Pd–Ce/ZIF-8 Nanocomposite for Catalytic Extraction of Sinomenine from Sinomenium acutum

Abstract

Sinomenine is a naturally occurring alkaloid and commonly used as one of the bioactive drug components in rheumatoid arthritis (RA) treatment in the clinic. Varying supported palladium-based catalysts have been synthesized and examined as heterogeneous catalysts for catalytic extraction of sinomenine from Sinomenium acutum. Among various examined supported catalysts, Pd–Ce/ZIF-8 (zeolitic imidazolate framework-8) demonstrates promising catalytic activity in the extraction reaction with an improved yield of 2.15% under optimized conditions. The catalyst composite can be recovered by centrifuging, and reused. A total of three catalyst recycling processes were performed with constant activity. The catalyst Pd–Ce/ZIF-8 has a particle size range of 2–12 nm and a total Pd–Ce loading amount of 5.1 wt% (ZIF-8).

1. Introduction

Rheumatoid arthritis (RA) is a chronic and destructive inflammatory disorder, affecting approximately 1% of the world’s population regardless of age, gender, race, and ethnicity, and it is growing [1]. RA usually affects the lining of the joints and shows inflammation with painful swelling, thickens the synovium, and eventually destroys the cartilage and erodes bone within the joint [2,3,4]. As of today, there is no cure for RA. Currently, the main clinical treatment option for RA is medication. Four types of medications, nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, disease-modifying anti-rheumatic drugs (DMARDs), and biologic agents, are used in RA treatment [2,3,4,5]. Sinomenine (9α,13α,14α-7,8-didehydro-4-hydroxy-3,7-dimethoxy-17-methylmorphinan-6-one) is a naturally occurring bioactive alkaloid, which has been extracted from the root of the climbing plant Sinomenium acutum [6,7,8]. Sinomenine-based pharmaceutical products have been well-developed in China, and used in the clinic to effectively relieve symptoms of rheumatoid arthritis and stop inflammation. In addition, sinomenine has also been found to be a key element of other therapeutic drugs, such as drugs for heart failure (HF) and dysrhythmias [9].

Sinomenine is a morphinan derivative. It has a phenanthrene core structure with an aromatic ring, two saturated rings, and an additional nitrogen-containing, six-membered saturated ring. The complicated molecular structure makes it a major challenge to totally synthesize and modify the molecular skeleton. In industry, sinomenine is currently mass-produced by a solvent-extraction technology [10]. However, the existing methods have inherent limitations to be overcome, such as being non-environmentally friendly and having a low yield [10]. Therefore, there is a high expectation to develop new technology to improve the current methods to extract sinomenine more effectively. Unfortunately, there are few reports detailing how to improve the technology. In one report, Tian et al. reported the application of imidazolium ionic liquids to improve the extraction performance of sinomenine with ultrasonic assistance [11]. On the other hand, various compounds, such as pentacyclic triterpenoids, alkanoids, and benzene derivatives, have been isolated from the dry powder of the S. acutum root [12]. These reports strongly suggest that S. acutum roots have partially similar chemical components to lignin. Lignin and other biomasses have been used as renewable bio-sources to manufacture aromatic compounds in the presence of a catalyst, such as a palladium-based catalyst [13,14,15,16,17]. Therefore, we first adopted the idea of catalytic conversion of biomass to value-added chemicals in the extraction procedure of a naturally occurring bioactive alkaloid from a traditional Chinese medicinal herb. Herein, we reported our preliminary results of using palladium-based supported catalysts in sinomenine extraction.

2. Results and Discussion

Palladium-based catalysts have been widely used in biomass conversion to produce high-value-added products. Therefore, we designed and synthesized Pd-based mono- and bis-metal catalysts to break the corresponding lignin-like dry powder of S. acutum. On the other hand, the commonly used catalyst supports, such as activated carbon, silicon, and metal–organic frameworks, were employed in this work. To investigate the catalytic effect on the sinomenine extraction, various palladium-based catalyst composites were prepared. The transition metal nanoparticle (NP)-based catalysts used in this work, including palladium NPs, palladium–gold NPs, palladium–ruthenium NPs, palladium–nickel NPs, and palladium–cerium NPs, were supported on the carriers by commonly used co-precipitation methods, in which the precursors Pd(acac)₂, AuCl, Ru(acac)₃, Ni(acac)₂, and Ce(acac)₃ were reduced by sodium borohydride, respectively. Different supports, including activated mesoporous carbon, multiple-walled carbon nanotubes, silicon, gamma (γ)-aluminum oxide, titanium oxide, mesoporous molecular sieves Ti-MCM-41 [18], porous aluminum terephthalate (MIL-53) [19], and Zeolitic imidazolate framework-8 (ZIF-8) (Basolite^® Z1200, 2-Methylimidazole zinc salt), were employed. The solid supports were either purchased from Sigma-Aldrich Lit. Co. or prepared according to literature methods [18,19]. The heterogeneous catalysts with promising activity and potential application for sinomenine extraction were fully characterized. The total amount of metal loading of all supported catalysts was controlled at around 5.0 wt% and that was further confirmed by inductively coupled plasma (ICP) analysis. All of the synthetic catalysts were evaluated for sinomenine extraction, and ZIF-8-supported Pd–Ce demonstrated an improved product amount. The Pd–Ce/ZIF-8 was fully characterized.

ZIF-8, Pd(acac)₂, Ce(acac)₃, AuCl, Ru(acac)₃, Ni(acac)₂ (acac = 2,4-pentanedionate), and solvents were purchased from Sigma and used without further purification. In this work, a widely used wet co-precipitation method was adapted to immobilize NP-based catalysts, such as Pd, Pd–Au, Pd–Ru, Pd–Ni, and Pd–Ce NPs, on various supports. The nanoparticles were synthesized by reducing the corresponding metal complex precursors Pd(acac)₂, Ce(acac)₃, AuCl, Ru(acac)₃, and Ni(acac)₂ (acac = 2,4-pentanedionate), respectively, using sodium borohydride. The products were fully characterized. Under the same conditions, catalyst Pd–Ce/ZIF-8 showed a higher activity, and its structure was analyzed by transmission electron microscopy (TEM), ICP, Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). The TEM images of the supported catalyst Pd–Ce NPs/ZIF-8 are shown in Figure 1. It was observed that the Pd–Ce NPs were small and dispersed on the support with slight aggregation. The average particle size was ~6 nm (determined from the measurement of ~120 particles). The Pd–Ce/MWCNTs (multiple-walled carbon nanotubes), Pd–Ru/ MWCNTs, and Pd/ZIF-8 catalysts were also analyzed by TEM to determine their particle size. As shown in Figure 2, the particle size of the loaded metals was around 5 nm and thus comparable with the metal particles on the Pd–Ce/ZIF-8 catalyst.

The oxidation states of the supported metal nanoparticles in catalyst Pd–Ce NPs/ZIF-8 were determined by XPS analysis (see Figure 3). Figure 3 (a) shows the spectrum for Pd 3d core electrons in the binding energy range of 330 eV to 350 eV. In a standard spectrum of metallic Pd (0), the Pd 3d_5/2 peak and the Pd 3d_3/2 peak are found at ~335 eV and ~340 eV, respectively [20,21]. Similar peaks were observed here and further confirmed from the careful Gaussian fitting of the spectra. The results suggest that Pd (0) metal has been successfully supported. Figure 3b shows the XPS spectrum for the Ce 3d electrons and the fitting peak positions in the range of 880 eV to 920 eV. The peaks at 885.66 eV and 903.91 eV were assigned to Ce³⁺ 3d_5/2 and Ce³⁺ 3d_3/2, respectively [21,22,23]. It is possible to explain that Ce⁴⁺ is also present because the fingerprint component at 917.68 eV of Ce⁴⁺ can be observed with a low percentage of 4.45% (AreaIntgP) [21,22,23]. Therefore, the XPS results suggest the coexistence of Ce⁴⁺, but to a very low extent as compared to the Ce³⁺ as shown in the Supplementary Materials. In the XRD spectra, the pristine ZIF-8 support catalyst Pd-Ce NPs/ZIF-8 shows similar absorption patterns due to a low loading amount and the ultrafine particle size of the transition metals. According to the ICP analysis, all loading amounts of the transition metals were in the range of 4.3–5.4 wt%. For the infrared spectroscopic analysis, normal absorption was observed in the FT-IR spectra for functional groups such as imidazole species, a component of the ZIF-8 support, both in pristine ZIF-8 and Pd-Ce/ZIF-8.

The catalysts were examined for sinomenine extraction, conducted in a mixed media of tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide and dimethyl sulfoxide (DMSO) in a volume ratio of 4:1. The mixture was found to be a good solvent to dissolve S. acutum powder. The dry powder of S. acutum was pretreated and dissolved in the above-described reaction media at 50 °C. The extraction was operated in a batch reactor with 1 atm H₂ at 60 °C for 2.5 h. After extraction, the reaction mixture was subjected to a GC-FID analysis and the results are listed in

Table 1. Results for catalytical reductive extraction of Sinomenium acutum.

Entry	Catalyst	Support Surface Area (m2/g)		Total Metal Loading Amount (wt%)	Sinomenine Yield (%)	TOF (Pd-based, h−1)
		Before Loading	After Loading
1	Contrast	NA	NA	NA	0.24	NA
2	PdAc2	NA	NA	47.4	1.73	0.16
3	Pd/C	200	ND	4.3	0.75	0.75
4	Pd/MWCNTs	220	195	4.6	1.02	0.96
5	Pd-Au/MWCNTs	220	140	4.5	1.05	1.76
6	Pd-Ru/MWCNTs	220	163	4.4	1.13	1.95
7	Pd-Ni/MWCNTs	220	ND	5.4	0.69	1.19
8	Pd-Ce/MWCNTs	220	152	5.2	1.25	2.15
9	Pd/SiO2	200	ND	4.7	0.84	0.72
10	Pd/γ-Al2O3	100	ND	5.2	0.92	0.79
11	Pd/TiO2	50	ND	4.8	0.77	0.66
12	Pd/MCM-41(mole ratio Si/Ti = 25:1)	915[18]	ND	4.5	0.59	0.51
13	Pd/MIL-53 (Al)	1590[19]	ND	5.4	0.72	0.62
14	Pd/ZIF-8	1550	1253	4.6	0.85	0.73
15	Pd-Ce/ ZIF-8	1550	1304	5.1	2.15	3.70
152nd	Pd-Ce/ ZIF-8	1550	ND	5.1	2.10	3.62
153rd	Pd-Ce/ ZIF-8	1550	ND	5.1	2.08	3.58
154th	Pd-Ce/ ZIF-8	1550	ND	5.1	2.11	3.64

Reaction conditions: solvent = Tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl) amide/ dimethyl sulfoxide (DMSO) (v/v = 4/1), cat loading amount = Pd 0.5 wt% of S. acutum, T = 60 °C, P_H2 = 1 atm, t = 2.5 h; yield based on Agilent gas chromatograph (GC-FID) analysis. Mass balance is 93–98%. ND = Not detected.

. Under the same conditions, catalyst Pd-Ce/NIF-8 showed higher activity in comparison with the other catalysts. A yield of 2.15% was reached by using the catalyst, and that is slightly higher than literature results [11]. Other types of palladium-supported catalysts, including Pd/C, Pd/MWCNTs, Pd–Au/MWCTs, Pd–Ru/MWCNTs, Pd–Ni/MWCNTs, Pd–Ce/MWCNTs, Pd/SiO₂, Pd/γ-Al₂O₃, Pd/TiO₂, Pd/MCM-41, Pd/MIL-53(Al), and Pd/ZIF-8, were also prepared and evaluated with lower yields of less than 1.25%. As seen in Table 1, the catalytic activities of the palladium-supported catalysts varied according to the combined metal. Under the same conditions of catalyst preparation and reaction, the catalytic activity followed a sequence of Pd–Ce > Pd–Ru > Pd–Au > Pd > Pd–Ni for the sinomenine extraction reaction. With the homogeneous catalyst PdAc₂, a yield of 1.73% was reached and that is slightly lower than that of the Pd-Ce/ZIF-8 catalyst. The optimized combination of the supported Pd–Ce NPs also showed advantages for other reactions [19]. The underlying mechanism remains unknown. Further investigation is underway in our laboratory to elucidate the phenomenon. Nevertheless, the co-supporting Ce³⁺ species could play crucial roles in improving the Pd catalyst. The Ce³⁺ species could enhance the catalyst activity by activating the Pd’s active center or the substrate. Furthermore, a catalytic lignin cleavage mechanism in a biomass conversion reaction could be partially applicable in the catalytic extraction reaction [24]. These results also suggest that Pd and Ce worked synergistically in the catalytic process, and that benefits the sinomenine extraction. It also can be seen that other metal combinations, such as Pd–Ni and Pd–Ru, did not provide an advantage; the catalyst support also played an important role in the extraction process.

Similar to other heterogeneous catalysts, both centrifugation and filtration techniques can be used to conveniently and effectively recover Pd–Ce/ZIF-8 from the reaction system. In this work, catalyst Pd–Ce/ZIF-8 was recycled three times with sustained activity for the sinomenine extraction reaction. In addition, we studied the leaching of the Pd–Ce nanoparticles from the catalyst support ZIF-8. The samples of the filtrate and washings obtained from the above standard reactions were collected and analyzed by an inductively coupled plasma optical emission spectrometer (ICP-OES). Based on the ICP results, the leaching concentrations of Pd and Ce were less than 4.0 and 5.3 ppm, respectively, in each measurement. Further, the extraction reaction was conducted with the recovered filtrate to identify the actual catalyst. However, the product yield was not increased in comparison with a control reaction. The results could prove that the actual catalytic species are contributed by the supported Pd–Ce NPs, rather than the leached metals. The homogeneous catalyst PdAc₂ was also explored for sinomenine extraction reactions with a yield of 1.73%. However, removal of the soluble catalyst PdAc₂ from the product mixture turned out to be a difficult problem. Indeed, catalyst recycling remains a major challenge for homogeneous catalytic processes. This limitation could prevent widespread applications of homogeneous catalysis, particularly in the pharmaceutical industry.

3. Materials and Methods

3.1. Materials and Equipment

Palladium acetate (PdAc₂), palladium (II) acetylacetonate (Pd(acac)₂), cerium (III) acetylacetonate (Ce(acac)₃), ruthenium (III) acetylacetonate (Ru(acac)₃), nickel (II) acetylacetonate (Pd(acac)₂), gold (I) chloride (AuCl), mesoporous carbon, multi-walled carbon nanotubes, silica, γ-aluminum oxide, titanium (IV) oxide, ZIF-8 (Basolite®Z 1200), Tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide, DMSO, and other solvents were purchased from Sigma-Aldrich, Singapore. S. acutum was purchased from Beijing Tong Ren Tang Science Arts Co. Pte Ltd, Singapore, and dried and crushed to powder with a lab ball mill. Supports MCM-41 (mole ratio of Si/Ti = 25:1) and aluminum terephalate (MIL-53 Al) were prepared according to previously reported methods [18,19]. Deionized water was used in this work.

3.2. Analysis Methods

TEM measurements were carried out on a JEOL Tecnai-G², FEI analyzer at 200 Kv, Singapore FT-IR samples were prepared by the KBr self-support pellet technique, and FT-IR spectra were recorded on a BIO-RAD spectrophotometer, Singapore. They were presented in the sequence of signal strength as strong (s), medium (m), and weak (w), and the peak pattern as single (s), multiple (m), and broad (br). Nuclear magnetic resonance (NMR) spectra were analyzed by a Bruker Fourier-Transform multinuclear NMR spectrometer at 400 and 100.6 MHz, respectively in Singapore, and Me₄Si (TMS) was used as the internal standard. ICP analysis was carried out using a VISTA-MPX, charge-coupled device (CCD) Simultaneous ICP-OES analyzer, Singapore. The oxidation states of metals were determined by an ESCALAB 250 XPS, Singapore.

3.3. General Procedure for Synthesizing Transition Metal Nanoparticles-Based Supported Catalysts

The supporting of metal nanoparticles on various supports was performed using the transition metal complex precursors Pd(acac)₂ (13.3 mg) and Ce(acac)₃ (19.1 mg) dissolved in tetrahydrofuran (THF) (35.0 mL). Two hundred milligrams of ZIF-8 was added to the above solution, and the resulting mixture was stirred for one hour. The mixture was then cooled to 0 °C by ice. After, 3.0 mL of NaBH₄ solutions were slowly added into the mixture. The final product was obtained by centrifuge, washed with methanol (2 × 10.0 mL) and diethyl ether (2 × 10.0 mL) in sequence, and dried in vacuum to obtain 210.0 mg of gray powder product. The products were subjected to analysis by TEM, ICP, XPS, and FT-IR. The same procedure was used to prepare Pd/C, Pd/MWCNTs, Pd–Au/MWCTs, Pd–Ru/MWCNTs, Pd–Ni/MWCNTs, Pd–Ce/MWCNTs, Pd/SiO₂, Pd/γ-Al₂O₃, Pd/TiO₂, Pd/MCM-41, and Pd/MIL-53(Al) catalysts. For bimetallic catalysts, the quantity of the starting material was calculated according to a mole ratio of Pd/M (M = Au, Ru, Ni) of 1:1 and a total theoretical metal loading amount of 5.0 wt%. IR (KBr pellet, cm^-1), 3348(br, s), 3020(s, s), 2930(s, s), 2712(s, m), 2361(s, m), 1653(s, vs), 1558(s, s), 1506(s, s), 1418(s, s), 1373(s, vs), 1296(s, m), 1254(s, s), 1157(s, s), 1111(s, m), 1074(s, m), 951(s, w), 914(s, w), 872(w, s), 827(s, m), 770(s, s), 691(s, m), 669(s, m), 567(s, m), 527(s, m), 502(w, s), 492(s, m), 419(s, m). All of the exact metal loading amounts were identified by an ICP-OES analyzer. The results are shown in Table 1.

3.4. General Procedure for Sinomenine Extraction Reactions

A 30.0 mL solution of Tetradecyltrihexylphosphonium bis(trifluoromethylsulfonyl)amide/DMSO (v/v=4/1) was used to dissolve S. acutum powder (1.0 g) at 50 °C. The resulting solution was filtered to remove any insoluble species. In a batch reactor equipped with a magnetic stirring bar, 30.0 mg of each catalyst was mixed with 6.0 mL of the above solution. The containers were then placed in a pressure chamber and flushed with hydrogen. The reaction was conducted at 60 °C and 1 atm of hydrogen for 2.5 h. After reaction, the reactor was cooled to room temperature spontaneously. The reaction vessels were taken out for the next workup. A solid catalyst was removed from the reaction mixture through a syringe filter. For Pd–Ce/ZIF-8, the catalyst was recovered by centrifuging, and reused for subsequent runs. The filtrates were collected in separate round bottom flasks and extracted with THF/EtOH (v/v = 1:1). The extraction solvent was removed under reduced pressure by using a rotary evaporator, and the resulting product mixtures were analyzed using gas chromatography (Agilent gas chromatograph (GC-FID), Singapore) 6850, column: Agilent 19091Z-433, flow rate: 1.5 mL/min). The yield was calculated according to the amount of S. acutum added and the results are listed in Table 1.

4. Conclusions

In this work, various palladium-supported heterogeneous catalysts were synthesized and employed in sinomenine extraction reactions for the first time. Catalyst Pd-Ce/ZIF-8 showed good activity for the extraction process to produce sinomenine in a high yield of 2.15 wt%. Based on their high catalytic activity and constant recyclability, the prototype catalysts can reasonably be expected to find broader applications in both academia and pharmaceutical chemistry. This new technology could particularly promote conversions of traditional Chinese medicinal herbs to high-value-added bioactive products, such as sinomenine.