1 Introduction

Suzuki and Heck coupling reactions are the most powerful one-step methods for carbon–carbon bond formation in organic synthesis, especially these reactions are employed in the preparation of a large variety of complex organic molecules, having prevalent applications in pharmaceutical, crop-protection, polymers, materials, and fine chemical applications [1,2,3,4,5,6,7,8,9,10,11,12]. The current state-of-the-art process relies on precious homogeneous palladium complexes with a combination of sophisticated and sensitive ligands [13,14,15]. Despite their excellent activity, selectivity, and tolerant to broad substrate scope, the catalyst separation and recycling of Pd are the major obstacles for large scale applications [16]. Moreover, these catalytic processes require a tedious workup procedure for the isolation of the product, resulting in large amounts of waste, leading to the expensive manufacturing process [17]. More importantly, the contamination of Pd and ligand residues in the end product limits their application in pharmaceutical industries [9, 18]. In contrast to homogeneous catalysts, solid catalysts are often preferred due to their easy separation and reusability [12, 19]. In this regard, a commercially available Pd/C catalyst was employed for Suzuki and Heck coupling reactions and achieved up to 36,000 ton [20]. But, functional group tolerance and too much leaching of Pd/C are the major problems [21]. Therefore, the research has been directed towards the development of green and eco-friendly heterogeneous Pd catalysts for C–C coupling reactions. In this sense, several methods have been developed for the immobilization of Pd complexes on solid supports/polymers to combine the benefits of both homogeneous and heterogeneous catalysts [12, 22]. However, these catalysts are also encountered with several issues pertaining to activity/selectivity and leaching of palladium into the reaction medium [22]. In addition, several chemists have reported heterogeneous palladium catalysts on various supports such as metal oxides [22,23,24,25,26,27], silica [28,29,30,31,32], zeolites [33,34,35], carbon materials [36,37,38], polymers [39,40,41,42,43], MOFs [44,45,46] and others [47,48,49,50,51,52,53] to facilitate the recovery and reuse of precious Pd catalysts. But, the issues related to the use and cost of catalyst supporter stabilizers, reaction conditions, and reproducibility are questioning the practical applicability in the industrial point of view.

On the other hand, the replacement of fossil resources by renewable materials has been given the top priority in fuel/chemical industries. In recent years, the use of renewable feedstocks emerges as a promising strategy for the production of fuels, chemicals, and materials in a more sustainable approach [54,55,56]. In this regard, highly abundant and low-cost agricultural waste by-product rice husk has received particular attention in academia and industry. In general, rice husk is produced from rice, as 1-ton rice produces 0.2 ton rice husk. Hence, the mass production of rice in the world results a huge volume of rice husk, nearly 148 million metric tons of rice husk from 740 million metric tons of rice annually, and treating as a low-value fuel for bio-refineries [57]. Rice husk is mainly composed of cellulose (30–35%), hemicelluloses (20–25%), lignin (15–20%), and ash (10–15%). Interestingly, more than 90% of rice husk ash is in the form of hydrated amorphous silica. Since the high content of amorphous silica, rice husk is being considered as an excellent precursor for the preparation of nano and mesoporous silica materials [57,58,59]. Recently, rice husk derived carbon-supported catalysts have been developed and applied successfully for C–C coupling reactions [60, 61]. Therefore, the preparation of rice husk derived amorphous SiO2 supported Pd catalysts is considered to be a green and low-cost approach for C–C coupling reactions.

Inspiring from the aforementioned works, herein, we demonstrate a simple and facile method for the preparation of inexpensive and environmentally benign biomass waste rice husk derived amorphous silica-supported palladium nano-catalyst (PdNPs/RH-SiO2). These RH-SiO2 supported Pd nanoparticles to create a stable and reusable catalyst for Suzuki–Miyaura and Heck–Mizoroki C–C cross-coupling reactions under mild reaction conditions.

2 Experimental section

2.1 Materials

Unless otherwise noted, all chemicals were purchased from M/s. Sigma–Aldrich and used as received. Gel-Silica (G-SiO2) with 60–120 mesh purchased from M/s. SD Fine Chemicals Pvt. Ltd. Rice-Husk was obtained from the local rice mill, Telangana. 35.4% HCl purchased from M/s. Rankem Pvt. Ltd. Phenylboronic acid (98%) purchased from M/s. Alfa Aesar, sodium carbonate (99.5%), Hydrazine (99%) purchased from M/s. SD Fine Chemicals Pvt. Ltd.

2.2 Preparation of catalysts

2.2.1 Preparation of rice-husk derived silica support (RH-SiO2)

RH-SiO2 support was prepared according to the reported procedure with modifications and details are shown in Scheme 1 [57, 60]. In a typical procedure, a required amount of raw rice husk was added to 0.1 M HCl solution and stirred at 100 °C for 6 h. Then, the mixture was cooled down to room temperature, and the solids were filtered and washed with distilled water to reach the pH ~ 7. The resultant solids were oven-dried at 100 °C for 12 h. The dried solids were calcined at 600 °C for 3 h under airflow with a 90 ml/min flow rate and yielded the RH-SiO2 support. The obtained silica was symbolized as RH-SiO2.

Scheme 1
scheme 1

Preparation of rice husk derived silica (RH-SiO2)

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2.2.2 Preparation of RH-SiO2-supported palladium nanoparticles (PdNPs/RH-SiO2)

The PdNPs/RH-SiO2 catalysts with 0.5%, 1%, and 1.25% Pd on RH-SiO2 support were prepared as per the reported method [62]. Typically, the required amount of PdCl2 was dissolved in distilled water, and 2–3 drops of HCl were added. Then, the requisite amount of RH-SiO2 support was added to the above solution and stirred at room temperature for 30 min. Subsequently, hydrazine hydrate was added drop wise under vigorous stirring for 1 h. The resulted reaction mixture was left for stirring for another 2 h, and the reaction mixture turns into black. The dark solids were filtered and washed several times with water followed by methanol and diethyl ether, respectively. The black solids were dried under vacuum at 100 °C for 12 h. In a similar procedure, 1% Pd on G-SiO2 (1% Pd/G-SiO2) and F-SiO2 (1% Pd/FM-SiO2) catalysts were prepared.

2.3 Catalytic activity

2.3.1 Procedure for Suzuki cross-coupling reaction

All reactions were performed in a 25 ml round bottom flask. In a typical experiment, Bromobenzene (1 mmol), phenylboronic acid (1.25 mmol), and Na3PO4 (1 mmol) were added to 2 ml EtOH:H2O (1:1 v/v) solvent. Then, 20 mg of the respective catalyst was added to the reaction mixture and stirred at 80 °C for the required time. After the reaction, the catalyst was recovered by simple filtration and washed numerous times with ethyl acetate followed by drying.

2.3.2 Procedure for Heck–Mizoroki coupling reaction

A mixture of iodobenzene (1 mmol), styrene (1.2 mmol), Na2CO3 (1 mmol), and 20 mg of catalyst in 2 ml of DMF solvent was taken into 25 ml round bottom flask and stirred at 120 °C for the required time. After completion of the reaction, the reaction mixture cooled down to room temperature and diluted with ethyl acetate solvent. The catalyst was recovered by filtration and washed with ethyl acetate and dried.

The products of all the above reactions were analyzed and identified by HP-GC and GC–MS (QP-2010 model, M/s. Shimadzu Instruments, Japan) equipped with EB–5 MS capillary column (30 m × 0.25 mm × 0.25 μm).

3 Result and discussions

Pd/RH-SiO2 nano-catalyst were prepared by a simple two-step method, (1) in the first step; rice husk biomass-derived SiO2 support was obtained by calcination of rice husk biomass under airflow at 600 °C, (2) secondly, the palladium nanoparticles (PdNPs) were deposited on RH-SiO2 support by chemical reduction with hydrazine hydrate.

3.1 Catalyst characterization

In order to understand the structure-activity of the 1% Pd/RH-SiO2 nano-catalyst for selective C–C coupling reactions, the catalyst was extensively characterized with comprehensive analytical techniques such as XRD, XPS, BET, TEM, solid-state 29Si CP-MAS NMR, and ICP-OES analysis.

3.1.1 N2 adsorption–desorption analysis

The textural characteristics of various silica-based catalysts are determined by N2 adsorption–desorption analysis. The BET surface area, pore-volume, and pore diameter and are depicted in Table 1. The commercially available Fumed-silica (F-SiO2) and G-SiO2 have the BET surface area of 552 and 318 m2/g with 0.3, 0.5 cm3/g pore volume and 3.2, 3.1 nm pore diameters (Table 1, entry 2 and 3). However, the rice husk derived SiO2 support has a low BET surface area (127 m2/g) and pore volume (0.1 cm3/g), but a large pore diameter (11.8 nm) was noticed with respect to commercial SiO2 supports (Table 1, entry 1). Further, a decrease in the surface area of all SiO2 supports has been observed after the introduction of PdNPs. A similar trend has been monitored for pore volume and pore diameter, respectively (Table 1, entry 4–8). These results suggest the pore blockage by the deposition of PdNPs. The Pd content in the catalysts was determined by ICP-OES analysis, and the results are close to the theoretical values (Table 1, entry 4–8).

Table 1 Textural properties of different SiO2 and supported Pd catalysts
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3.1.2 XRD analysis

The wide-angle XRD pattern of 1% Pd/RH-SiO2, 1% Pd/G-SiO2, and 1% Pd/F-SiO2 nano-catalysts are shown in Fig. 1. The 1% Pd/RH-SiO2 nanocatalyst exhibits a broad peak between 20° and 30° on the 2θ scale, which is a characteristic peak of amorphous silica. The 1% Pd/RH-SiO2 nanocatalyst displays a diffracted peak at 2θ = 40.1°, 2θ = 44.8° are referred to (111) (200) planes of palladium in zero oxidation state (JCPDS #05-0681). However, the peak corresponding to (220) plane is not observed, indicating the fine and homogeneous distribution of PdNPs throughout the SiO2 matrix. The similar XRD patterns observed with G-SiO2 and F-SiO2 supported Pd nanocatalysts, respectively (Fig. 1).

Fig. 1
figure 1

Wide-angle XRD patterns of different SiO2 and supported Pd catalysts

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3.1.3 XPS analysis

XPS analysis was performed to ascertain the oxidation state of palladium and metal-support interactions in the different SiO2 supported Pd catalysts, and the corresponding spectrum is shown in Fig. 2. As shown in Fig. 2, the Pd 3d spectra of 1% PdNPs/RH-SiO2 catalyst display two peaks, the binding energies centered at 343.2 and 337.8 eV are attributed to 3d5/2 and 3d3/2 of palladium in zero oxidation state, which is consistent with the reported values for palladium in zero-valent [60]. In the case of 1% PdNPs/FM-SiO2 and 1% PdNPs/G-SiO2 catalysts, the Pd 3d peaks are shifted to lower binding energies (342.9, 337.1 eV and 341, 336 eV). These results are indicating that the 1% PdNPs/RH-SiO2 nanocatalyst has strong metal-support (Pd–SiO2) interactions.

Fig. 2
figure 2

XPS patterns of PdNPs/RH-SiO2, PdNPs/FM-SiO2, and PdNPs/G-SiO2 catalysts

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3.1.4 Solid-state 29Si CP-MAS NMR

The 29Si CP-MAS NMR analysis provides information regarding the structural environment of silicon atoms in the SiO2 matrix. The 29Si CP-MAS NMR spectra of the different SiO2 samples are shown in Fig. 3. The spectrum of RH-SiO2 can be fitted into two peaks at −100 and −110 ppm ascribed to silanol groups [Q3, Si(Si–O)3(OH)] and (Si–O)4 of the silica framework [Q4, Si(Si–O)4] [63]. FM-SiO2 and G-SiO2 also exhibited the same NMR pattern (Fig. 3) but, the lines are with much larger line widths.

Fig. 3
figure 3

29Si CP-MAS NMR of a G-SiO2, b FM -SiO2, and (c) RH-SiO2

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3.1.5 TEM analysis

The size and morphology of Pd nanoparticles in 1% PdNPs/RH-SiO2, 1% PdNPs/FM-SiO2, and 1% PdNPs/G-SiO2 catalysts were investigated by using transmission electron microscopic (TEM) analysis, and the resultant TEM image is displayed in Fig. 4a. The Pd particles in 1% PdNPs/RH-SiO2 catalysts are spherical with an average particle diameter in the range of 3–5 nm and uniformly dispersed throughout the RH-SiO2 matrix. In the case of 1% PdNPs/G-SiO2 and 1% PdNPs/FM-SiO2 catalysts, the aggregation of Pd particles was observed in (Fig. 4b, c).

Fig. 4
figure 4

TEM images of a 1PdNPs/RH-SiO2, b 1PdNPs/FM-SiO2, c 1PdNPs/G-SiO2 catalysts

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3.2 Catalyst screening

The catalytic activity of as-prepared 1% Pd/RH-SiO2 nanocatalyst was systematically examined for the Suzuki–Miyaura cross-coupling reaction with bromobenzene (BB) and phenylboronic acids as model substrates and the corresponding results are shown in Table 2. Control experimental data shows that the reaction did not proceed in the absence of a catalyst (Table 2, entry 1). Similarly, no reaction was observed when SiO2 supports were directly used as catalysts (Table 2, entry 2–4), indicating the need for active metal species to perform the reaction. The yield of biphenyl (BP) was only 42% with 1% PdNPs/G-SiO2 catalyst, while the biphenyl yield was increased to 57% with 1% PdNPs/FM-SiO2 catalyst (Table 2, entry 5 and 6). To our delight, the reaction with 1% PdNPs/RH-SiO2 at 80 °C for 6 h using Na3PO4 base in H2O: Ethanol solvent mixture, the yield was significantly increased to >99% and without the formation of any undesired side products (Table 2, entry 7). The remarkable catalytic activity of 1% PdNPs/RH-SiO2 catalyst can be attributed to the formation of small-sized nanoparticles (3–5 nm) with uniform distribution on the RH-SiO2 surface and strong metal-support interactions. In order to optimize the reaction parameters, the reaction has been performed in different solvents (Table 2, entry 8–10). In non-polar toluene solvent, only 41% yield obtained (Table 2, entry 8), while the yields were improved to 68–78% in polar DMF and ethanol solvents (Table 2, entry 9 and 10). The maximum biphenyl yield (>99%) was achieved in H2O:Ethanol solvent mixture within the 6 h of reaction time (Table 2, entry 7). Next, we evaluated the effect of the base on the product yields. In general, the base plays a crucial role in the product yields and selectivity in Suzuki–Miyaura C–C coupling reactions [64]. From Table 2, we found that the reaction in inorganic bases proceeded smoothly than the organic base and obtained good to excellent desired product yields (Table 2, entry 11–14). Furthermore, the selectivity and yields depend on the strength of the base, as the yield was increased with decreasing the strength of the base. Further, we studied the effect of Pd loading on RH-SiO2 support for the Suzuki cross-coupling reaction of bromobenzene (BB) and phenylboronic acid (Table 2, entry 15 and 16). With an increase, the Pd loading on the RH-SiO2 support, the desired biphenyl product yields were also increased and obtained a maximum of >99% yield with 1% Pd loading (Table 2, entry 7). A further increase in the Pd loading, from 1 to 1.5 wt%, there is a significant drop in the biphenyl yield observed (Table 2, entry 16), which could be due to the aggregation of Pd nanoparticles on the surface of RH-SiO2 support with increasing Pd loading.

Table 2 Suzuki–Miyaura C–C cross-coupling reaction of bromobenzene (BB) and phenylboronic acids over different supported Pd catalysts
figure b
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With an optimized reaction condition in hand, Suzuki–Miyaura C–C cross-coupling of aryl halides and phenylboronic acids with a diverse functional group in the presence of 1% PdNPs/RH-SiO2was evaluated, and the results are depicted in Table 3. The reaction with simple iodobenzene proceeds smoothly and obtained 99% conversion and selectivity within 3 h of reaction time, whereas the bromobenzene required 6 h reaction time (Table 3, entry 1 and 6). No reaction was observed with chlorobenzene (Table 3, entry 15). This result suggesting the iodobenzene has more reactivity than the Bromo and chlorobenzene, which is due to the –I is better leaving group than the –Br and –Cl groups. Next, we investigated the electronic effect of substituted aryl halides on the activity and selectivity of 1% PdNPs/RH-SiO2 catalyst. The aryl halides with electron-withdrawing groups at para-position need short reaction times for complete conversion with 99% selectivity (Table 3, entry 4, 7, 10). In contrast, the electron-donating groups require a longer reaction time (Table 3, entry 3, 9, 14). Further, the aryl halides with electron-withdrawing/donating groups at ortho-position are less reactive than the para-position (Table 3, entry 2, 8, 11, 13). This phenomenon could be due to the substitute groups at the ortho position create a steric hindrance, which further restricts the reactivity of aryl halides. Also, the –OCH3 substituted phenylboronic acids were successfully applied for the Suzuki–Miyaura C–C cross-coupling reaction in the presence of 1% PdNPs/RH-SiO2 catalyst (Table 3, entry 5, 12).

Table 3 Substrate scope of Suzuki cross-coupling over 1% PdNPs/RH-SiO2 catalyst
figure c
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In the present context, the exact mechanism is not clear. However, according to the literature and the current results, we have proposed the plausible mechanism, as shown in Scheme 2. Initially, oxidative addition of aryl halide to Pd(0)/RH-SiO2 (I) to form a phenyl Pd halide complex (II). Next, the trans-metalation step involves the transfer of phenyl group from phenylboronic acid to phenyl Pd halide complex (II) to form complex III. Finally, the reductive elimination step leads to the formation of the desired biphenyl product and regeneration of Pd(0)/RH-SiO2 (I) catalyst.

Scheme 2
scheme 2

Proposed mechanism for Suzuki–Miyaura coupling over PdNPs/RH-SiO2 catalyst

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3.2.1 Heck–Mizoroki coupling

Since the remarkable activity and selectivity of 1% PdNPs/RH-SiO2 catalyst for C–C bond formation via Suzuki–Miyaura cross-coupling reaction of aryl halides with phenylboronic acids, we further investigated the activity of 1% PdNPs/RH-SiO2 catalyst for Heck–Mizoroki coupling reaction of iodobenzenes with styrene for the preparation of stilbenes. The reactions were carried out at 120 °C in DMF solvent using Na2CO3 base over 1% PdNPs/RH-SiO2 catalyst for required reaction time, and the results are shown in Table 4. The catalyst is tolerant of a variety of iodobenzene derivatives and afforded good yields of stilbenes with excellent selectivity (Table 4, entry 1–5).

Table 4 Substrate scope of Heck-coupling over 1% PdNPs/RH-SiO2 catalyst
figure d
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3.3 Reusability

To assess the stability and activity of the 1% PdNPs/RH-SiO2 for Suzuki–Miyaura C–C cross coupling reaction of aryl bromide with phenylboronic acid, the catalyst was separated by centrifugation and washed several times with ethyl acetate followed by acetone solvents. The catalyst was vacuum dried at 60 °C, reused four times under the optimized reaction conditions (Fig. 5). Within repeated cycles, there is a little drop in the bromobenzene (Bb) conversion observed, but the selectivity of biphenyl (Bp) remains constant up to four successive cycles. Palladium leaching test was conducted by hot filtration method, in which the reaction was carried out for half of the reaction time (after 3 h) and separated the catalyst from the product mixture by centrifugation and continued the reaction for 6 h, but no conversion or product formation was observed. Further, the reaction solution was analyzed by ICP-OES and found no trace of Pd suggesting the no leaching of Pd nanoparticles during the reaction. From these results, it is concluded that this catalyst involves in a complex heterogeneous mechanism [52].

Fig. 5
figure 5

Reusability of 1% PdNPs/RH-SiO2 catalyst. Reaction conditions: Bromobenzene (Bb): 1 mmol, phenylboronic acid (1.25 mmol), catalyst (20 mg), Na3PO4 (1 mmol), 80 °C, 6 h

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4 Conclusions

In summary, we have developed a green and environmentally benign biomass waste rice husk derived amorphous silica-supported palladium (1% PdNPs/RH-SiO2) nanocatalyst via the deposition of PdNPs on rice husk derived amorphous silica obtained by chemical reduction method with an average particle size of 3–5 nm. This novel catalyst exhibits an excellent activity for Suzuki–Miyaura and Heck–Mizoroki C–C cross-coupling reactions for the selective preparation of biaryl and stilbene compounds under mild reaction conditions. Furthermore, the catalyst is tolerant of various functional groups under the optimized reaction conditions and reused four times for the Suzuki–Miyaura C–C coupling reaction of bromobenzene and phenylboronic acid. However, a little drop in the conversion was monitored, but the selectivity remains constant in all four cycles.