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Article

Pd-HPW/SiO2 Bi-Functional Catalyst: Sonochemical Synthesis, Characterization, and Effect on Octahydroquinazolinone Synthesis

by
Md. Afroz Bakht
1,*,
Mshari. Alotaibi
1,
Abdulrahman I. Alharthi
1,
Israf Ud Din
1,
Abuzer Ali
2,
Amena Ali
3 and
Mohamed Jawed Ahsan
4
1
Department of Chemistry, College Science and Humanities, Prince Sattam Bin Abdulaziz University, P.O. Box 83, Al-Kharj 11942, Saudi Arabia
2
Department of Pharmacognosy, College of Pharmacy, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
3
Department of Pharmaceutical Chemistry, College of Pharmacy, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
4
Department of Pharmaceutical Chemistry, Maharishi Arvind College of Pharmacy, Ambabari Circle, Rajasthan 302 039, Jaipur, India
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(11), 1273; https://doi.org/10.3390/catal11111273
Submission received: 18 September 2021 / Revised: 14 October 2021 / Accepted: 20 October 2021 / Published: 22 October 2021
Browse Figures
Versions Notes

Abstract

:
A Palladium-doped silica-supported heteropoly acid (HPW) (1%Pd-HPW/SiO2) bi-functional catalyst was produced using ultrasonic and conventional procedures. Both forms of catalyst were characterized with distinct analytical approaches in order to access the advantages of each one. The presence of the required functional groups in the catalyst was confirmed using FT-IR. The crystallinity of ultrasonically generated 1%Pd-HPW/SiO2 was confirmed with XRD. The existence of necessary elements in the catalyst was also suggested by XPS and EDX data. BET was used to calculate the surface area of the ultrasonically synthesized catalyst (395 m2 g−1), and it was found to be greater than that of the non-ultrasonic synthesized catalyst (382 m2 g−1). The N2 adsorption-desorption isotherm indicated mesoporous structures. The SEM morphology at a similar magnification exhibited quite different shapes. In comparison to traditional methods, ultrasonic approaches produce higher yields in less time and use less energy. Furthermore, the effect of the preparation method of the 1%Pd-HPW/SiO2 catalyst was extensively studied with respect to the synthesis of octahydroquinazolinones. Excellent product yields, a fast reaction time, and simple work-up methods are some peculiarities associated with the ultrasonically synthesized catalyst. The recycling study was also investigated and found suitable for up to four reaction cycles.

Graphical Abstract

1. Introduction

The properties and applications of catalysts are mainly correlated with the preparation process, so an adequate variety of preparation techniques can essentially determines the efficiency of the catalysts [1]. Scientists have developed varieties of synthetic techniques to prepare materials/catalysts [2]. The synthetic processes that have been used for catalyst preparation so far are the gas-phase, liquid-phase, and mixed-phase methods [3]. These methods often entail harsh reaction conditions due to the organic solvents, surfactants, and potent reducing agents, which generate plenty of harmful waste [4,5]. Additionally, the conventional techniques are also very expensive. Hence, to bypass these hazardous approaches, a greener and more cost-effective alternative method is desirable for adequate catalyst preparation.
The ultrasound-assisted method is the most valuable synthetic tool for preparing nano-sized materials; for example, Cu/Zn substitution in Malachite–Rosasite [6] and SBA-15@ADMPT/H5PW10V2O40 were reported recently [7]. The cavitation effect was produced during the sonochemical production of a Cu/ZnO-based catalyst for methanol synthesis [8]. Ultrasound was applied to co-precipitation and the ageing phase of precipitation during preparation of a Cu/ZnO/Al2O3 catalyst for a water-gas shift reaction [9]. In the fabrication of the Zirconia catalyst, a high ultrasonic frequency generates the vibration of ions in a solution, rapid collisions, and spontaneous particle movement, and it eventually regulates the size of the catalyst [10].
Polyoxometalates that have been used as catalysts in various applications [11,12] are inherently multifunctional compounds of substantial structural mobility [1,2]. Heteropoly acids of Keggin form Hn [XM12O40] (HPA), where the most common polyoxometalates are X = Si4+, P5+ and M = Mo6+ W6+, V5+, etc. These are strong Brønsted acids and efficient redox agents. Their acid and redox properties can be tuned by varying their polyanion composition, whereas the molecular (primary) structure of HPA remains unchanged. Solid HPA causes a large change in their texture in a regulated way (secondary structure), and it can be chemically changed to add a metal function, Lewis acidity, basicity, and so on [11,12]. As multifunctional catalysts, the ability of polyoxometalates has long been recognized. Heteropoly acid (HPA) is more active than traditional solid acids, such as SiO2, Zeolites, Al2O3 [13], and especially H3PW12O40 (PW) or H4SiW12O40 (SiW). The regeneration of HPA, however, restricts it to having a limited use as a catalyst [14]. In SiO2 or Al2O3 [15], regeneration can be easily managed by heating up to 500 °C. Due to its poor thermal stability, such a regeneration technique is not feasible for HPA [16]. The efficient use of HPA as a heterogeneous catalyst could be made possible by adding certain transition metals, such as Pd or Pt, to increase its regeneration capacity [15,16]. Efficient bi-functional catalysts for alkane isomerization were thus found to be solid HPA doped with Pd and Pt [17]. A Pd-doped HPA catalyst was developed with the Showa-Denko process to produce ethane-oxidizing acetic acid [18]. The difficulty in extracting a catalyst from a product has led to economic and environmental issues, which is problematic in continuous production. Any acidic or neutral supports, such as SiO2 [18,19], TiO2 [20,21], ZrO2 [22], acidic ion exchange resin, or active carbon resin, are impregnated with HPA to solve these problems [18].
The use of multicomponent reactions to synthesize octahydroquinazolinone derivatives has gained popularity due to its easy synthetic procedure, economic efficiency, and good selectivity [23]. Octahydroquinazolinone derivatives also have well-known moiety for the top medicinal agents. They have been used for antimicrobial [24], anti-inflammatory analgesic [25], anticancer [26], and antiviral [27] purposes. Pietro Biginelli first achieved the synthesis of MCRs through acid-catalyzed cyclo-condensation of aldehyde, ethyl acetoacetate, and urea, and these reactions are known as the Biginelli reactions [28]. Bigenelli reactions preferably use β-diketone rather than open-chain dicarbonyl compounds to synthesize octahydroquinazolinone [29]. Much of the literature has shown the synthesis of octahydroquinazolinone using different catalysts or chemical agents, such as Conc. HCl [30], Conc. H2SO4 [29], trimethylsilyl chloride (TMCl) [31], SOCl2 [32], and Lewis acids (La(OTf)3, L2O3, ZrCl4) [29,33]. However, these chemicals are associated with one or more of the following disadvantages: generation of side reactions, harsh reaction conditions, handling difficulties, and expensive costs [34,35,36]. In other way, heterogeneous catalysts, such as silica sulfuric acid [37] or vanadium n-propyl amino phosphate, are used because of their non-toxic nature and ability to promote selective reactions for octahydroquinazolinone synthesis [38]. In many multicomponent organic reactions, the catalysts that are needed must have the qualities of numerous active sites, nano range size, and a large surface area. To improve these qualities, materials can be subjected to an ultrasound technique. In fact, the synthesis of inorganic nanomaterials (used as catalysts) with ultrasound effects can produce abundant active sites. Since polyoxymetalate-type metal-based catalysts are made by sonochemically reducing metal ions like Pd(II), Pt(IV), Ni(II), and others [39], as a result overall morphology can be controlled. Meanwhile, solid supports and the heterogeneous metal-based catalysts produced with sonochemical effects can provide a highly selective surface by increasing the surface area [40,41].
This study concerns the synthesis of a 1%Pd-HPW/SiO2 catalyst with the ultrasound technique and the traditional method in order to improve the catalysts’ performance in octahydroquinazoline synthesis. The performance of the catalysts was compared according to the characteristics of each methods of preparation. Both forms of catalysts were well characterized. Furthermore, the effects of the catalysts were checked with respect to the synthesis of some octahydroquinazolinone derivatives.

2. Results and Discussion

2.1. Characterization of the Pd-HPW/SiO2 Bi-Functional Catalyst

2.1.1. FT-IR Spectroscopy

The ultrasonically and non-ultrasonically synthesized catalysts had peaks at 1066 and 1077 cm−1 and at 792 and 796 cm−1 in the FT-IR spectrum (Figure 1) for 1% Pd-HPW/SiO2 with silica. In both cases, vibrations at 981 and 982 appeared at the usual positions with unchanged relative intensity due to the W=O band. The peak at 792 and 796 cm−1 corresponded to the corner sharing band of W-O-W. This suggests that, regardless of the preparation process, the HPW retained the Keggin structure, which was already confirmed by our previous efforts [42,43]. The peak assigned at 1066 and 1077 cm−1 may have been due to Si-O-Si stretching vibrations [42]. Some other peaks for SiO and SiO2 may not have been visible due to the overlapping of HPW as result of impregnation with the SiO2 support [42]. However, a band above 1600 cm−1 was not observed due to the reduction of heteropoly acid after Pd loading, which is consistent with other work [44]. Furthermore, Pd does not substantially change the primary structure of heteropoly acid [44].

2.1.2. XRD Patterns

Figure 2 delineates the XRD patterns of the ultrasonically and non-ultrasonically synthesized catalysts. It can be seen that some sharp peaks punctuated a broad peak at 2θ ≈ 13°–35°, which was related to amorphous silica, whereas the sharp peaks were ascribed to HPW phases. With caution, a peak at around 2θ ≈ 38° suggested a distinct metallic palladium phase of the fcc structure and was aligned with the HPW peak, which matched with JCPDS No. 46-1043 for Pd. It may be concluded that this peak could be attributed to palladium or HPW, or that there was an overlap of the peaks. It was reported that when the content of Pd increased from 1% to any higher level, the crystallinity of the peak became more apparent and prominent [45]. In the present case, the Pd loading was only 1 wt.%, revealing a relatively weak XRD signal. The other XRD diffractogram postulated the presence of heteropoly anions of HPW that were distributed over the SiO2 [46]. The XRD band at 2θ ≈ 26° could be attributed to SiO2, which matches with JCPDS No. 41-1413 and was confirmed by our earlier work [42]. Moreover, the XRD pattern of the ultrasonically synthesized catalyst showed a somewhat higher intensity of its peaks than the non-ultrasonically synthesized catalyst, indicating that it was slightly more crystalline than the other. This was further confirmed by the FWHM values of 0.370 (25 nm) and 0.350 (22 nm) for the non-ultrasonically and ultrasonically synthesized catalysts, respectively.

2.1.3. N2 Adsorption-Desorption Isotherm

Figure 3 displays the nitrogen adsorption–desorption isotherm for the synthesized 1%Pd-HPW/SiO2 (US) and 1%Pd-HPW/SiO2 (non-US) catalysts. Both catalysts demonstrated isotherm type IV with an H1 hysteresis loop, thus representing a mesoporous solid with non-uniform shape and size. This isotherm resembled the isotherm of silica. Since the current catalyst contained more than 90% of silica, the nature of the adsorption-desorption isotherm was justified [42,47].
Additionally, the steep increase in nitrogen adsorption in the low-pressure region indicated the existence of micropores and mesopores in the catalysts.

The Measurements of the BET Surface Area

The BET catalyst surface was analyzed based on multiple points. It was found that the surface area of the non-ultrasonically synthesized Pd-HPW/SiO2 was 382 m2 g−1, whereas it was 395 m2 g−1 for the ultrasonically synthesized catalyst (Table 1). Despite the slight increase in surface area of the ultrasonically synthesized catalyst, it seems that the ultra-sonication treatment did not play a significant role in the increment in the catalyst surface area.

2.1.4. XPS and EDX Analysis of Non-US Pd-HPW/SiO2 Catalyst

The XPS and EDX spectra of the non-US catalyst are shown in Figure 4 and Figure 5, respectively. As reported in the literature [48], there are usually no substantial variations depending on the preparation method, hence the spectra of the non-US catalyst are only presented.
Both analyses identified W, Si, Pd, and all of the elements present in their stoichiometric amounts.
The chemical composition of the 1%Pd-HPW/SiO2 catalyst was further investigated with X-ray photoelectron spectroscopy (XPS). The surveyed XPS spectrum of the 1%Pd-HPW/SiO2 catalyst is shown in Figure 4. The spectrum was dominated by characteristic peaks of W 4f, Si 2p, Pd 3d, and O 1s, corresponding to the elements of tungsten, silicon, palladium and oxygen O 1s, respectively. The absence of other elements indicated the high purity of the examined 1%Pd-HPW/SiO2 catalyst. Further, the EDX analysis shown in Figure 5 indicates that Pd reacted well with HPW-SiO2. The quantitative surface composition was found to be O = 62.36%, W = 0.29%, Si = 31.59%, and Pd = 0.03%. The high percentage ratio of the O 1s peak reflects the high oxidation state of the 1%Pd-HPW/SiO2 catalyst.

2.1.5. SEM Images of Pd-HPW/SiO2

Figure 6A,B presents the SEM images for ultrasonically and non-ultrasonically synthesized Pd-HPW/SiO2. From the images, the morphologies of the two samples were slightly different. Generally, both samples revealed an irregularly shaped aggregate and diverse particles of different sizes and shapes. In addition, it is worth noting that the particle size of 1%Pd-HPW/SiO2 (US) was smaller than that of 1%Pd-HPW/SiO2 (non-US). Despite the different sizes of the particles in the two samples, their surface areas were very close (Table 1). A portion of the 1%Pd-HPW/SiO2 (US) catalyst’s particles had a smaller size, which played a role in the fact that the surface area of the catalyst was close to that of the other sample of 1%Pd-HPW/SiO2 (non-US).

2.2. Study of the Attributes of Ultrasound Effects Significant to the Preparation of 1%Pd-HPW/SiO2

The 1%Pd-HPW/SiO2 catalysts were prepared with the conventional (with stirring) and ultrasonic methods. It took 1 h to prepare the desired catalyst with the non-ultrasonic method with an obtained yield of 80%. On the contrary, the preparation of the same catalyst with the ultrasonic method resulted in a high yield of 95% at the expense of only 20 min (Table 2).
The superiority of one of the two methods was further validated by the energy measurements, as shown in Table 2. In the non-ultrasonic method, the total energy consumption was found to be 1.64 kJ/g, while only 0.92 kJ/g was used for the ultrasonic method. Thus, by using an ultrasonic procedure, a total of 44% of the energy was saved. (Yield and energy calculations are available in Supplementary Materials from page no S20–S22). The ultrasonic method’s excellent yield and energy efficiency were attributed to the extreme microscopic conditions created by rapid bombardment and material collapse, which eventually formed small particles within the reaction system [49].

2.3. Catalytic Activity

In a multicomponent reaction, the catalytic activity of the ultrasonically and non-ultrasonically prepared 1% Pd-HPW/SiO2 was assessed for the synthesis of octahydroquinazolinone (4a–f) from dimedone (1 mmol), urea/thiourea (1.5 mmol), and aldehydes (1 mmol) (Scheme 1).
Initially, the hit and trial method was adopted in order to obtain the compound with the highest yield with both catalyst forms. The compound containing 4-flurobenzaldehyde was chosen as the model compound since it produced the maximum yield when utilizing the ultrasonically generated catalyst.
The effects of the ultrasonically and non-ultrasonically synthesized catalysts are presented in Table 3 for the synthesis of model compound 4a in aqueous conditions.
In the absence of catalysts, trace amounts of the target model-octahydroquinazolinone produced by reacting dimedone (1 mmol), urea (1.5 mmol), and 4-flurobenzaldehyde (1 mmol)—were produced for up to 2 h. (Table 3, entry 1). The conversion of the desired model compound was achieved by increasing order of the catalyst load (Table 3, entries 2–5). A maximum yield was found with the model compound when utilizing 100 mg of the catalyst for with only 20 min of reaction time (Table 3, entry 5). It also discovered that with a further rise in the catalyst load (120 mg), no improvement in the yield was observed when using the ultrasonically (US) and non-ultrasonically synthesized 1%Pd-HPW/SiO2 (entry 6, Table 3). Therefore, it was assumed that 100 mg would be the optimal quantity with which to obtain the target octahydroquinazolinone.
The use of different aldehydes and urea/thiourea with dimedone in a typical multi-component reaction to obtain a series of octahydroquinazolinones (4a-f) was investigated (Table 4, entries 1–6). In general, the yield was better under the influence of the ultrasonically synthesized catalyst than with the non-ultrasonic form for all of the products listed (Table 4, entries 1–6). The better performance of the ultrasonically synthesized catalyst could be attributed to the moderately elevated BET surface area (Table 1) and small particle size compared to the non-ultrasonically prepared catalyst (Figure 6).
Under optimized conditions, the investigation scrutinized the substrate selection (aldehydes, urea, and thiourea). The compound with a monosubstituted electron-withdrawing group (4a, 4d) displayed the highest yield in just 20 min (Table 4, entry 1) compared to the compound with an electron-donating substitution (Table 4, entries 2, 3 and 5, 6). In addition, the compounds with urea components gave a higher yield than their thiourea counterparts. Interestingly, the catalyst’s performance was superior in terms of the yield and reaction time for all of the compounds studied [33,50,51].
The significance of the catalyst (1%Pd-HPW/SiO2) was further evaluated by comparing it with tungstophosphoric acid (HPW) and silica-supported tungstophosphoric acid (HPW/SiO2) with respect to the synthesis of the model compound. When the reaction was performed and triggered with HPW and HPW/SiO2, the yield was found to be 71 and 88%, respectively, at the expense of 3 and 1.5 h (Table 5, entries 1,2). However, the yield abruptly increased with the introduction of 1% Pd, and the reaction was completed in just 20 min (Table 5, entry 3). The improvement in the results observed with 1%Pd-HPW/SiO2 could be attributed to the active sites of the catalyst being exposed to the reactant more uniformly, resulting in a higher product yield.

2.4. Catalyst Reusability

The reusability of the sonochemically synthesized catalyst (1% Pd-HPW/SiO2) in the model reaction under identical reaction conditions was checked in four runs. In each cycle, the catalyst was dried in an oven before being reused in the subsequent reaction.
The compound yield was marginally reduced after each run owing to the decrease in the amount of catalyst after every reaction (Table 6).

2.5. Probable Mechanism

A predicted mechanism that was verified by the 1% Pd-HPW/SiO2-catalyzed octahydroquinazolinone synthesis is shown in Scheme 2. The mechanism begins with the catalyst activating the carbonyl group of an aldehyde to form intermediate I, followed by condensation with urea/thiourea to form iminium intermediate (III) by removing the water molecule, and finally, it reacts with dimedone to form another intermediate (IV). The removal of another water molecule and the subsequent cyclization result in the desired octahydroquinazolinone.

3. Materials and Methods

3.1. Materials and Instrumentations

The Thermo Science iD5 ATR diamond Nicolet iS 5 FT-IR Spectrometer was used to record the FT-IR spectra for the catalyst. The data spacing with a single beam with the DOMINIC software was 0.482 cm−1. The catalyst XRD diffractogram was defined by using an X-ray diffractometer from ULTIMA NR, Tokyo, Japan. A scanning electron microscope (Business (FEI), Model: Quanta FEG 250, GH Eindhoven, Holland) was used to assess the morphological attributes of the catalyst. The specially designed Thermocraft built-in tube furnace was used for catalyst calcination. Catalyst synthesis was carried out using the UP 20Ht (serial no. 351219415), a Hielscher ultrasonic processor with a 26 kHz frequency and a 200 W ultrasonic probe (dimensions = 300 mm 190 mm 90 mm), which was produced in Berlin, Germany. The BET surface area of the catalyst was assessed with QuantachromeASiQwin, version 5.2., BOYNTON BEACH, FL, USA. X-ray photoelectron spectroscopy (XPS, Thermo Scientific, Waltham, MA, USA) was performed by using a flood gun for charge compensation. The analysis of the XPS spectra and data was performed with the Thermo Avantage software (version 5.932). Energy-dispersive X-ray spectroscopy (EDX) was used for an elemental analysis of the prepared catalyst (Jeol, Tokyo, Japan, Model, ASEM-6360A). The 1H-NMR and 13C spectra of the synthesized octahydroquinazolinones were recorded with a Bruker-Plus (400 MHz) NMR instrument, with tetramethylsilane as an internal standard. All of the chemicals used in this study were bought from Sigma Aldrich in the United States (Burlington, MA, USA). These compounds were employed in the studies without being purified beforehand.

3.2. Preparation of the 1%Pd-HPW/SiO2 Bi-Functional Catalyst

A magnetic stirrer was used for the non-ultrasonic method and a 26 kHz ultrasonic reactor was used for the ultrasonic method to prepare the catalyst. In the non-ultrasonic method, a 200 mL beaker and 2.96 g of 25% HPW/SiO2 powder with a 0.02 M Pd(OAc)2 solution in benzene (14 mL) were stirred at room temperature for 1 h in a magnetic stirrer, followed by gradual evaporation of the benzene in a rotary evaporator. The procedure was repeated for 20 min in a specially built jacketed flask using an ultrasonic reactor. The catalyst was calcined under vacuum at 150 °C/0.1 kPa after drying and then reduced in an oven for 2 h with a hydrogen flow at 250 °C.

3.3. General Procedure for the Synthesis of Octahydrquinazoline Derivatives (4a–f)

In a 100 mL round-bottom flask, a mixture of dimedone (1) (1 mmol), urea/thiourea (2) (1.5 mmol), and different aldehydes (3) (1 mmol) was stirred in the presence of an optimized amount (100 mg) of 1%Pd-HPW/SiO2 catalyst in 10 mL of water at 100 °C. In a solvent system of ethyl acetate and acetone (3:7), the progress of the reaction was continuously monitored.
4-(4-Fluorophenyl)-7,7-dimethyl-1,2,3,4,5,6,7,8-octahydroquinazoline-2,5-dione (4a). White solid; FT-IR (cm−1, ATR); 3433 and 3336 (NH), 1594 (C=O, ring), 1461 (C=O, urea), 1372 (C=C); 1H NMR (DMSO-d6, 400 MHz): 11.88 (s, 1H, NH), 9.96 (s, 1H, NH), 8.08–8.08 (d, 1H, J = 2.84 Hz, Ar-H), 7.90 (s, 1H, Ar-H), 7.25–6.94 (m, 2H, Ar-H), 5.49 (1H, s, CH), 2.40–2.37 (d, 2H, J = 13.88 Hz, CH2), 2.28–2.21 (d, 2H, J = 25.48 Hz, CH2), 1.07 (s, 6H, 2×CH3); 13C NMR (DMSO-d6, 100 MHz): δ 195.55 (C=O), 190.55 (NC=O), 162.44 (C-F), 159.79 (NC=C), 133.65, 133.62, 128.30, 115.52, (ArC), 101.41 (OC-C=C), 64.31 (C-NH), 50.66 1C, CH2), 42.94 (1C, CH2), 31.40 (1C, CH), 28.25 (2CH3); 13C-DEPT-135 NMR (DMSO-d6, MHz, δ ppm) 130.32, 128.51, 115.10, 114.90, −50.46, −47.01, 31.11, 30.78, 29.09, 28.23, 26.96.
4-(3,4,5-Trimethoxy)-7,7-dimethyl-1,2,3,4,5,6,7,8-octahydroquinazoline-2,5-dione (4b). Light yellow solid; FT-IR (cm−1, ATR); 3305 and 2948 (NH), 1658 (C=O, ring), 1457 (C=O, urea), 1361 (C=C); 1H-NMR (DMSO-d6, 400 MHz): 9.86 (s, 1H, NH), 7.24 (s, 1H, NH), 6.60 (s, 1H, Ar-H), 6.40 (s, 1H, Ar-H), 4.46 (1H, s, CH), 3.68(s, 9H, 3×OCH3), 2.55–2.51 (d, 2H, J = 12.56 Hz, CH2), 2.29–2.25 (d, 2H, J = 16.25 Hz, CH2), 2.12–2.08 (d, 2H, J = 16.28 Hz, CH2), 1.03–0.92 (s, 6H, 2xCH3 ); 13C NMR (DMSO-d6, 100 MHz): δ 197.10 (C=O), 163.76 (NC=O), 152.76(C-OCH3), 140.39 (NC=C), 136.27, 114.67 (ArC), 105.80 (OC-C=C), 60.41(3C, 3×OCH3), 56.19(1C, CH), 50.46 1C, CH2 ), 32.28 (1C, CH2 ), 29.15 (1C, CH ), 26.66 (2CH3 ); 13C-DEPT-135 NMR (DMSO d6, MHz, δ ppm) 105.80, 60.41, −50.46, −40.09, 31.66, 29.15, 26.66.
4-(o-Tolyl)-7,7-dimethyl-1,2,3,4,5,6,7,8-octahydroquinazoline-2,5-dione (4c). Light yellow solid; FT-IR (cm−1, ATR); 3299 and 2966 (NH), 1628 (C=O, ring), 1547 (C=O, urea), 1480 (C=C); 1H-NMR (DMSO-d6, 400 MHz): 10.22 (s, 1H, NH), 9.33 (s, 1H, NH), 7.92 (s, 1H, Ar-H), 7.03-6.86 (m, 3H, Ar-H), 4.59 (1H, s, CH), 2.68–2.41 (s, 2H, CH2), 2.31–2.03 (s, 2H, CH2), 1.99(3H, s, CH3), 1.02–0.85 (s, 6H, 2×CH3); 13C NMR (DMSO-d6, 100 MHz): δ 197.19 (C=O), 163.36 (NC=O), 143.91 (NC=C), 136.78, 130.04, 126.46, 126.22, 116.07, 114.91 (ArC), 101.48 (OC-C=C), 60.49 (1C, CH), 50.47(1C, CH2), 47.33 1C, CH2), 32.29 (1C, CH2), 29.16 (1C, CH), 26.63 (2CH3), 19.82 (1C, CH3); 13C-DEPT-135 NMR (DMSO-d6, MHz, δ ppm) 130.04, 127.99. 126.46, 60.49, −50.47, −47.19, 27.62, 26.63, 19.82.
4-(4-Fluoro)-7,7-dimethyl-5-oxo-1,2,3,4,5,6,7,8-octahydroquinazoline-2-thione (4d). White solid; FT-IR (cm−1, ATR); 3383 and 2952 (NH), 1647 (C=O, ring), 1511 (C=S, thiourea), 1456 (C=C); 1H-NMR (DMSO-d6, 400 MHz): 8.01 (s, 1H, NH), 7.19 (s, 1H, NH), 7.17–6.96 (m, 4H, Ar-H), 5.92 (1H, s, CH), 2.337–2.331 (s, 4H, 2×CH2), 1.03–0.90 (s, 6H, 2×CH3); 13C-NMR (DMSO-d6, 100 MHz): δ 196.57 (C=O), 187.69 (NC=S), 163.44 (NC=C), 140.91, 137.39, 128.59, 128.52, 115.10 (Ar C), 114.93 (OC-C=C), 59.31 (1C, CH), 50.46 (1C, CH2), 47.01 1C, CH2), 32.34 (1C, CH2), 31.77 (1C, CH), 28.24, 26.96 (2CH3); 13C-DEPT-135 NMR (DMSO-d6, MHz, δ ppm) 130.32, 128.51, −50.46, −47.01, 31.11, 30.78, 29.09, 28.23, 26.96.
4-(3,4,5-Trimethoxy)-7,7-dimethyl-5-oxo-1,2,3,4,5,6,7,8-octahydroquinazoline-2-thione (4e). Light yellow solid; FT-IR (cm−1, ATR); 3313 and 2944 (NH), 1660 (C=O, ring), 1509 (C=S, thiourea), 1455 (C=C); 1H-NMR (DMSO-d6, 400 MHz): 8.12 (s, 1H, NH), 7.29 (s, 1H, NH), 6.99-6.83 (m, 1H, Ar-H), 6.22(s, 1H, Ar-H), 5.84 (1H, s, CH), 2.51–2.50 (t, 2H, J = 3.44 Hz, CH2), 2.32 (s, 2H, CH2), 1.10–0.82 (s, 6H, 2×CH3); 13C NMR (DMSO-d6, 100 MHz): δ 197.09 (C=O), 188.25 (NC=S), 152.81 (NC=C), 137.13, 135.39, 114.88, 114.68, 105.81 (ArC), 104.19 (OC-C=C), 60.45 (1C, CH), 56.00 (3C, OCH3), 46.95 (1C, CH2), 31.74 (1C, CH2), 28.13 (1C, CH), 26.67 (2CH3); 13C-DEPT-135 NMR (DMSO-d6, MHz, δ ppm) 105.81, 104.19, 60.46, 56.00, −50.46, −46.96, 31.40, 28.13, 26.13.
4-(o-Tolyl)-7,7-dimethyl-5-oxo-1,2,3,4,5,6,7,8-octahydroquinazoline-2-thione (4f). Light yellow solid; FT-IR (cm−1, ATR); 3348 and 2954 (NH), 1657 (C=O, ring), 1614 (C=S, thiourea), 1460 (C=C); 1H-NMR (DMSO-d6, 400 MHz): 10.22 (s, 1H, NH), 9.54 (s, 1H, NH), 7.04–6.86 (m, 4H, Ar-H), 5.83 (1H, s, CH), 2.68–2.51(m, 2H, CH2), 2.26–2.03 (m, 2H, CH2), 1.02–0.85 (s, 6H, 2×CH3); 13C NMR (DMSO-d6, 100 MHz): δ 197.22 (C=O), 186.87 (NC=S), 163.38 (NC=C), 143.91, 138.79, 136.78, 135.00, 130.05, 125.53, 116.07 (Ar C), 101.48 (OC-C=C), 60.49 (1C, H), 50.47 (1C, CH2), 47.09 (1C, CH2), 32.29, 28.14, 26.62, 19.81 (1C, CH3); 13C-DEPT-135 NMR (DMSO-d6, MHz, δ ppm) 130.05, 127.99. 125.53, 124.99, 60.50, −50.47, −47.09, 29.16, 28.14, 26.62, 20.51.
Details of spectroscopic data are available from Figure S1–Figure S18 in Supplementary Materials).

4. Conclusions

In conclusion, 1%Pd-HPW/SiO2 catalysts were prepared with both ultrasonic and non-ultrasonic methods. Both catalysts were analyzed in order to determine their qualitative characteristics. The ultrasonic method was found to be a moderately superior, more cost-effective, less time-consuming, high-yielding, and energy-saving process. The synthesis of octahydroquinazolinone derivatives was investigated and compared when using both of the catalysts. The ultrasonic methodology presented here offers attractive features in terms of product yield, reaction time, and recyclability when compared with non-ultrasonic catalysts, as well as with other catalysts that have a wide scope in organic synthesis. Moreover, this is a simple procedure that involves aqueous conditions combined with easy recovery of the catalyst, which makes this method economically and environmentally safe. In future research, the bi-functional properties of Pd doping on heteropoly acid with silica support could be used in various organic syntheses.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal11111273/s1. Figure S1: 4a 1H-NMR, Figure S2: 4a 13C-NMR, Figure S3: 4a 13C-DEPT 135 NMR, Figure S4: 4b 1H-NMR, Figure S5: 4b 13C-NMR, Figure S6: 4b 13C-DEPT 135 NMR, Figure S7: 4c 1H-NMR, Figure S8: 4c 13C-NMR, Figure S9: 4c 13C-DEPT 135 NMR, Figure S10: 4d 1H-NMR, Figure S11: 4d 13C-NMR, Figure S12: 4d 13C-DEPT 135 NMR, Figure S13: 4e 1H-NMR, Figure S14: 4e 13C-NMR, Figure S15: 4e 13C-DEPT 135 NMR, Figure S16: 4f 1H-NMR, Figure S17: 4f 13C-NMR, Figure S18: 13C-DEPT 135 NMR.

Author Contributions

Conceptualization, M.A.B., M.A. and A.I.A.; Catalyst preparation and characterization, M.A., A.I.A. and I.U.D.; Synthesis of octahydroquinazolinones with the ultrasonic method and characterization, M.A.B. and A.A. (Amena Ali); Conventional method of octahydroquinazolinones synthesis, A.A. (Amena Ali) and M.J.A.; Funding acquisition, A.A. (Abuzer Ali); Writing—original draft, M.A.B., M.A. and A.I.A.; Writing—review and editing, M.A.B., A.A. (Abuzer Ali), I.U.D. and M.J.A. All authors have read and agreed to the published version of the manuscript.

Funding

One of the authors, Abuzer Ali, is thankful to Taif University Research Supporting Project Number (TURSP-2020/124), Taif University, Taif, Saudi Arabia.

Acknowledgments

One of the authors, Abuzer Ali, is thankful to Taif University Research Supporting Project Number (TURSP-2020/124), Taif University, Taif, Saudi Arabia. The authors are also thankful to the the Central Laboratory Facilities, Prince Sattam bin Abdulaziz University, for the characterization of the catalysts.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 11 01273 g001 550
Figure 1. FT-IR analysis of the ultrasonically and non-ultrasonically synthesized 1%Pd-HPW/SiO2 bi-functional catalysts.
Figure 1. FT-IR analysis of the ultrasonically and non-ultrasonically synthesized 1%Pd-HPW/SiO2 bi-functional catalysts.
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Catalysts 11 01273 g002 550
Figure 2. XRD analysis of the ultrasonically and non-ultrasonically synthesized Pd-HPW/SiO2.
Figure 2. XRD analysis of the ultrasonically and non-ultrasonically synthesized Pd-HPW/SiO2.
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Catalysts 11 01273 g003 550
Figure 3. N2 adsorption–desorption isotherm of the ultrasonically (A) and non-ultrasonically (B) prepared 1%Pd-HPW/SiO2 bi-functional catalysts.
Figure 3. N2 adsorption–desorption isotherm of the ultrasonically (A) and non-ultrasonically (B) prepared 1%Pd-HPW/SiO2 bi-functional catalysts.
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Catalysts 11 01273 g004 550
Figure 4. XPS spectra of the 1%Pd-HPW/SiO2 bi-functional catalyst.
Figure 4. XPS spectra of the 1%Pd-HPW/SiO2 bi-functional catalyst.
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Catalysts 11 01273 g005 550
Figure 5. EDX analysis of the 1%Pd-HPW/SiO2 bi-functional catalyst.
Figure 5. EDX analysis of the 1%Pd-HPW/SiO2 bi-functional catalyst.
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Catalysts 11 01273 g006 550
Figure 6. SEM images of the ultrasonically (A) and non-ultrasonically (B) prepared 1%Pd-HPW/SiO2 bi-functional catalysts.
Figure 6. SEM images of the ultrasonically (A) and non-ultrasonically (B) prepared 1%Pd-HPW/SiO2 bi-functional catalysts.
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Catalysts 11 01273 sch001 550
Scheme 1. 1%Pd-HPW/SiO2 catalyzed Octahydroquinazolinone synthesis.
Scheme 1. 1%Pd-HPW/SiO2 catalyzed Octahydroquinazolinone synthesis.
Catalysts 11 01273 sch001
Catalysts 11 01273 sch002 550
Scheme 2. Proposed mechanism for octahydroquinazolinone synthesis.
Scheme 2. Proposed mechanism for octahydroquinazolinone synthesis.
Catalysts 11 01273 sch002
Table
Table 1. The texture and Pd dispersion of the catalysts studied.
Table 1. The texture and Pd dispersion of the catalysts studied.
CatalystSBETa
m2 g−1		Pore volume b
cm3 g−1		Pore Size c
Å
1%Pd-HPW/SiO2 (US)3950.2718.13
1%Pd-HPW/SiO2 (Non US)3820.2318.18
a BET surface area. b Multipoint total pore volume. c Average BET pore diameter.
Table
Table 2. Synthesis of 1%Pd-HPW/SiO2 with the non-ultrasonic (non-US) and ultrasonic (US) methods.
Table 2. Synthesis of 1%Pd-HPW/SiO2 with the non-ultrasonic (non-US) and ultrasonic (US) methods.
EntryReaction TimeYield (%)Energy Utilized (kJ/g)Energy Saved (%)
1NUSUSNUSUSNUSUS44
1 h20 min80951.640.92
Table
Table 3. Effects of the 1%Pd-HPW/SiO2 catalyst for the synthesis of the octahydroquinazolinone model (4a) a.
Table 3. Effects of the 1%Pd-HPW/SiO2 catalyst for the synthesis of the octahydroquinazolinone model (4a) a.
EntryCatalyst (mg)(US)Yield (%) b
(NON-US)		Time
(Minute)
1-TraceTrace120
250726760
370787040
480908530
5100979320
6120928920
a Dimedone (1 mmol), 4-fluoro benzaldehyde (1 mmol), and urea (1.5 mmol) with 100 mg of 1%Pd-HPW/SiO2 as a catalyst in 10 mL of distilled water at 100 °C. b Yield of isolated products.
Table
Table 4. The synthesis of octahydroquinazolinones (4a-f) derivatives catalyzed with 1%Pd-HPW/SiO2.
Table 4. The synthesis of octahydroquinazolinones (4a-f) derivatives catalyzed with 1%Pd-HPW/SiO2.
Catalysts 11 01273 i001
EntryRXProductTime (min)Yield (%) a,b M.P (°C)Ref. No.
USNon-USObservedReported
14-(F)-C6H4O4a209794268–270274–75[50]
23,4,5-(CH3O)-C6H2O4b259592135–137139–140[51]
32-( CH3)-C6H4O4c259391238–240240–241[51]
44-(F)-C6H4S4d209190261–263260–262[33]
53,4,5-(CH3O)-C6H2S4e308986132–134134–136[52]
62-(CH3)-C6H4S4f258684156–158--
a All reactions were performed with 100 mg of 1%Pd-HPW/SiO2 as a catalyst in a stoichiometric ratio of each reacting species in 10 mL of distilled water. b Isolated yields.
Table
Table 5. Catalyst optimization for the synthesis of the model octahydroquinazolinone 4a a.
Table 5. Catalyst optimization for the synthesis of the model octahydroquinazolinone 4a a.
EntryCatalystTimeYield b
1HPW3 h71
2HPW/SiO21.5 h88
31%Pd-HPW/SiO220 min97
a Dimedone (1 mmol), 4-fluoro benzaldehyde (1 mmol), and urea (1.5 mmol) with 100 mg of 1%Pd-HPW/SiO2 as a catalyst in 10 mL of distilled water at 100 °C. b Yield of isolated products.
Table
Table 6. Reusability studies of the 1%Pd-HPW/SiO2 catalyst a.
Table 6. Reusability studies of the 1%Pd-HPW/SiO2 catalyst a.
RunTime (min)Yield (%)Catalyst Amount (mg)
12097100
2209690
3209583
4209377
a Reactions performed under optimized conditions reactions in a stoichiometric ratio of each reacting species with 100 mg of 1%Pd-HPW/SiO2 as a catalyst.
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Bakht, M.A.; Alotaibi, M.; Alharthi, A.I.; Din, I.U.; Ali, A.; Ali, A.; Ahsan, M.J. Pd-HPW/SiO2 Bi-Functional Catalyst: Sonochemical Synthesis, Characterization, and Effect on Octahydroquinazolinone Synthesis. Catalysts 2021, 11, 1273. https://doi.org/10.3390/catal11111273

AMA Style

Bakht MA, Alotaibi M, Alharthi AI, Din IU, Ali A, Ali A, Ahsan MJ. Pd-HPW/SiO2 Bi-Functional Catalyst: Sonochemical Synthesis, Characterization, and Effect on Octahydroquinazolinone Synthesis. Catalysts. 2021; 11(11):1273. https://doi.org/10.3390/catal11111273

Chicago/Turabian Style

Bakht, Md. Afroz, Mshari. Alotaibi, Abdulrahman I. Alharthi, Israf Ud Din, Abuzer Ali, Amena Ali, and Mohamed Jawed Ahsan. 2021. "Pd-HPW/SiO2 Bi-Functional Catalyst: Sonochemical Synthesis, Characterization, and Effect on Octahydroquinazolinone Synthesis" Catalysts 11, no. 11: 1273. https://doi.org/10.3390/catal11111273

APA Style

Bakht, M. A., Alotaibi, M., Alharthi, A. I., Din, I. U., Ali, A., Ali, A., & Ahsan, M. J. (2021). Pd-HPW/SiO2 Bi-Functional Catalyst: Sonochemical Synthesis, Characterization, and Effect on Octahydroquinazolinone Synthesis. Catalysts, 11(11), 1273. https://doi.org/10.3390/catal11111273

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