Knoevenagel Condensation Of Benzaldehyde Research Articles

Studies of the Solvent-Free Knoevenagel Condensation over Commercial NiO compared with NiO Drived from Hydrotalcites

In this study, we compared the effect of the commercial NiO, synthesis NiAl-HT and NiO-HT drived from hydrotalcite in Knoevenagel condensation reaction. The NiAl-HT sample was synthesized by the coprecipitation method with a molar ratio M2+/M3+ = 2 at constant basic pH. X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) were utilized to identify crystalline phases present in NiAl-HT, NiO-HT and commercial NiO. The chemical composition of the obtained solids was determined by Atomic Absorption Spectroscopy (AAS). Other techniques, such as Thermogravimetric Thermal Analyzer (TGA), Scanning Electron Microscopy (SEM) and Brunauere Emmette Teller Method (BET) were also used. As well as the BET showed the increase of the specific surface for the solid NiO-HT. The performance of the catalysts were studied in Knoevenagel condensation of benzaldehyde with ethyl acetoacetate without solvent to synthesis of organic compounds such as intermediates of dihydropyridines derivatives. The influence of different parameters, such as catalyst amount, reaction temperature and reaction time were optimized for studied the activity, the selectivity and the stability of the solids. Catalytic activity was in its lowest in the presence of NiAl-HT (26% of benzaldehyde conversion) whereas the benzaldehyde conversion increased to 77% in case of NiO-HT which can be explained by the presence of the basic sites of the NiO-HT oxides, a high surface area and a small crystallite size. Therefore, the lower increase in benzaldehyde conversion was noticed using commercial NiO (84%), perhaps owing to its high purity. A reaction mechanism is proposed by using density functional method (DFT). Copyright © 2023 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

Aminoguanidine Derived N‐Rich Mesoporous Carbon Nitrides with Tunable Nitrogen Contents for Knoevenagel Condensation

AbstractNitrogen‐rich carbon nitrides are desired materials for base‐catalysed transformations; however, their synthesis is challenging due to the volatile nature of N at high temperatures. Herein, we report on the temperature‐controlled synthesis of ordered N‐rich mesoporous carbon nitrides (MCNs) via pyrolysis of aminoguanidine by using SBA‐15 as a hard template. The properties and the nitrogen content of the materials were tuned by varying the carbonization temperature in the range of 350–500 °C. At 350 and 400 °C, higher amounts of N could be retained in the MCN framework with the predominant formation of C3N6 having a six‐membered aromatic ring with diamino‐s‐tetrazine moiety, whereas C3N5 with 1‐amino/imino‐1,2,4‐triazole moieties was produced at 450 and 500 °C. The base catalytic activity of MCNs in Knoevenagel condensation of benzaldehyde with malononitrile revealed that the MCN‐400 exhibited the highest catalytic performance by displaying a 96.4 % product yield with toluene as a solvent. The superior catalytic activity of MCN‐400 is attributed to high N content (62.6 wt%), high surface area (235 m2 g−1), and large pore volume (0.74 cm3 g−1). The optimum temperature for obtaining the highest yield of the products is 80 °C, and the catalyst showed good cycling stability for 5 consecutive cycles.

A Continuous-Flow Microreactor for Knoevenagel Condensation of Ethyl Cyanoacetate with Benzaldehyde: The Effect of Grafted Amino Groups on Catalytic Activity.

Ethyl α-cyanocinnamate was synthesized in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate in flow monolithic microreactor of 0.63 cm3 volume. The catalytically active core was made of silica monolith modified with various amine group precursors. Structural properties of the support, surface density of NHx groups, and catalytic activity were investigated. It was found that the poly- or di-amine groups attached to the silica surface appeared to be more effective than the aminopropyl groups. Microreactors grafted with diamine functional groups, accompanied by hydrophobic methyl groups, showed the highest activity and stability. It was proved that the decisive role on the activity of catalysts was exerted by the presence of primary amines in diamine chain. The reaction conditions were optimized and it was found that almost full substrate conversion could be achieved in 6 min at 50 °C in the microreactor with low concentration of diamine groups equal to 0.33 mmol g-1 .

Study of the catalytic properties of MgNi(Cu)Al LDH in the one-pot cascade oxidation-Knoevenagel condensation reaction

In this work, the structural and bifunctional catalytic properties of MgNi(Cu)Al layered double hydroxides (LDH) with different molar ratios between the cations were studied. The LDH materials were prepared by co-precipitation and were characterized using X-ray fluorescence (XRF), X-Ray diffraction (XRD), diffuse reflectance infrared Fourier-transform spectroscopy (DRIFT), diffuse reflectance ultraviolet-visible spectroscopy (DR-UV-VIS), scanning electron microscopy (SEM), nitrogen adsorption-desorption and acidity-basicity measurements. They were tested in a one-pot cascade oxidation-Knoevenagel condensation reaction used to synthesize 2-Benzoyl-3-phenylprop-2-enenitrile also known as 2-Benzoyl-3-phenylacrylonitrile (BPA). This involved the benzyl alcohol (BnOH) oxidation under an oxygen atmosphere followed by the Knoevenagel condensation of benzaldehyde with 3-oxo-3-phenylpropanenitrile also known as benzoyl acetonitrile (BzACN). Three different non-polar solvents, i.e., toluene, benzene and 1,3,5-trimethylbenzene (mesitylene), were used as reaction media. For the LDH catalysts containing both nickel and copper, the alcohol conversion and BPA yield increased with Ni/Cu molar ratio. The acid-base properties of the solids and the nature of the solvent strongly influence the reaction performance.

Synthesis, crystal structure, DFT, Hirshfeld surface analysis, energy framework, docking and molecular dynamic simulations of (E)-4-(4-methylbenzyl)-6-styrylpyridazin-3(2H)-one as anticancer agent

In this work, a novel crystal, (E)-4-(4-methylbenzyl)-6-styrylpyridazin-3(2H)-one (E-BSP) was synthesized via Knoevenagel condensation of benzaldehyde and (E)-6-(4-methoxystyryl)-4,5-dihydropyridazin-3(2H)-one. The molecular structure of E-BSP was confirmed by using FT-IR, 1H-NMR, 13C-NMR, UV-vis, ESI-MS, TGA/DTA thermal analyses and single crystal X-ray diffraction. The DFT/B3LYP methods with the 6-311++G(d,p) basis set were used to determine the vibrational modes over the optimized structure. Potential energy distribution (PED) and the VEDA 4 software were used to establish the theoretical mode assignments. The same approach was used to compute the energies of frontier molecular orbitals (HOMO-LUMO), global reactivity descriptors, and molecular electrostatic potential (MEP). Additionally, experimental and computed UV spectral parameters were determined in methanol and the obtained outputs were supported by FMO analysis. Molecular docking and molecular dynamics (MD) simulation analyses of the E-BSP against six proteins obtained from different cancer pathways were carried out. The proteins include; epidermal growth factor receptor (EGFR), Estrogen receptor (ERα), Mammalian target of rapamycin (mTOR), Progesterone receptor (PR) (Breast cancer), Human cyclin-dependent kinase 2 (CDK2) (Colorectal cancer), and Survivin (Squamous cell carcinoma/Non-small cell lung cancer). The results of the analyses showed that the compound had less binding energies ranging between −6.30 to −9.09 kcal/mol and formed stable complexes at the substrate-binding site of the proteins after the 50 ns MD simulation. Therefore, E-BSP was considered a potential inhibitor of different cancer pathways and should be used for the treatment of cancer after experimental validation and clinical trial. Communicated by Ramaswamy H. Sarma

Coordination Polymers Constructed from an Adaptable Pyridine-Dicarboxylic Acid Linker: Assembly, Diversity of Structures, and Catalysis.

4,4'-(Pyridine-3,5-diyl)dibenzoic acid (H2pdba) was explored as an adaptable linker for assembling a diversity of new manganese(II), cobalt(II/III), nickel(II), and copper(II) coordination polymers (CPs): [Mn(μ4-pdba)(H2O)]n (1), {[M(μ3-pdba)(phen)]·2H2O}n (M = Co (2), Ni (3)), {[Cu2(μ3-pdba)2(bipy)]·2H2O}n (4), {[Co(μ3-pdba)(bipy)]·2H2O}n (5), [Co2(μ3-pdba)(μ-Hbiim)2(Hbiim)]n (6), and [M(μ4-pdba)(py)]n (M = Co (7), Ni (8)). The CPs were hydrothermally synthesized using metal(II) chloride precursors, H2pdba, and different coligands functioning as crystallization mediators (phen: 1,10-phenanthroline; bipy: 2,2'-bipyridine, H2biim: 2,2'-biimidazole; py: pyridine). Structural networks of 1-8 range from two-dimensional (2D) metal-organic layers (1-3, 5-8) to three-dimensional (3D) metal-organic framework (MOF) (4) and disclose several types of topologies: sql (in 1), hcb (in 2, 3, 5), tfk (in 4), 3,5L66 (in 6), and SP 2-periodic net (6,3)Ia (in 7, 8). Apart from the characterization by standard methods, catalytic potential of the obtained CPs was also screened in the Knoevenagel condensation of benzaldehyde with propanedinitrile to give 2-benzylidenemalononitrile (model reaction). Several reaction parameters were optimized, and the substrate scope was explored, revealing the best catalytic performance for a 3D MOF 4. This catalyst is recyclable and can lead to substituted dinitrile products in up to 99% product yields. The present study widens the use of H2pdba as a still poorly studied linker toward designing novel functional coordination polymers.

Differences in Catalytic Activity and Durability of Nitrogen Sites on Nitrided SBA-15 or Porous Carbon Nitride

Nitrided SBA-15 (NSBA-15) is a solid base catalyst for several reactions, in which the basicity arises from a lone pair on a nitrogen atom substituted in the silicate framework. Porous carbon nitride (PCN) formed on a SBA-15 template is also a solid base catalyst with a nitrogen atom as the active site. In this study, the catalytic activity and durability of nitrogen atoms in nitrided mesoporous silica and porous carbon nitride were compared to evaluate the origin of basicity by using the Knoevenagel condensation of benzaldehyde with active methylene compounds. With malononitrile (pKa of 11.1) as the methylene compound, both NSBA-15 and PCN showed a high product yield. Using ethyl cyanoacetate (pKa of 13.1), only NSBA-15 catalyzed the reaction, probably because the active site of NSBA-15 is a quasi-aliphatic amine and that of PCN is an aromatic amine. On the other hand, PCN showed high durability as the content of nitrogen was not decreased by the catalytic reaction and long-term storage, whereas the Si–N bond in NSBA-15 easily underwent hydrolysis.

Knoevenagel condensation reaction in supercritical carbon dioxide medium using a Zn(II) coordination polymer as catalyst

The replacement of conventional organic solvents by the so-called “green solvents” is a usual approach to more sustainable chemical processes. Supercritical carbon dioxide (scCO2) has been successfully regarded as a candidate for such a purpose. Moreover, it possesses moderate critical pressure and temperature conditions and may be easily separated from the catalytic system by a simple depressurization. In the present work, the new Zn(II) coordination polymer [Zn(L1)(NMeF)]n·n(NMeF) (Zn-CP 1) is reported, being prepared by the solvothermal reaction of 5-{(pyren-4-ylmethyl)amino}isophthalic acid (H2L1) with Zn(NO3)2·6H2O in N-methylformamide (NMeF). The SCXRD analysis revealed that Zn-CP 1 has a 1D double chain type of structure. Its potential as catalyst was studied in the Knoevenagel condensation of benzaldehyde and malononitrile as a model reaction under mild conditions, in scCO2 medium. An increasing trend was observed in the reaction yield as we moved from aprotic (THF) to protic (EtOH and H2O) polar co-solvents, reaching the full conversion in the case of water. It was found that scCO2, in the absence of a protic co-solvent, is not the most suitable medium for this reaction. The catalyst can be recycled without a considerable loss of activity; SEM, PXRD, FT-IR and TGA analyses indicate its high stability throughout the process.

Ce-Containing MgAl-Layered Double Hydroxide-Graphene Oxide Hybrid Materials as Multifunctional Catalysts for Organic Transformations.

The combination of layered double hydroxides (LDH) with graphene oxide (GO) enables the formation of nanohybrids with improved properties. This work focuses on the structural and catalytic properties of Ce-containing MgAl LDH-GO composites bearing different concentrations of GO in the range of 5–25 wt.%. The synthesis of the composites was performed by co-precipitating the LDH phase in the presence of GO, while their characterization was performed using XRF, XRD, DRIFT, Raman, SEM, nitrogen adsorption-desorption, and acidity-basicity measurements. The LDH-GO composites, showing redox, basic, and acid catalytic functions, were tested in two different types of organic transformations: (i) Knoevenagel condensation and (ii) one-pot cascade oxidation-Knoevenagel condensation. (i) The cinnamic acid was synthesized by the Knoevenagel condensation of benzaldehyde with diethylmalonate. The composites showed catalytic performances in strong contrast to neat LDH or GO, suggesting a synergistic interaction between the two components. During Knoevenagel condensation, the catalytic activity increased with the GO content in the hybrids up to 15 wt.% and decreased afterwards. (ii) 2-Benzoyl-3-phenylacrylonitrile was synthesized by the aerobic oxidation of benzyl alcohol followed by the Knoevenagel condensation with benzoyl acetonitrile using three different non-polar solvents, i.e., toluene, benzene, and mesitylene. The conversion of benzyl alcohol was higher for the hybrid materials compared to the individual components but decreased with the increase of the graphene oxide concentration.

One-step non-templating synthesis of hybrid zeolite catalyst for Knoevenagel condensation at room temperature efficiently

From the view of energy saving and environment protection, green catalysis has become one of the most important goals of chemists, including both the green preparation of catalyst and the highly efficient catalysis under mild reaction condition. Herein, N,N-dimethylaminopropyl-functionalized hybrid zeolite is firstly synthesized by one-step non-templating method, which is characterized by powder X-ray diffraction, Fourier-transform IR, thermogravimetric analysis, element analysis, scanning electron microscope, water contact angle measurements, inductively coupled plasma atomic emission spectrometry, and N2 adsorption–desorption measurements. The functionalized hybrid zeolite can be directly used as base catalyst without any post modification and achieve high yields of corresponding products in the Knoevenagel condensation of benzaldehyde (or substituted benzaldehyde) with ethyl cyanoacetate at room temperature by using ethanol as solvent. By the comparison with other zeolite catalysts, the unique and excellent catalytic properties of N,N-dimethylaminopropyl-functionalized hybrid zeolite was ascribed to not only the synergic effect of basic property and hydrophobic/lipophilicity of the organic groups, but also the easier accessibility of bulky substrate molecules with highly exposed catalytic active sites over the solid catalyst. The recycling and control experiments show that it owes excellent reusable stability.

Incorporating highly basic polyoxometalate anions comprising Nb or Ta into nanoscale reaction fields of porous ionic crystals.

Polyoxometalates (POMs) are oxide cluster anions composed of high-valence early transition metals and are widely used as catalysts. Yet base catalysis of POMs remains an ongoing challenge; group V (V, Nb, and Ta) elements form more negatively charged POMs than group VI (Mo and W) elements, and in particular, polyoxoniobates and polyoxotantalates are known to show strong basicity in solution due to the highly negative surface oxygen atoms. Herein, we report for the first time porous ionic crystals (PICs) comprising Nb or Ta. The PICs are composed of Dawson-type Nb/W or Ta/W mixed-addenda POMs with oxo-centered trinuclear CrIII carboxylates and potassium ions as counter cations to control the crystal structure. Among the PICs, those with Nb or Ta tri-substituted POMs exhibit the highest yield (78-82%) and selectivity (99%) towards the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate (353 K, 6 h), which is a typical base-catalyzed reaction, as reusable solid catalysts, and they can also catalyze the reaction of other active methylene compounds. A detailed investigation into the crystal structures together with DFT calculations and in situ IR spectroscopy with methanol as a basic probe molecule shows that the exposure of [Nb3O13] or [Ta3O13] units with highly negative surface oxygen atoms to the pore surface of PICs is crucial to the catalytic performance. These findings based on the composition-structure-function relationships show that Nb- and Ta-containing PICs can serve as platforms for rational designing of heterogeneous base catalysts.

Ordered Mesoporous Carbon Nitrides with Tuneable Nitrogen Contents and Basicity for Knoevenagel Condensation

AbstractWe report on the preparation of ordered mesoporous carbon nitrides (MCN) with a 3D porous structure, tuneable nitrogen contents and basicity and their basic catalytic activities on the Knoevenagel condensation of benzaldehyde with malononitrile. The chemical structure and the nitrogen contents of the materials are finely tuned with the simple adjustment of the calcination temperature from 350 to 550 °C. The samples prepared at 350 and 400 °C exhibit C3N5 structures with 1‐amino/imino‐1,2,4‐triazole moieties, whereas the samples synthesised at 450, 500, and 550 °C possess C3N4 structures with 2‐amino/imino‐1,3,5‐triazine moieties. The materials prepared at the temperature lower than 400 °C show much higher activity than those of the samples prepared at 450, 500 and 550 °C. The change in the catalytic activities of these materials is linked with the change of the structure, nitrogen contents and the functional groups on the surface of the materials prepared at different temperatures. CO2 temperature programme desorption study reveals that 1‐amino/imino‐1,2,4‐triazole moieties in C3N5 samples provide high basicity due to the strain in 5‐membered 1,2,4‐triazole rings; however, the samples with 1,3,5‐triazine moieties have limited basicity which significantly affects the catalytic activity of the material. The optimised C3N5 catalyst shows an enhanced catalytic activity when compared to other mesoporous basic catalysts and C3N4. It is also found that the optimised catalysts are highly stable and can be recycled several times and no major change in the activity is observed.

Crystal growth study of nanosized K-MER zeolite from bamboo leaves ash and its catalytic behaviour in Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate

A detailed study on the formation of nanosized K-MER zeolite from a template-free hydrogel system is reported. By using a new recipe and bamboo leave ash (BLA) as a reactive silica source, the K-MER zeolite crystallization time has been shortened from 15 days to 6 h at 180 °C, enabling the investigation of induction, nucleation and crystal growth of nanoscale K-MER zeolite using spectroscopy and microscopy techniques. The resulting zeolite nanorods ((45.1 ± 14.5 nm length) × (3.9 ± 1.9 nm width)) have a Si/Al ratio of 2.31 and they tend to assemble in parallel forming bulkier bundle-like secondary structure ((728 ± 171 nm length) × (251 ± 49 nm width). The nanocrystals (84.3% conversion and yield, 100% selective to ethyl 2-cyano-3-phenylacrylate (ECP)) show better catalytic performance than other homogeneous base catalysts (92–99% conversion, 82–84% yield) in Knoevenagel reaction of benzaldehyde and ethyl cyanoacetate under non-microwave instant heating condition, thus offering an eco-friendly approach for synthesizing highly selective solid base catalyst for synthesizing pure fine chemicals.

Effect of delamination on active base site formation over nitrided MWW-type zeolite for Knoevenagel condensation

Abstract High surface area zeolite is advantageous in base catalysis for large molecule synthesis via C–C bond formation. Delamination of layered zeolite precursors is one of the effective approaches to modify the microporous zeolite nature. In this study, a MWW-type zeolite precursor was delaminated under controlled treatment temperature and time and subsequently nitrided by ammonia. All nitrided products were employed to catalyze a model reaction, Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate. Among several catalytic properties, the initial reaction rate was proportional to the external surface area. This indicates that there is no relation between the number of active sites and the amount of nitrogen on the entire surface. Together with the experimental results and the structural aspect of the MWW framework, it is proposed that delamination increases the external surface and eventually increases the unique Si–OH groups, which might be a precursor to the active Si–NH2 sites for the base-catalytic reaction.

Selenotungstates incorporating organophosphonate ligands and metal ions: synthesis, characterization, magnetism and catalytic efficiency in the Knoevenagel condensation reaction.

Three sandwich-type TM-containing (TM = transition metal) organophosphonate-based polyoxotungstate clusters, [TM(H2O)4(SeW6O21)2{Co(OOCCH2NCH2PO3)2}3]12- (TM = Co, Ni), have been successfully synthesized, which are the first reported TM-containing organophosphonate-based selenotungstates. They were structurally characterized by PXRD analyses, IR spectroscopy, TGA analyses, etc. Magnetic measurements show that all three compounds exhibit antiferromagnetic interactions. Besides, Co1 can be used as an efficient heterogeneous catalyst in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate under mild conditions (60 °C), and exhibits satisfactory conversion (93%) and high selectivity (99%).

Clean synthesis of benzylidenemalononitrile by Knoevenagel condensation of benzaldehyde and malononitrile: effect of combustion fuel on activity and selectivity of Ti-hydrotalcite and Zn-hydrotalcite catalysts

Benzylidene malononitrile find applications in pharmaceutical industries, pharmacology, biotech, specialty chemicals, perfumery, and for fluorescence-based assay to determine methane and is produced by polluting routes. Hydrotalcites (HT) have been very effective as solid bases in different reactions and their properties can be changed by using different synthetic methods. In this work, the effect of additional metal in the synthesis of Al-Mg hydrotalcite was systematically studied to prepare Ti-Al-Mg (Ti modified hydrotalcite) and Zn-Al-Mg HT (Zn modified hydrotalcite) using combustion method with glycine as well as glycerol as a fuel. All synthesized catalysts were evaluated in Knoevenagel condensation of benzaldehyde with malononitrile to give benzylidene malononitrile. The catalysts were completely characterized by SEM, EDXS, $$\text {N}_{2}$$ Adsorption, $$\text {CO}_{2}$$ -TPD and $$\text {NH}_{3}$$ -TPD and XRD techniques. Ti-Al-Mg hydrotalcite using glycine as a fuel was found to be the most active, selective and reusable catalyst. Langmuir-Hinshelwood-Hougen-Watson (LHHW) model was used to establish the reaction mechanism and kinetics. All species were weakly adsorbed leading to the second order power law model. Using mole ratio of 1:3 of benzaldehyde to malononitrile with ethyl acetate as a solvent and $$2.5 \times 10^{-4}$$ g/ $$\text {cm}^{3}$$ catalyst loading, 67.1% conversion of benzaldehyde and 97.6% selectivity to benzylidene malononitrile were achieved in 4 h at 60 $${^{\circ }}$$ C. The apparent activation energy was 10.01 kcal/mol. The process is green. Ti-Al-Mg and Zn-Al-Mg hydrotalcites were prepared using combustion method with glycine and glycerol as fuel and used in the Knoevenagel condensation of benzaldehyde with malononitrile to give benzylidene malononitrile. Ti-Al-Mg hydrotalcite (glycine) was the most active, selective and reusable catalyst.

Structural Studies of Multifunctional SrTiO3 Nanocatalyst Synthesized by Microwave and Oxalate Methods: Its Catalytic Application for Condensation, Hydrogenation, and Amination Reactions.

The present study deals with the synthesis of SrTiO3 (STO) nanocatalysts by conventional oxalate and microwave-assisted hydrothermal methods. Thorough characterization of the nanocatalysts synthesized has been done by using various techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy, N2 physisorption, transmission electron microscopy, total acidity by pyridine adsorption method, and acidic strength by n-butylamine potentiometric titration, respectively. Structural parameters were estimated by Rietveld refinement analysis from XRD data which confirms cubic structure of SrTiO3. Traces of impurities such as TiO2 and SrCO3 were found in conventional catalysts, whereas these are absent in microwave catalyst. Brunauer–Emmett–Teller (BET) surface area of the microwave catalyst was enhanced 14-folds compared to conventional catalyst. Increase in Lewis acid sites and their strength were also observed in STO microwave catalyst. Catalytic performance of the catalysts was evaluated for various reactions, such as Knoevenagel condensation of benzaldehyde, catalytic transfer hydrogenation of nitrobenzene, and amination of benzaldehyde. Catalytic results reveal that microwave-synthesized catalyst showed 100% conversion and selectivity (>99% yield) for the chosen reactions than the conventional catalyst. Excellent catalytic activity of the STO microwave catalyst was due to high BET surface area, pore volume, and acidity of the catalyst, as compared to conventional catalyst. The present study marks the first-time application of perovskite-based SrTiO3 as a potential multitasking cost-effective catalyst for the above reactions and synthesized using environment friendly microwave synthesis method.

Polyoxoniobates as a superior Lewis base efficiently catalyzed Knoevenagel condensation

The outstanding basicity of negative charged Lindqvist type Polyoxoniobate K7HNb6O19·13H2O (Nb6) have been proved experimentally as well as by theoretical NBO calculations, the results insights high electron density on terminal and bridging oxygen atoms of niobate anion. The most negative Natural Bond Orbital charge (NBO) of oxygen in Nb6 is −1.001, which is a much more negative value than those reported in other polyoxometalates, that corroborates its high basicity thus likely to be employed as a strong base catalyst. Experimental study suggests that Nb6 can efficiently catalyze Knoevenagel condensation of various carbonyl compounds with active methylene compounds neglecting the steric and electronic effect of aromatic aldehydes under mild conditions. Kinetic test shows that Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate exhibits second-order kinetics in the presence of Nb6 and the calculated activation energy is 43.3 kJ mol−1. Meanwhile, a proper mechanism according to the NBO study speculates that the most negative charged terminal oxygens in Nb6 would be pivotal in this transformation.

Direct synthesis of magnesium doped mesoporous silica: Their catalytic activity in Knoevenagel Condensation

Mesoporous magnesium silicate has been synthesized by co-condensation method using unconventional source of magnesium, namely magnesium ethoxide, in the presence of supramolecular assembly of a mixture of surfactants. Three different samples have been synthesized with magnesium to silicon ratio of 1:100, 1:50 and 1:10. The samples produced had high surface area and uniform pore size distribution and the loading of magnesium in each case was in conformity with the amount of magnesium introduced in the reaction gel during the syntheses of the respective samples. The nanostructures of the materials have been characterized by powder X-ray diffraction, transmission electron microscopy and nitrogen adsorption-desorption studies. The insertion of magnesium into the silica framework has been confirmed by UV-visible spectroscopy whereas the magnesium content of the samples has been estimated by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Catalytic studies revealed high activity of these magnesium-doped mesoporous silica in Knoevenagel condensation of benzaldehyde, 1-naphthaldehyde and 2-hydroxy naphthaldehyde with ethyl cyanoacetate. Knoevenagel reaction takes place in presence of a base and in this case the alkaline earth metal sites act as those basic centres. Such high catalytic efficiency cannot be achieved over basic sites in conventional microporous zeolites due to space constraint. The catalysts were found to be reusable for up to three reaction cycles and their efficiency can be attributed to the improved mass transfer rate which arises due to the presence of mesopores. Hot filtration test carried out for a representative reaction further established the heterogeneity of the catalytic reaction.

Supported choline hydroxide (ionic liquid) on mesoporous silica as heterogeneous catalyst for Knoevenagel condensation reactions

In this work hybrid mesoporous SBA-15 and mesoporous silica microspheres functionalized with choline hydroxide ionic liquid have been prepared. A two-step process inspired by the industrial production of choline hydroxide has been developed to synthesize the hybrid SBA-15 material with choline hydroxide ionic liquid (Chol-SBA-15) and the hybrid SBA-15 material with choline hydroxide ionic liquid and hexamethyldisilizane as capping agent (Chol-HMDS-SBA-15). For the preparation of hybrid mesoporous silica microspheres (Chol-MSMs), a co-condensation method has been employed with an ionic liquid prepared by reaction of 3-(glycidyloxypropyl)trimethoxysilane and trimethylamine in aqueous media. The synthesized materials have been fully characterized by elemental analysis, X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), N2 adsorption-desorption, transmission electron microscopy (TEM) and 13C CP/MAS NMR spectroscopy. These materials have been tested successfully as catalysts in Knoevenagel condensation of benzaldehyde with RCH2C≡N (R = CN (malononitrile), CO2Et (ethyl cyanoacetate)). The catalyst Chol-MSMs showed high catalytic activity, 99% of conversion and 99% of selectivity towards ethyl 2-cyano-3-phenylacrylate in 2 h of reaction and using ethanol as solvent. In addition, their reusability without loss of activity has been demonstrated.