Hydrogenation of α-Pinene over Platinum Nanoparticles Reduced and Stabilized by Sodium Lignosulfonate.

A one-pot clean preparation procedure and catalytic performance of platinum nanoparticles (NPs) reduced and stabilized by sodium lignosulfonate in aqueous solution are reported. No other chemical reagents are needed during the metal reduction and stabilization step, thanks to the active participation of sodium lignosulfonate (SLS). UV-vis, Fourier transform infrared (FT-IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), 1H NMR, 195Pt NMR, and two-dimensional heteronuclear single-quantum coherence (2D HSQC) NMR studies were thoroughly performed to analyze the formation, particle size, and main lattice planes of NPs, the valence-state changes of the metal, and structural changes of SLS. An ecofriendly selective synthesis of cis-pinane from an abundant renewable natural resource, α-pinene, was developed in the presence of the prepared Pt NP aqueous system. Furthermore, this catalyst system was proved to show easy recovery and stable reusability by five-run tests. The synergistic effect of SLS reduction and stabilization not only avoided the introduction of conventional reducing agents and stabilizers but also made full use of the byproducts of the pulp and paper industry. This proved to be an environmentally friendly method for converting the natural resource α-pinene to cis-pinane.

Introduction

When low-cost natural biomass resources are employed to prepare high-value-added chemicals, materials, and intermediates, environment-friendly, productive processes are likewise greatly desired for the sake of green chemistry principles. Pine resin, known as “petroleum growing on trees”, is one of the important biomass resources. As the base of the light component of pine resin, α-pinene can be selectively and catalytically hydrogenated into cis-pinane, which is a significant platform chemical compound for pharmaceutical and perfume industries.[1−3] The common catalysts of α-pinene hydrogenation, such as Pd/C, Raney-Ni, and supported catalysts, always suffered from high temperature and pressure requirements, low selectivity, easy coking, etc.[4] In the last few decades, metal nanoparticles (NPs) have exhibited perfect catalytic performance in some organic redox reactions and the hydrogenation of α-pinene as well.[5−7] However, the general preparation of metal nanoparticles involves the reduction of metal ions to the zero-valent state using nonenvironmentally friendly reagents, such as sodium borohydride, hydrazine, or dimethylformamide, or a harsh reductive gaseous environment and the stabilization of these metal nanoparticles in an organic solvent, using various polymers, dendrimers, surfactants, polyoxometalates, or organic ligands as capping or encapsulating agents.

We have previously developed clean hydrogenation processes of α-pinene under mild conditions based on aqueous micelle microreactors with metal nanoparticles stabilized by amphipathic synthetic or natural polymers, ionic liquids, or mesoporous materials. It was noted that favorable reuse stability of catalysts could be achieved under the above conditions.[8−12] Still, in the synthesis of such metal nanoparticles stabilized by amphipathic substances, a variety of not-so-benign chemical reagents are also necessary as reductants and capping or dispersing agents. As a result, a greener and recyclable procedure in aqueous solution for the transformation of α-pinene into cis-pinane is expected eagerly.

Lignin is the second most abundant and only biomass natural polymer with aromatic units and multifunctional groups in its structure on the earth.[13] It is available in large amounts as a waste product of the wood pulp industry.[14,15] Xie[16] found that lignosulfonic acid (LSA) can effectively catalyze the conversion of fructose to 5-hydroxymethylfurfural (HMF) in ionic liquids, which provides a novel idea following the green chemistry principles. Nanoparticles of metals such as Au, Pd, Ag, or Pt have been prepared and stabilized in an aqueous or organic phase by lignin derivatives from the “black liquor” obtained during the pulp-making process in recent years and were, accordingly, used in C–C coupling or electrocatalytic reactions.[13,17−20] However, little research has been reported on the application of the above metal catalysts stabilized by lignosulfonates to meet the practical green chemistry demand. Particularly, there is no report yet on hydrogenation catalyzed by such nanoparticle systems to achieve the cleaner production goal of biomass chemicals. Herein, we demonstrate a Pt nanoparticle aqueous catalytic system without any other reagents but sodium lignosulfonate (SLS), which can act as both reducing and stabilizing reagents, and its efficient catalytic hydrogenation of α-pinene. In this way, only H2PtCl6·6H2O and several clean and low-cost natural resources, such as water, sodium lignosulfonate, and α-pinene, were involved in the production of a significant product, cis-pinane, which made it a perfect ecofriendly process.

Results and Discussion

Behavior of Metal Nanoparticles Reduced and Stabilized by Sodium Lignosulfonate

Table shows that the aqueous solution of SLS exhibited almost no activity for α-pinene hydrogenation. No obvious change in the UV–vis diagnostic absorption (274 nm, attributed to the phenolic units in the SLS structure[17,21] of this SLS solution) was observed after stirring at 80 °C for 3 h under the reaction conditions given in Table (Figure S1). Accordingly, the reduction of metal ions in solution can be estimated by the disappearance of their UV–vis diagnostic absorption peaks.[22,23] There was no obvious change in appearance when an aqueous solution of H2PtCl6 was added to SLS solution (SLS + H2PtCl6-0h in Figure ). After a self-reduction at 80 °C for 2–3 h, the mixture solution changed into a black transparent one (SLS + H2PtCl6-3h in Figure ). At the same time, as shown in Figure , the UV–vis diagnostic absorption strength of H2PtCl6 at 259 nm[24] degraded notably. As a result, the SLS diagnostic absorption at 274 nm, covered by the H2PtCl6 band previously, became visible with lower strength than that of the pure SLS solution, due to partial oxidation of the phenolic units.[17,25]

Table 1

Catalytic Hydrogenation Behavior of Pt Nanoparticles Reduced and Stabilized by SLS in Aqueous Solution

					hydrogenation performance
					conv. of α-pinene (%)	selectivity (%)
entry	samples	water (mL)	SLS (mg)	SLS concentration (g L–1)		cis-pinane	trans-pinane
1	SLSa	6	60	10	4.46	52.69	47.31
2	H2PtCl6b	6	0	0	99.78	93.93	6.07
3	H2PtCl6b*	6	0	0	85.10	94.41	5.59
4	SLS + H2PtCl6-0hb	6	60	10	73.90	94.00	6.00
5	SLS + H2PtCl6-0hb*	6	60	10	60.12	94.22	5.78
6	SLS + H2PtCl6 – NaBH4b	6	60	10	99.55	94.96	5.04
7	SLS + H2PtCl6 – NaBH4b*	6	60	10	86.25	93.76	6.24
8	SLS + H2PtCl6-3hb	6	60	10	84.31	93.86	6.14
9	SLS + H2PtCl6-3hb*	6	60	10	83.30	93.24	6.76
10	SLS + H2PtCl6-3ha	6	60	10	99.44	93.91	6.09
11	SLS + H2PtCl6-3hb	6	20	3.33	77.65	91.26	8.74
12	SLS + H2PtCl6-3hb	6	40	6.67	79.43	91.86	8.14
13	SLS + H2PtCl6-3hb	6	80	13.33	75.84	92.81	7.19
14	SLS + H2PtCl6-3hb	6	100	16.67	69.75	91.47	8.53
15	SLS + H2PtCl6-3hb	3	60	20	85.77	95.54	4.46
16	SLS + H2PtCl6-3hb	9	60	6.67	83.33	92.45	7.56
17	SLS + H2PtCl6-3hb	12	60	5	82.34	92.73	7.27
18	SLS + H2PtCl6-3hb	15	60	4	82.02	89.07	10.93

Reaction conditions: n(α-pinene)/n(catalyst) = 100, 10 mmol α-pinene, 3 MPa H2, 100 °C, 1.5 h.

n(α-pinene)/n(catalyst) = 400, 10 mmol α-pinene, 1 MPa H2, 70 °C, 1.5 h. *Catalysts after a five-run recycling test.

Figure 2

Changes in appearance during catalyst preparation: (a) SLS; (b) SLS + H2PtCl6-0h; (c) SLS + H2PtCl6-3h; and (d) SLS + H2PtCl6-3h#.

Figure 1

UV–vis plots of Pt NPs reduced by SLS.

The above Pt NP aqueous catalytic system exhibited excellent catalytic performance for the hydrogenation of α-pinene. A 99.44% conversion of α-pinene and a 93.91% selectivity for cis-pinane were achieved under the following conditions: n(α-pinene)/n(catalyst) = 100, 10 mmol α-pinene, 3 MPa H2, 100 °C, 1.5 h (entry 10 in Table ). Furthermore, when the hydrogenation was performed under much milder conditions, n(α-pinene)/n(catalyst) = 400, 10 mmol α-pinene, 1 MPa H2, 70 °C 1.5 h (entry 8 in Table ), the conversion and selectivity could still reach 84.31 and 94.36%, respectively. Moreover, good reusability (entry 9 in Table ) and steady appearance (see Figure , SLS + H2PtCl6-3h#, the slight muddiness was attributed to the dispersion of trace organics in the aqueous solution) of the separated Pt NP aqueous catalytic system were observed.

Although the initial hydrogenation activity of the H2PtCl6 aqueous solution was pretty high (99.78%), thanks to the rapid in situ reduction action by hydrogen, the precipitate of the metal would be observed after the hydrogenation process without the help of prior stabilization by SLS. When SLS was added to the H2PtCl6 aqueous solution and the hydrogenation was started straight away (Table , entry 4), the complexation by SLS molecules would play a competitive role with the reduction by H2. As a result, the reduction of metal would be slowed down. Accordingly, the initial hydrogenation activity of SLS + H2PtCl6-0h is lower (73.90%) than that of the H2PtCl6 aqueous solution. Moreover, because the prior stabilization by SLS was still not enough, the reuse activity of SLS + H2PtCl6-0h, together with that of the H2PtCl6 aqueous solution, decreased considerably (85.10 and 60.12%, respectively).

Likewise, when NaBH4 was introduced into the SLS + H2PtCl6-0h system as a reductant (Table , entry 6), the high catalytic activity could be attributed to the rapid reduction of Pt by NaBH4. However, in a similar way, since the stabilization by SLS molecules would not be as fast as the reduction of Pt by NaBH4, agglomerated particle precipitation of Pt and decreased reuse activity would also be observed (Table , entry 6). It can be distinctly seen from Table and Figures and 2 that only the Pt metal nanoparticles formed via a one-pot reduction–stabilization synergistic process by SLS at 80 °C exhibited excellent recyclable catalytic performance for hydrogenation of α-pinene. Thus, a clean preparation method of the catalyst and its catalytic hydrogenation in the aqueous phase were developed under simple conditions.

Characterization of Pt NPs Reduced and Stabilized by SLS

The X-ray photoelectron spectroscopy (XPS) analysis indicates that there was only Pt4+ in the SLS + H2PtCl6-0h sample (Figure b, BE = 73.08 eV (Pt 4f7/2) and BE = 76.53 eV (Pt 4f5/2)). However, the diagnostic peaks of both Pt4+ (BE = 73.01 and 76.41 eV) and zero-valent Pt (BE = 71.66 and 75.01 eV) were observed in the SLS + H2PtCl6-3h sample (Figure c, 85.56% Pt4+and 14.44% Pt0), which was due to the reducing action of SLS. Furthermore, it was found that the recycled sample (SLS + H2PtCl6-3h#) possessed more zero-valent Pt (Figure d, 70.58%) after catalytic hydrogenation.

Figure 3

XPS spectra of SLS + H2PtCl6-0h, SLS + H2PtCl6-3h, and SLS + H2PtCl6-3h#: (a) survey scan and (b–d) Pt 4f.

The surface oxygen-containing group of catalysts can also be detected by XPS analysis.[26] High-resolution C 1s spectra of the samples (Figure a–d) were deconvoluted into four peaks at BE = 284.7(C–C, sp3), 285.1(C–C, sp2), 286.3(C–O), and 288.5–288.9 eV(C=O), respectively. It was found that the C–O content of SLS in the SLS + H2PtCl6-3h sample was lower than that in the SLS + H2PtCl6-0h sample, accompanied by an increase in C=O content, which verified the transformation of hydroxyls of SLS into quinone or aldehyde ketone groups during the reduction and stabilization of the metal. The decrease in C=O content in the SLS + H2PtCl6-3h# sample was attributed to the re-reduction by H2.[25,27]

Figure 4

C 1s core-level spectra (a) and fitted C 1s XPS spectra for SLS + H2PtCl6-0h (b), SLS + H2PtCl6-3h (c), and SLS + H2PtCl6-3h# (d).

High-resolution O 1s spectra of SLS + H2PtCl6-0h, SLS + H2PtCl6-3h, and SLS + H2PtCl6-3h# samples (Figure S2a–d) exhibited decreasing BE values in sequence, which accounted for the coordination stabilization of Pt NPs by O atoms in SLS structures.[28] The increasing C=O proportion in the SLS structure shown in Table S1 would offer more stabilization sites for the Pt metal and then improve the stability of NPs.[29,30]

195Pt NMR analysis of SLS + H2PtCl6-0h, SLS + H2PtCl6-3h, and SLS + H2PtCl6-3h# samples was performed to distinguish the existential state of the Pt metal in various preparation steps. From Figure , the characteristic peaks of a standard K2PtCl6 sample at 10 and 508 ppm were attributable to the chloroplatinate PtCl62– and PtCl5(H2O)− species, respectively.[31−33] However, only diagnostic signals of PtCl62– species were observed in the SLS + H2PtCl6-0h sample within the chemical shift range from 1700 to 2700 ppm due to the slower water ligand exchange in the acidic solution.[34,35] Furthermore, no diagnostic signals were found in SLS + H2PtCl6-3h and SLS + H2PtCl6-3h# samples. Together with the results of Figures and S2a, it can be estimated that the remaining PtIV in SLS + H2PtCl6-3h and SLS + H2PtCl6-3h# samples undergo steady complexation with SLS.[36,37]

Figure 5

195Pt NMR spectra of SLS + H2PtCl6-0h, SLS + H2PtCl6-3h, SLS + H2PtCl6-3h#, and K2PtCl6.

It has been found that the main frame structures of lignin are quite stable. Over the Pt/C catalyst, the β-O-4 bonds of the lignin structure needed 350 °C, 15 MPa H2, or more harsh conditions (such as in the presence of supercritical ethanol) to undergo hydrodeoxygenation and depolymerization.[38−40]1H NMR analysis of SLS + H2PtCl6-0h, SLS + H2PtCl6-3h, and SLS + H2PtCl6-3h# samples was performed to study the structural change of SLS during the preparation and catalysis process of Pt NPs. It can be learned from Figure S3 that except for the signal strength of methoxyl at 3.0–4.4 ppm, no other distinguishable change was observed from the 1H NMR spectra of these three samples.[41−44] Furthermore, chloroform was employed to extract the possible small organic molecules in SLS + H2PtCl6-0h and SLS + H2PtCl6-3h samples (before and after treating under the conditions mentioned in Table , but without the participation of α-pinene), and no product from the depolymerization of SLS was detected by GC.

Two-dimensional (2D) NMR spectroscopy is a powerful tool to gain important structural information of complex molecular skeletons such as lignin.[45,46] In this study, 2D heteronuclear single-quantum coherence (2D HSQC) NMR spectroscopy was employed to understand the fine structure changes of SLS in SLS + H2PtCl6-0h, SLS + H2PtCl6-3h, SLS + H2PtCl6-3h#, and SLS + H2PtCl6-3hb* samples. HSQC cross-signals at δC/δH = 50–90/2.5–5.5 (side-chain regions of SLS) and δC/δH = 100–135/5.5–8.5 (aromatic regions of SLS) are observed in Figures and S4. At the same time, Figure S5 shows the well-resolved anomeric correlation signals of associated carbohydrates at δC/δH = 90–105/3.9–5.4. Tables S2 and S3 also list the major lignin cross-signals and the distribution of related carbohydrate cross-signals assigned in HSQC spectra, respectively, by comparison with published literature.[47−53]

Figure 6

Side-chain (a)(b) (δC/δH = 50–90/2.5–6.0) and aromatic regions (c)(d) (δC/δH = 100–135/5.5–8.5) in the 2D HSQC NMR spectra. (a)(c) SLS + H2PtCl6-0h; (b)(d) SLS + H2PtCl6-3h. Symbols are taken from drawing: (A) β-aryl ether formed by β-O-4′ linkages; (A′) β-aryl ether with Cα-oxidized β-O-4′ substructures; (B) resinol; (C) phenylcoumaran; (D) spirodienone; (I) p-hydroxycinnamyl alcohol end groups; (PCA) p-coumarate substructures; (FA) ferulate substructures; (H) p-hydroxyphenyl units; (G) guaiacyl units; (G′) oxidized guaiacyl units with a Cα ketone; (S) syringyl units; (S′) oxidized syringyl units with a Cα ketone. See Table S2 for signal assignment.

By the reduction operation, the Aβ(S/G) structure was gradually weakened to the oxidized (Cα=O) β-O-4′ structure A′ appeared at δC/δH = 83.4/5.20, and the Aβ(S/G) structure basically disappeared after the hydrogenation reaction and repeated use, indicating that the benzyl alcohol structure was partially oxidized to a benzyl carbonyl structure through the reduction process and hydrogenation process.[54−56] Moreover, with the steady reduction of Pt, the peak intensity of structure B was gradually weakened. Meanwhile, the reduction process will lead to the decomposition of some resinol substructures. It is important that the C2–H2 and C6–H6 correlations of oxidized α-ketone structures G′ could be found at δC/δH = 110.5/7.55 and 123.9/7.55 after the reduction and hydrogenation operations. In addition, some other fine structures have not changed significantly. The extraction results and 2D NMR characterization data of SLS + H2PtCl6-0h and SLS + H2PtCl6-3h showed that the guaiacyl and syringyl groups of SLS did not oxidize to the corresponding quinone structure. After reduction and stabilization of the catalytic system for hydrogenation and repeated use, the structure of SLS did not change significantly.

Fourier transform infrared (FT-IR) spectra of the catalyst samples before and after reduction and hydrogenation are presented in Figure S6. According to the results of XPS, only the diagnostic absorption strength of C=O groups at 1715 cm–1[57,58] increased after the reduction and hydrogenation procedures because of the oxygenation of hydroxyls by the metal salt. Meanwhile, the diagnostic absorption of sulfonic groups at 1216 and 1036 cm–1, wide band of hydroxyls at 3405 cm–1,[59,60] stretching vibration of −CH2– at 2939 and 2839 cm–1,[61] characteristic peaks of benzene rings at 1611, 1513, and 1423 cm–1, stretching vibration of C–O bonds at 1330 cm–1[62] of all samples indicated the relative stability of the SLS frame structure.

From the X-ray diffraction (XRD) patterns shown in Figure S7, the SLS + H2PtCl6-0h sample showed the same wide diffraction peak at 2θ = 20–30° as SLS.[61,63,64] The face-centered cubic Pt(111), (200), (220), and (311) crystal peaks of SLS + H2PtCl6-3h, SLS + H2PtCl6-3h#, and SLS + H2PtCl6-3hb* samples at 39.9, 46.4, 67.8, and 81.6° confirmed the formation of Pt NPs.[65] Besides, the weaker diffraction peak at 2θ = 20–30° indicated the interaction between the SLS structure and Pt species. In addition, the observed NaCl crystal pattern in the latter samples was the result of PtCl62– hydrolysis, which gave Cl– to couple with Na+ in the SLS structure during the reduction procedure of H2PtCl6. Irregular form of Pt NPs of size 3.2 ± 1.1 nm could be directly observed via transmission electron microscopy (TEM) of the SLS + H2PtCl6-3h sample (Figure ), while Pt NPs in SLS + H2PtCl6-3h# were smaller in size (2.4 ± 0.7nm) due to the reduction process. High-resolution TEM (HR-TEM) analysis indicated that the main lattice planes of Pt NPs prepared in this work were (111) and (200).

Figure 7

TEM images and particle size distribution (PSD) of Pt NPs of SLS + H2PtCl6-3h (A, B) and SLS + H2PtCl6-3h# (D, E); HR-TEM images (the inset images are the corresponding FFT patterns of Pt NPs) of SLS + H2PtCl6-3h (C) and SLS + H2PtCl6-3h# (F).

Reducing and Stabilizing Action of the Functional Groups in the SLS Frame

Guaiacyls, syringyls, and para-hydroxyphenyls were found to be the main construction units in the lignin structure, accompanied by phenolic hydroxyls, alcoholic hydroxyls, and methoxyls as the major functional groups. In the case of SLS, sulfonic groups are also involved (Scheme ).[66] Herein, anisole, phenol, 2,6-dimethoxyphenol, 4-propylphenol, 2-methoxyphenol, and 2-methoxy-4-propylphenol were used, respectively, to simulate the reducing action of various functional groups in SLS. From the UV–vis spectra shown in Figure S8, no reducing capacity toward H2PtCl6 was observed by the methoxyls in anisole. However, the hydroxyls in phenol were found to be effective reductants for H2PtCl6. Furthermore, both propyls and methoxyls could enhance the reducing capacity of phenolic hydroxyls, which accounted for the formation of Pt0 NPs in the SLS solution.

Scheme 1

Typical Structure of SLS

When lignosulfonic acid was used to replace SLS in the preparation of the Pt NP aqueous catalytic system, precipitation emerged in the prepared solution, along with a pretty low catalytic activity for hydrogenation of α-pinene. Although lignosulfonic acid and SLS share almost the same structure, except for anti-ions, the favorable dispersing and stabilizing capability of SLS, a well-known anionic surfactant, mainly comes from the good dissociation of −SO3Na. Meanwhile, the anti-ionic action of Na+ also contributes to the colloidal behavior and critical micelle concentration (CMC). Since the weaker dissociation ability and higher CMC of lignosulfonic acid cannot sufficiently meet the needs of dispersion and stabilization of Pt NPs, precipitation would be observed. Moreover, fewer available active sites due to precipitation, together with complexation by other groups of lignosulfonic acid molecules, result in the low catalytic activity of the lignosulfonic acid system.

That is to say, SLS can provide not only steric and coordination stabilization by aromatic rings and hydroxyl oxygen,[67,68] but also stabilization of micelles in water solution (Figure S9). It can be learned from Table that although the initial activity of the H2PtCl6 solution was lowered in the presence of SLS, the formed SLS micelles in the case of a higher concentration of SLS than CMC[69] would provide effective stabilization to Pt NPs. As a result, the catalyst solution with SLS achieved perfect reusability. However, excessive SLS would also hinder the effective collision of NPs and α-pinene and was expected to result in a decrease in activity. Table also reveals that the dosage of water slightly affects the catalytic activity.

Effect of Reaction Conditions on the Hydrogenation of α-Pinene Catalyzed by the Pt NP Aqueous System

As can be observed in Figure a, the conversion of α-pinene increased gradually with the dosage of the Pt NP catalyst and reached the maximum at a 1:400 dosage ratio of Pt after 2 h of reaction. Meanwhile, the selectivity of cis-pinane underwent a slight decrease with the increase of Pt dosage ratio to α-pinene and reaction time. Figure b shows that the reaction temperature played a key role in the hydrogenation of α-pinene catalyzed by the prepared Pt NPs (1:400 dosage ratio to α-pinene). The conversion of α-pinene increased gradually with the reaction temperature and reached equilibration at 70 °C after 2 h of reaction. It can also be observed from Figure c that 1 MPa hydrogen pressure resulted in the highest catalytic activity within 2 h. However, a higher hydrogen pressure than that needed a longer reaction time to achieve the highest conversion of α-pinene.

Figure 8

Optimization of hydrogenation conditions catalyzed by the Pt NP system: catalyst molar ratio (a), reaction temperature (b), hydrogen pressure (c), and recycling performance (d).

Reusability of the Pt NP Aqueous Solution System

Pretty good reusability of the prepared Pt NP catalytic system is observed from Table and Figure d after a five-run test. The FT-IR and XRD data of the Pt NP catalyst samples after five runs (SLS + H2PtCl6-3hb*) revealed almost no change compared with those of the fresh catalyst samples, which was attributed to the stable structure of SLS and particle size of Pt NPs.[70] TEM images and metal PSD of the SLS + H2PtCl6-3hb* sample (Figure S10) confirmed the stable size distribution of Pt NPs during the catalytic hydrogenation. In a word, SLS is a remarkable reductant and stabilizer to obtain an excellent Pt NP aqueous hydrogenation catalyst for α-pinene without any other reagents.

Hydrogenation of Other Alkenes over Pt NPs Reduced and Stabilized by SLS

The same Pt NP catalyst reduced and stabilized by SLS has also been used for the hydrogenation of other terpenes and unsaturated hydrocarbon compounds. It can be seen from Table S4 that the prepared Pt NP aqueous system presented similarly excellent catalytic activities for the hydrogenation of various monoterpenes and sesquiterpenes, except for p-cymene, the one containing a benzene ring structure. In addition, Table S5 shows that under the same conditions, the hydrogenation activities toward paraffins and cycloparaffins were extremely high in the presence of the Pt NP catalyst reduced and stabilized by SLS. However, the catalytic activities toward benzene and toluene were pretty low. Therefore, the Pt NP system prepared in this research is not an effective catalyst for benzene compounds.

Conclusions

In this work, a stable Pt NP aqueous catalyst was prepared using an aromatic renewable resource, sodium lignosulfonate, acting as both stabilizing and reducing agents. This NP system was found to be a highly efficient catalyst for α-pinene hydrogenation to develop a clean production method for cis-pinane. Through a series of characterizations, it was shown that the free -OH of the side chain in the lignin structure mainly participated in the reduction process of H2PtCl6·6H2O. During the formation of Pt NPs, the hydroxyl group was oxidized to a ketone structure, and the rest of the structure did not change significantly. Moreover, the prepared Pt NP system exhibited excellent stability and renewability for convenient reuse of the catalyst. In the whole process of preparation and application of the catalyst, no other agents were involved, except H2PtCl6·6H2O, sodium lignosulfonate, water, α-pinene, and hydrogen.

Experimental Section

Materials

Hexachloroplatinic acid (H2PtCl6·6H2O) and potassium chloroplatinate (K2PtCl6·6H2O) were purchased from Aladdin. Sodium lignosulfonate was purchased from Macklin and dried in a vacuum oven for 24 h before use. Sodium type 732 cation exchange resins were purchased from Sinopharm. α-Pinene was purchased from Jiangxi Hessence Chemicals Co., Ltd., China. The remaining reagents used in the catalytic tests were purchased from Macklin and were used without any further treatment.

Preparation of the Metal NP Catalytic System and Hydrogenation of α-Pinene

To 60.0 mg of SLS (or lignosulfonic acid (LSA) as a contrast) in 6.0 mL of water, 0.10 mmol transition-metal salt was added. The solution was allowed to react for 3 h at 80 °C under magnetic stirring at 400 rpm. The color change of the aqueous system, UV–vis spectroscopy, and XPS analysis confirmed the formation of metal NPs.

To this NP solution in a stainless autoclave with Teflon lining, 10.0 mmol α-pinene was added and sealed. The autoclave was filled with 1 MPa H2 after atmosphere replacement with hydrogen five times and was heated to 70 °C for 2 h under magnetic stirring at 400 rpm. The product mixture at the top layer after centrifugation was analyzed by gas chromatography–mass spectrometry (GC–MS) and GC separately. To the separated lower aqueous layer with the NP catalyst, 10.0 mmol fresh α-pinene was added. Accordingly, the same reaction operation as above was carried out to test the reusability of the NP catalyst.

Characterization of Catalysts

The formation of NPs and the valence-state changes of the metal during the catalyst preparation were analyzed by UV–Vis spectroscopy (TU-1901) under the following conditions: 50 μL of NP solution was diluted with 9 mL of ultrapure water and then decanted into a quartz cuvette (optical path 10 mm, cubage 3.5 mL). The samples were measured using a TU-1901 spectrophotometer with a spectral window range of 200–600 nm.

Water solvent was evaporated off from the NP solution to prepare the samples for Fourier transform infrared (FT-IR), powder X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) characterization. FT-IR spectra for the resulting solid residue on KBr disks were recorded on a Nicolet iS10 FT-IR instrument, using a spectral window of 4000–400 cm–1. XRD analysis for the resulting solid samples was conducted on a Rigaku diffractometer equipped with a Cu Kα radiation source at 40 kV and 150 mA (D/MAX-2500/PC, Japan). Diffraction data were recorded in the 2θ range from 5 to 90° at a scanning speed of 5°/min. XPS spectra for the resulting solid residue were recorded on an X-ray photoelectron spectrometer (PHI 5702×, USA) with Al Kα radiation (hv = 1486.6 eV). The binding energies were calibrated using the C ls peak at 284.6 eV. Test conditions were as follows: voltage of 15 kV, power of 150 W, test area of 0.7 × 0.3 mm2, vacuum degree of 1 × 10–9 torr, and resolution of 0.4 eV.

The samples for transmission electron microscopy (TEM) analysis were prepared by dropping the NP solution onto copper grids and then evaporating the water solvent with an infrared lamp for 15 min. TEM measurements were performed using a JEM F200 transmission electron microscope (JEOL, Japan). Meanwhile, the particle size distribution (PSD) of Pt NPs was obtained using software ImageJ (Version 1.41o) to count at least 100 particles.

Samples for nuclear magnetic resonance (NMR) analysis were prepared by concentrating the NP solution 10-fold. A Bruker Avance III 600 spectrometer (14.09 Tesla) with a high-resolution BBO forward-looking broadband probe was used to obtain the NMR spectra. Briefly, 50 μL of D2O was added to the concentrated solution in the NMR tube and, at the same time, a co-axial capillary tube with a D2O solution of sodium 3-(trimethylsilyl)-1-propanesulfonate (DSS) was used as the reference to eliminate the dominant water signal in the 1H NMR spectra. A water solution of K2PtCl6 with an equivalent Pt content as the samples was used as the reference in the case of 195Pt NMR. For the 2D HSQC NMR, the system was freeze-dried at a low temperature and dissolved in dimethyl sulfoxide (DMSO)-d6 to prevent the influence of water peaks. The central solvent peak of DMSO-d6 at δC/δH = 39.5/2.50 was employed as the internal reference. The spectral widths were 5000 and 20 000 Hz for the 1H and 13C dimensions, respectively. Bruker’s “hsqcetgpsisp2.2” adiabatic pulse program was used in the HSQC experiments with spectral widths from 0 to 11 ppm (6602 Hz) and from 0 to 180 ppm (27165 Hz) for the 1H and 13C dimensions. The number of collected complex points was 2048 for the 1H dimension with a recycle delay of 1.5 s. The number of transients was 64 and 256 time increments were always recorded in the 13C dimension. Prior to Fourier transformation, the data matrices were zero-filled to 1024 points in the 13C dimension.