Turkish perlite supported nickel oxide as the heterogeneous acid catalyst for a series of Claisen–Schmidt condensation reactions

Potentially active and eco-friendly solid acid catalysts have been synthesized by loading different weight percentages (10, 15, and 50) of nickel oxide on thermally activated Turkish perlite through the deposition-precipitation method. Structural features of prepared catalysts were analyzed using BET surface area analysis, X-ray diffraction, scanning electron microscope (SEM), SEM-EDX, transmission electron microscopy (TEM), Fourier-transform infrared (FT-IR), pyridine adsorbed FT-IR, UV-Vis diffuse reflectance spectroscopy (DRS), and thermogravimetric analysis (TGA) techniques. Pyridine adsorbed FT-IR analysis confirmed the presence of the optimum amount of Bronsted acidic sites in a catalyst having 15 wt. % loading of nickel oxide, which was tested for catalyzing a series of Claisen–Schmidt condensation of cyclohexanone and aromatic aldehydes to produce good isolated yield (90%–93%) of 2,6-bis(substituted benzylidene)cyclohexanones, significantly used in anti-tumor and cytotoxic activities. The high catalytic efficiency of the chosen catalyst remains almost intact up to six reaction cycles. On higher wt. % loading of nickel oxide, crystallite size increases along with agglomeration of larger nickel oxide particles on catalyst surface resulting in pore blockage and poor catalytic activity. Loading of NiO on the surface of thermally activated Turkish perlite was confirmed by SEM-EDX analysis, and TEM observations show that the particle size of the preferred catalyst was less than 50 nm. Based on results drawn from XRD, FT-IR, pyridine adsorbed FT-IR, UV-Vis DRS studies, model structures were proposed for Turkish perlite and all prepared catalysts. During this work, the catalytic potential of the preferred catalyst was compared with other previously reported catalysts, and it showed appreciable results. The formed products were further confirmed by their melting point and 1H-NMR analysis.

16% of its total production [14]. It is, thus, assumed that Turkish perlite, which has high silica and alumina composition might possess significant stable surface-active sites and can be converted into a potential heterogeneous acid catalyst for catalyzing industrially beneficial organic transformations. Nickel-based catalysts supported on different substrates like silica, alumina, zeolites have attracted research attention because of their potential applications in many important reactions such as hydrogenation, diesel steam reforming, ozonation of 2,4-dichlorophenoxyacetic acid, dry reforming of methane [31][32][33][34], etc. Particularly, supported nickel oxide catalysts have been utilized in numerous potential reactions such as hydrogen production, oxidative dehydrogenation, hydrogen peroxide decomposition, etc. [35][36][37], while fewer records are found for its application in condensation reactions [38]. Catalyst support is vital as it facilitates higher dispersion of nickel oxide and prevents its aggregation [39], which, in turn, enhances the efficiency of catalyst, and, thus, maximum conversion and yield % of desired products are achieved. With the increasing social interests over environmental degradation and future resources, it is of great importance for chemists to come up with new approaches that are less hazardous to human health and the environment. In this series, we have tried to explore stable surface-active sites of a novel silica-alumina enriched natural waste-Turkish perlite and its efficiency as catalyst support.
In the current work, a novel, efficient Turkish perlite supported nickel oxide catalyst (NPC-15) was prepared by loading nickel oxide on thermally activated Turkish perlite (TAP), and it successfully catalyzed a series of Claisen-Schmidt condensation reactions giving a higher isolated yield % of desired products, 2,6-bis(substituted benzylidene) cyclohexanones, used in anti-tumor, anti-cancer, and cytotoxic activities [40] and also serve as an important precursor for the synthesis of potentially bioactive pyrimidine derivatives [41,42]. Table 1 summarizes some nickel catalysts supported on economical substrates already precedented in literature, utilized in different reactions including Turkish perlite supported nickel oxide catalyst  presented in this work. Our work is different and innovative in terms of sustainability and reusability of NPC-15 up to six consecutive reaction cycles, also the catalytic potential of NPC-15 was tested for a series of Claisen-Schmidt condensation reactions, which is quite an uncommon field of catalysis for supported nickel oxide catalysts. Use of less expensive support, recyclable catalyst, solvent-less reaction conditions, green methodology, ease of product purification are the key features of this protocol, and, thus, it may be considered as an efficient alternative to the existing, high-priced, less-effective procedures. This work is aimed to act as a stepping-stone for the prospective researchers into the rewarding field of utilization of waste materials like Turkish perlite as the catalyst support in the development of heterogeneous catalysts.

Chemicals
Nickel nitrate, sodium hydroxide, nickel oxide, cyclohexanone, and aromatic aldehydes were purchased from Sigma Aldrich, Bengaluru, Karnataka, India. Turkish perlite was supplied by Indica Chem. Ind. Pvt. Ltd., Kotdwar, Uttarakhand, India. this procedure, a mixture of cyclohexanone and aromatic aldehydes (molar ratio = 1:2) was placed in a RBF and mixed with the activated catalyst. The reaction mixture was stirred at different times and temperatures. Once the reaction is over, the catalyst was removed from the reaction mixture by simple filtration. Melting points of products were detected by using Melting Point System MP30, Mettler Toledo India Pvt. Ltd., Vasai, Maharashtra, India, melting apparatus. Table 2 shows the surface properties of Turkish perlite and NPC-10, 15, and 50 catalysts with different nickel oxide loadings (10, 15, and 50 wt. %). The results indicate that when the nickel oxide loading increased from 10 to 15 wt. % in NPC catalyst, a decrease in the surface area was observed. Further increase in nickel oxide loading resulted in a significant decrease in surface area, which may be due to the blockage of pores of Turkish perlite by nickel oxide dispersion. Thus, from these results, it can be established that an increase in nickel oxide loading is responsible for the agglomeration of nickel oxide species to form bigger particles, and, thus, their diffusion decreases. These results are in good agreement with XRD results. Despite this, it was also observed that in NPC-10 and 15 the crystallite size remains unaltered, but, on further increase in nickel oxide loading, crystallite size also increased. The above discussion indicated that higher wt. % loading of nickel oxide does not provide sufficient active sites due to their agglomeration as inferred from increased crystallite size [46,47]. The XRD of Turkish perlite ( Figure 1a) shows the existence of a hump between 2θ = 10-15° attenuated to amorphous silica [48]. However, in TAP, a small crystalline peak appears at 2θ = 27.642° (Figure 1b) which confirms the formation of quartz crystalline phase in the sample on heating [49] along with a broad hump at 2θ = 22-23° showing amorphous content of silica [50,51]. The absence of crystalline peaks in X-ray diffraction patterns of NPC-10 and 15 (Figures 2a and 2b) is due to the dispersion of nickel oxide particles on the Turkish perlite surface in the amorphous phase and tiny in size to give an XRD peak [52] in lower wt. % catalysts. While, in case of NPC-50 (Figure 2c) [53], which are evident of formation of Ni-O phase in the sample. However, because of the weak, broad, and partially overlapping peaks, the crystallite size determination of the nickel oxide containing phase is difficult in NPC-50 [54].   Figure 3a) revealed randomly placed irregular shred particles with uneven edges, open pores also shown in other reports [55,56]. SEM image of TAP ( Figure 3b) is mainly fragmatic, glassy, and random [57], but the morphology is less irregular indicating removal of water at a high temperature, which leads to a reduction in porosity and also agglomeration of smaller particles. SEM images of NPC-10, 15, and 50 (Figures 3c, 3d, 3e) demonstrate the dispersion of shiny, fine nickel oxide particles on the surface of Turkish perlite. It was also found that the porosity of Turkish perlite particles was decreased on the loading of nickel oxide.

Characterization of catalysts
EDX analysis of Turkish perlite, NPC-10, 15, and 50 (Table 3) shows the presence of SiO 2 , Al 2 O 3 , K 2 O, Na 2 O, and other minor metal oxides in the samples. However, the presence of nickel oxide in all NPC catalysts confirms its effective loading on the Turkish perlite surface.
A more realistic vision of irregular morphology, ragged shreds of Turkish perlite can be furnished by its TEM image (Figure 4a). The micrograph of NPC-15 ( Figure 4b) shows fine dispersion of nickel oxide particles on the Turkish perlite surface. These results are in agreement with SEM images. TEM observations also show that the particle size of NPC-15 is less than 50 nm.
FT-IR spectrum of Turkish perlite is shown in Figure 5a. A broad band in the range between 3700-3000 cm -1 shows the presence of surfacial hydroxyl groups (-Si-OH) with strong intermolecular hydrogen bonding and physisorbed water molecules [58,59]. The intensity and broadness of this band diminished in TAP (Figure 5b), which is due to the loss of water as a consequence of thermal activation. A peak at 1178 cm -1 , attenuated to the Si-O-Si asymmetric vibrational frequency, confirms the presence of silica skeleton in Turkish perlite. After thermal activation, this peak gets shifted to 1227 cm -1 ,     which is a common phenomenon in amorphous silica samples [60]. An intense peak around 1630 cm -1 in Turkish perlite, characteristic of bending mode (δ O-H ) of water molecule [61,62] again highly decreased in TAP.
While loading nickel oxide on TAP, its surface gets activated by hydroxyl groups because aqueous NaOH was used as a basification agent during catalyst preparation. Consequently, the FT-IR spectra of NPC-10, 15, and 50 ( Figure 6) show an increase in broadness and peak intensity of the band attributed to -OH groups. A new band appeared around 1050 cm -1 can be assigned to ≡ Si-O-Ni stretching vibration [63], which is normally observed in the range of 1100-1000 cm -1 , but, here, it cannot be resolved because of its overlap with the absorbance of Si-O-Si stretching, appearing in the range of 1300-1000 cm -1 . An absorption band due to surficial Si-O-Ni stretching vibration has appeared at 964 cm -1 [64]. It is also found that the intensity of the band was increased to some extent after increasing nickel oxide incorporation.
The FT-IR spectra of NPC-10, 15, and 50, obtained after pyridine adsorption in the range of 1550-1400 cm -1 are shown in Figure 7. The peak at 1538 cm -1 (Figure 7a) indicates the presence of few Bronsted acidic sites in NPC-10. In NPC-15 (Figure 7b), a peak and a broad band appearing at 1438 and 1540 cm -1 , respectively [65][66][67] show the existence of some Lewis and optimal Bronsted acidic sites for suitable catalytic activity. NPC-50 has the highest Lewis acidic sites corroborated by the appearance of intense peaks at 1438 and 1450 cm -1 (Figure 7c).
UV-Vis DRS is used to determine the state of nickel oxide incorporated into the Si-O-Si skeleton of TAP. During steps of synthesis, due to the calcination of catalysts at 450 °C for 4 h, the presence of Ni(OH) 2 can be declined. A broad band around 250 nm (Figure 8), which becomes more intense with an increase in weight percentage of nickel oxide loading suggests that Ni +2 ions are in an octahedral local environment [68].
The TGA and DSC curve of NPC-15 ( Figure 9) comprises of two main regions having a major weight loss of about 8.96% up to 643.83 °C and a minor one of approximately 0.82% weight loss at higher temperature along with an exothermic DSC peak. The weight loss in the first region could be due to the removal of bulk water, physisorbed water, and loosely bound hydroxyl groups [69], and decomposition of nickel salt (used as a precursor) [70]. The second region shows a minor weight loss, which may be due to the conversion of Ni(OH) 2 into NiO.

Catalytic activity
The catalytic performance was preliminarily tested by Claisen-Schmidt condensation reaction of cyclehexanone and benzaldehyde to give 2,6-bis(benzylidene)cyclohexanone in a single step, one-pot reaction conditions. The reaction was carried out at 120 °C for 150 min, taking 0.2 g of catalyst, and cyclohexanone/benzaldehyde (molar ratio 1:2). Results given in Table 4 show that Turkish perlite, TAP, and commercial NiO do not possess any catalytic activity for this reaction. In the case of NPC-10, a lower isolated yield (58%) is obtained due to the presence of fewer  surface, while, in the presence of NPC-15, the isolated yield % is highest due to significant increment of active catalytic sites. The isolated yield % is again decreased on using NPC-50 due to blockage of surface-active Bronsted acidic sites by bulk deposition of NiO crystallites. As depicted from Table 5, NPC-15 gave a good isolated yield % of 2,6-bis(benzylidene)cyclohexanone in the first as well as in the last run with decent enough consecutive reaction runs when compared with some previously reported catalysts.

Regeneration of catalyst
The spent catalyst was regenerated by doing simple filtration, washing with acetone, drying at 110 °C in an oven for 12 h, and finally by calcination at 450 °C in a muffle furnace for 1 h. Thus, obtained regenerated catalyst showed efficient catalytic activity up to consecutive 6 reaction cycles giving almost similar isolated yield % in the range of 90%-83%, indicating the presence of stabilized acidic sites in the regenerated catalyst. After the sixth reaction cycle, the yield % was decreased significantly, this may be due to blockage of acidic sites of the catalyst by the deposition of carbonaceous residues  Table 6. NPC-15 catalyzed Claisen-Schmidt reaction between cyclohexanone and different aromatic aldehydes. a Reaction conditions:Temperature = 110-140 °C; molar ratio (cyclohexanone/aromatic aldehydes = 1:2); catalyst weight = 0.2 g. b Products (5a-5g) were characterized by melting point and spectral (IR and 1 H-NMR) data and compared with the literature. of organic reactants and products on the surface. The FT-IR spectrum ( Figure 10) of regenerated NPC-15 regenerated after the sixth reaction cycle is similar to that of fresh NPC-15 demonstrating the stability of nickel oxide loading on Turkish perlite.

Product identification by 1 H-NMR (CDCl 3 , 500 MHz)
All the products synthesized by Claisen-Schmidt condensation reactions were identified by melting point analysis and 1 H-NMR spectroscopy. δ-values of all synthesized products are summarized in Table 7.

Conclusion
15 wt. % nickel oxide loaded on Turkish perlite (NPC-15) was proven to be an effective and competent solid acid catalyst, possessing significant Bronsted acidity to catalyze condensation reactions between cyclohexanone and aromatic aldehydes with high isolated yield (90%-93%) of desired products in one-pot, liquid phase, solvent-free reaction conditions. The catalyst was easily filtered, regenerated, and recycled several times with analogous efficiency, suggesting the stability of acidic sites of the catalyst during the reactions. The results of X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET) surface area, pyridine adsorbed Fourier-transform infrared (FT-IR), etc. recommended that the optimum catalytic activity of NPC-15 was due to the presence of sufficient surface-active Brönsted acidic sites. While, at higher wt. % loading, NiO crystallites increased Lewis acidic sites and blocked the active Bronsted acidic sites, which ultimately decreased isolated yield % of desired products. XRD results depicted that, in NPC-15, nickel oxide particles were finely dispersed on the Turkish perlite surface in the amorphous phase. A fine dispersion of nickel oxide particles was also shown in scanning electron microscope (SEM) and transmission electron microscope (TEM) images of NPC-15. The novelty of this work is the utilization of abundant natural waste, Turkish perlite as solid support for the synthesis of highly efficient heterogeneous acid catalysts. This investigation also suggests that Turkish perlite could be an alternative to commercial silica for synthesizing novel solid acid catalysts, which can catalyze various industrially important reactions in a cost-effective manner. Step-I Step  Figure 10. FT-IR spectra of (a) fresh NPC-15 and (b) regenerated NPC-15 after the sixth reaction cycle.