption branches of the isotherm. Flame photometry analysis was performed in Systronics Flame Photometer 128.

The strengths of basic sites were determined by the Hammett indicator method, following the procedure reported by Fraile et al.[46] The following Hammett indicators were used: neutral red (H = 6.8–8.0), phenolphthalein (H = 8.0–9.6), Tropaeolin O (H = 11.1–12.7), and 2,4-dinitroaniline (H = 15). In this procedure, 25 mg of loaded catalyst was shaken with 1 mL of indicator solution (0.1% in methanol) and allowed to equilibrate for 2 h. The color of the catalyst was then noted. The base strength was reported as stronger than the weakest indicator, which exhibits a color change, and weaker than the strongest indicator, which exhibits no color change.[47] Total basicities of the samples were determined by titration with benzoic acid. In this method, a suspension of the loaded hydrotalcite (0.15 g) in a toluene solution of phenolphthalein (2 mL, 0.1 mg mL–1) was stirred for 30 min and titrated with a toluene solution of benzoic acid (0.01 M). To determine the leachable basicities, 100 mg of loaded hydrotalcite was added to 10 mL water and the mixture was shaken well at room temperature for 1 h. The catalyst was filtered off, and a methanol solution of phenolphthalein (1 mL, 0.1 mg mL–1) was added to the filtrate, which was then titrated with a methanol solution of benzoic acid (0.01 M).

Catalytic Reactions

The Knoevenagel condensation reaction was carried out in a 50 mL round-bottom flask at room temperature. The catalysts were oven-dried at 80 °C for 12 h prior to use. In the typical procedure, 0.025 g of catalyst was added to a mixture of 1 mmol aldehyde, 1 mmol active methylene compound, and 3 mL of DMF at room temperature. The whole mixture was stirred for the required time and monitored by thin-layer chromatography. After completion of the reaction, DMF was evaporated and the product was extracted with ethyl acetate, dried over anhydrous Na2SO4, evaporated, purified, and finally analyzed using a 1H NMR spectrometer (JEOL JNM-ECS 400) taking Me4Si as the internal standard and CDCl3 as solvent. The used catalysts were washed several times with acetone, dried, and then reused. The conversions (%) were determined from integration of 1H NMR signal of the crude reaction mixtures.