Highly efficient conversion of biomass-derived levulinic acid into γ-valerolactone over Ni/MgO catalyst

Abstract

Hydrogenation of levulinic acid (LA) was investigated with several supported Ni catalysts for the production of γ-valerolactone (GVL). The catalysts have been characterized with different techniques (XRD, N₂ adsorption–desorption, TPR, ICP-AES, TEM). Under optimal reaction conditions of 150 °C, 1.0 MPa and 20 mL i-PrOH for 2 h, the highest selectivity (93.3%) of GVL was obtained. The productivity reached 0.32 mol_GVL g_metal⁻¹ h⁻¹, which was highest compared with all reported Ni-based catalysts.

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Introduction

With fossil energy diminishing, biomass has been identified as a sustainable resource^1–3 for alternative fuels and added-value chemicals,⁴ decreasing the dependence on fossil resources and reducing the emissions of greenhouse gases.^5,6 Considering the chemical complexity of the feedstock and end product, biomass would typically convert to some intermediates – often referred to as platform chemicals – for downstream processes.^7,8 Levulinic acid (LA), being obtainable through acid catalyzed degradation of biomass,⁹ is an important platform chemical due to its high reactivity and extensive use in fine chemicals.^10,11 Moreover, LA can be converted into many valuable derivatives,^12–14 in which γ-valerolactone (GVL) has caught most attention in recent studies.^15,16 GVL is renewable, safe to store and can be employed in many fields, for example, as a kind of liquid fuel, food additive, solvent and as an intermediate for the synthesis of fine chemicals.¹⁶

Generally, GVL could be obtained by the reduction of LA or its esters (Scheme 1), which generated from the esterification of LA and several low-carbon alcohols, such as ethanol,¹⁷ butanol,¹⁸ sec-butyl alcohol,¹⁹ etc. The economical efficiency to be taken into account in the design of reaction pathway, it seems more favourable by the direct reduction of LA to GVL, which was commonly conducted with H₂ gas as a reductant. Recently, several groups explored formic acid (FA) as a hydrogen source instead of external H₂ gas,^20–23 since FA is a by-product of LA production from C6 sugars. However, the use of FA entails several unresolved disadvantages, such as the need for precious metal, homogeneous catalysts, and/or harsh reaction conditions et al.¹⁸

Scheme 1 Reduction of LA and its esters to γ-valerolactone.

Comparatively speaking, the hydrogenation of LA to GVL is especially suited to the commercial-scale manufacturing of GVL by using heterogeneous catalytic systems.²⁴ Group VIII metals, notably ruthenium,^{16,21,25–32} have been shown to facilitate the hydrogenation step using molecular H₂. Manzer et al. compared several noble metals for LA hydrogenation under the same reaction conditions. Results showed that the catalytic activity order was as follows: Ru > Ir > Rh ≈ Pd > Pt > Re.³³ Palkovits et al.¹¹ summarized the development of heterogeneous catalysts for the targeted conversion of LA to GVL and found the similar results that Ru-based catalysts performed best with the relatively highest productivity.

Although high yield of GVL could be obtained with Ru-based catalysts, the financial cost associated with precious metal is detrimental to the production of high volume and relatively low value liquid transportation fuels. So the cheap and efficient catalysts should be developed to replace precious metal catalysts. Hwang et al.³⁴ used the nano composite 5 wt% Cu/SiO₂ for the vapor-phase hydro-dehydration of LA and 99.9% yield of GVL was obtained at 260 °C. Yan et al.^35,36 used Cu-catalyst for the hydrogenation of LA. However, the harsh reaction conditions (200 °C and 7 MPa H₂) still remained unsatisfied.

Ni-based catalysts are widely used in both industry and academia owing to their low initial cost and high activity.³⁷ In the 1940s, RANEY® Ni was first employed for the conversion of LA and 94% yield of GVL was obtained at 200 °C and 5–6 MPa H₂.²⁷ Recently, improved work was still underway with RANEY® Ni in order to reduce the reaction conditions. However, the results are far from satisfactory due to low conversion efficiency (<20%)³¹ or complex reaction systems.³⁸ Rao etc. employed several supported Ni catalysts for vapour phase hydrocyclisation of LA.³⁹ Ni/SiO₂ showed better activity at 250 °C with good dispersion, but the productivity was comparatively lower with 0.028 mol_GVL g_metal⁻¹ h⁻¹. In the former study, we present an efficient technology for LA hydrogenation to GVL over RANEY® Ni prepared from melt-quenching Ni–Al alloys.⁴⁰ Excellent 97.4% GVL yield was obtained under the mild reaction conditions. In order to further reduce the dosage of Ni metal, we employed supported Ni catalysts for LA hydrogenation to increase the catalytic efficiency. Ni/MgO provides excellent 69.9% GVL yield under the mild reaction conditions and the productivity reached 0.32 mol_GVL g_metal⁻¹ h⁻¹, which was the highest compared with all reported Ni-based catalysts.^{24,27,31,37,38,40}

Experimental section

Materials and methods

All the reagents used in this study were analytical grade and used without further purification. NiSO₄ (Tianjin Guangfu Fine Chemical Research Institute), Ni(Ac)₂, NiCl₂ (ChengDu Kelong Chemical Co., Ltd), Ni(NO₃)₂ (Tianjin Damao Chemical Reagent Factory). NaOH, KOH, Na₂CO₃, NH₃·H₂O, (NH₄)₂CO₃ (Guangdong Xilong Chemical Co., Ltd), CO(NH₂)₂ (Tianjin Guangfu Fine Chemical Research Institute). SiO₂ (Degussa Aerosil 200), Al₂O₃ (Tianjin Chemical Research & Design Institute), TiO₂ (Degussa P25), ZnO (Guangdong Xilong Chemical Co., Ltd), MgO (Guangdong Xilong Chemical Co., Ltd) and ZrO₂ (Sinopharm Chemical Reagent Co., Ltd).

The supported nickel catalysts were prepared according to the deposition–precipitation (DP) method^41–43 and the experimental procedure is as follows. Firstly, the support was added into an aqueous solution containing nickel salt (0.017 M). Then the suspension was heated to 50 °C [90 °C for CO(NH₂)₂ system] in constant stirring for 1 h and a little silica sol was added into the suspension for another 1 h. The precipitant solution (0.017 M) was dripped into the above mixture. After a given time of deposition–precipitation, the suspension was cooled to 25 °C and then filtered. The sample was washed with distilled water to remove the possible adsorbed ions and dried at 60 °C for 24 h. The dried precursor was heated to 500 °C at a heating rate of 7.5 °C min⁻¹ under N₂ and reduced for 4 h in 20 mL min⁻¹ flow of H₂. The catalyst was cooled to room temperature under N₂ and gradually exposed to air in 10 min for further usage. The catalysts prepared using nickel sulfate, nickel acetate, nickel chloride, and nickel nitrate were named as Ni/Support-S, Ni/Support-A, Ni/Support-Cl and Ni/Support-N. The number stands for Ni loadings, ‘Support’ stands for all the oxide support being used.

The catalytic hydrogenation reaction was conducted in a stainless-steel autoclave (75 mL capacity). Typically, LA (2 or 4 mmol), solvent (20 mL) and catalyst (0.01 g) were directly added into the reactor, and then it was sealed and purged with N₂ and H₂ for 3 times respectively.

The products were quantitatively analysed by GC (Techcomp GC 7890F, FID detector, DB-FFAP (30 m × 250 μm × 0.25 μm) capillary column) with calibrated area normalization method and qualitatively analysed by GC-MS (Agilent 5979C).

Catalyst characterization

XRD. X-ray diffraction (XRD) patterns for all samples were recorded on a Rigaku D/max-2400 diffractometer (Cu Kα/λ = 0.1541 nm/40 kV/40 mA). The sample was scanned from 10° to 90° with a rate of 10° min⁻¹.

TEM. Transmission electron microscopy (TEM) images were obtained with an accelerating voltage of 300 kV (TEM, FEI TF30).

XPS. XPS experiment was performed on an Escalab 250Xi spectrometer of Thermo Scientific with an Al anode using K_α (hv = 1486.6 eV) radiation. To reduce differential surface charging effects, an electron flood gun was employed. Binding energies were corrected for charge effects by reference to C 1s peak at 284.6 eV.

N₂ adsorption–desorption. The N₂ adsorption–desorption isotherms were obtained by using a 3H-2000PS1 sorption system (Beishide Instrument-ST (Beijing) Co., Ltd). Firstly, the sample was dried in vacuum at 473 K for 6 h. The N₂ isotherms were carried out at 77 K. The specific surface areas were calculated from the isotherms using the Brunauer–Emmett–Teller (BET) method. The pore distribution and the cumulative volumes of pores were obtained by the Barrett–Joyner–Halenda (BJH) method from the desorption branches of the isotherms.

ICP-AES. The sample was dissolved in aqua regia (9 mL HCl and 3 mL HNO₃ dilute to 100 mL) and then test the concentration of Ni on an Optimi2000DV.

H₂-TPR. H₂-TPR was conducted in TP-5076 (Tianjin Xianquan Industry and Trading Co., Ltd). Firstly, the sample (0.05 g) was heated to 100 °C in N₂ and kept for 3 h. After being cooled to room temperature, N₂ was replaced by 5% H₂/Ar and the sample was reheated to 900 °C at a rate of 10 °C min⁻¹.

Results and discussion

Characterization of catalysts

XPS measurements for Ni/MgO were performed as shown in Fig. 1. Generally, the binding energy of metallic Ni⁰ 2p level was corresponded to 851 eV ± 0.5, which not being detected in the XPS analysis. It indicates the absence of metallic Ni⁰,⁴⁴ which might be caused by the surface oxidation of Ni particles in air due to its high activity. As a result, the binding energy of Ni 2p_3/2 level in the NiO status was detected at 855.7 eV⁴⁵ and its satellite peak was displayed at 861.8 eV,⁴⁶ in agreement with the reported results.⁴⁷ The catalyst had a higher Ni 2p_3/2 binding energy relative to pure NiO species (854.5 eV),⁴⁸ indicating that electron transfer took place from Ni to MgO.⁴⁹

Fig. 1 Ni 2p XPS of the Ni/MgO catalyst.

The H₂-TPR profile for (a) MgO support, (b) 0Ni/MgO and (c) 44Ni/MgO is shown in Fig. 2. The blank test for MgO was carried out according to the preparation procedure of catalysts (the only difference is aqueous solution containing no nickel salt) and the corresponding sample was denoted as 0Ni/MgO. MgO support and 0Ni/MgO almost do not show any hydrogen consumption under the measurement conditions. Comparatively speaking, 44Ni/MgO provides three TPR peaks: the peak at temperature lower than 400 °C was denoted as T_L, 400–750 °C as T_M and 750–850 °C as T_H. First, the T_L peak at 228.4 °C is assumed to represent the reduction of NiO located on the surface,^50–52 which has been ascertained in XPS measurement. The T_M peaks are usually assigned to the reduction of Ni²⁺ ions in the outermost layer or sub-surface layers of MgO lattice, while the T_H peaks can be attributed to the reduction of Ni²⁺ ions located deep in the MgO lattice.^39,51–54 In the literature, it is reported that such NiO seemed to be influenced by MgO and the NiO–MgO solid solution formed.

Fig. 2 H₂-TPR profiles of the catalysts.

Generally, the presence of T_M and T_H peaks is a clear symbol that NiO has undergone a progressive diffusion into the MgO lattice to form NiO–MgO solid solution.⁴⁹ According to researches, the Ni²⁺ ions located in the NiO–MgO solid solution were difficult to be reduced and only a small amount of NiO can be reduced to metallic nickel.^52,55 And the more NiO–MgO solid solution the catalyst had, the less NiO can be reduced to metallic status at low temperature, which would have an negative influence on the catalytic activity of Ni/MgO. Therefore, to avoid the formation of NiO–MgO solid solution,⁴⁹ we chose 500 °C to reduce 44Ni/MgO precursor in a flow of H₂ in this study.

Support effects on LA hydrogenation

We initiated the hydrogenation of LA by screening six supports and results are summarized in Table 1. All Ni-based catalysts were prepared by the same DP method and the actual loading was measured by ICP for each catalyst. To make direct comparisons, the same reaction conditions were employed in all experiments. The product distribution is as follows. GVL was obtained as the main product by the hydrogenation of LA and subsequent cyclization. The main by-product was isopropyl levulinate (ester), which was produced by the esterification of i-PrOH and levulinic acid. Pseudo-levulinic acid (P-LA) was an intermediate generating from the isomerization of LA and could be further converted into GVL.

Table 1 Hydrogenation of LA with supported Ni catalysts^a

Cat.	Actual loadings^b/%	Conv./%	Sel./%				Productivity^c/molGVL gmetal^−1 h^−1
			GVL	Ester	P-LA	Others
a Reaction conditions: 0.2302 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 2.0 MPa, 120 min, the nominal loading of each sample is 66 wt%.b Detected by ICP-AES.c Productivity = n(LA) × yield/[m(Ni)/t].
Ni/SiO2	47.89	19.7	58.9	9.1	29.9	2.1	0.024
Ni/Al2O3	45.95	37.2	85.8	4.3	7.3	2.6	0.069
Ni/TiO2	43.63	25.7	76.3	7.4	14.0	2.3	0.045
Ni/ZrO2	39.03	19.5	73.8	8.2	11.8	6.2	0.037
Ni/ZnO	48.19	41.3	57.6	33.2	6.8	2.4	0.049
Ni/MgO	47.48	58.1	92.4	2.4	2.9	2.3	0.11

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According to the reported results,⁵⁶ it could induce the solid acid when using Al₂O₃, SiO₂, TiO₂ and ZrO₂ as supports. Taking both the acid strength and catalytic activity into account, we can conclude that the activities of supported-Ni catalysts seem to be related to their acid strength, i.e. higher acid strength of the catalysts resulting in higher conversion. The order of catalytic activity is as follows: Ni/Al₂O₃ > Ni/TiO₂ > Ni/SiO₂ ≈ Ni/ZrO₂. However, it provides comparatively higher activities when using ZnO and MgO, which were usually regarded as solid bases⁵⁶ and could promote the esterification process.^57,58 The conversions with Ni/ZnO and Ni/MgO catalysts were 41.3% and 58.1% respectively.

Furthermore, the support was directly used for LA hydrogenation in order to investigate the support effect (Table 2). Results showed that the acid supports (Al₂O₃, TiO₂, SiO₂, ZrO₂) have little influence over the reaction, but the base supports (ZnO, MgO) could obviously promote the esterification of i-PrOH and levulinic acid, which was similar with the intramolecular esterification of γ-hydroxyvaleric acid. Based on the reaction mechanism, it could be concluded that Ni mainly provided the active sites for LA hydrogenation and ZnO (MgO) promoted the esterification process of γ-hydroxyvaleric acid. Considering the selectivity of GVL, Ni/MgO exhibited comparatively higher reactivity than Ni/ZnO in this study.

Table 2 Hydrogenation of LA with blank supports^a

Supports	Surface area (m^2 g^−1)	Pore		Conv./%	Sel./%
		V (cm^3 g^−1)	D (nm)		Ester	GVL	P-LA
a Reaction conditions: 0.2302 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 2.0 MPa, 120 min.
Blank	—	—	—	3.3	60.6	3.0	36.4
SiO2	202.6	0.68	9.86	4.0	60.0	2.5	37.5
Al2O3	213.0	1.49	19.78	3.3	60.6	3.0	36.4
TiO2	51.4	0.23	12.15	4.9	36.7	2.0	63.3
ZrO2	5.9	0.04	19.58	4.4	59.1	4.5	34.1
ZnO	7.9	0.05	16.89	20.7	84.1	0.5	15.5
MgO	18.6	0.11	15.15	15.1	71.5	2.0	26.5

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Effects of Ni loadings

Based on the above study about support effect, we concluded that Ni provided the reactive sites for hydrogenation and MgO promoted the esterification process. It seems that there is a balance between these two steps. So we investigated various loadings of Ni/MgO (22–66 wt%) for LA hydrogenation to GVL (Table 3). By increasing the loading of Ni, the conversion of LA firstly increased, and then decreased. When the nominal loading was 44%, the conversion reached the maximum value of 79.3% and the productivity was 0.32 mol_GVL g⁻¹ h⁻¹. Those phenomena are related to the status of Ni in the composite.

Table 3 Hydrogenation of LA to GVL with Ni/MgO^a

Cat.	Conv./%	Sel./%				Productivity/molGVL gmetal^−1 h^−1
		GVL	Ester	P-LA	Others
a Reaction conditions: 0.4604 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 1.0 MPa, 120 min.
0Ni/MgO	18.2	1.2	85.2	13.6	0	—
22Ni/MgO	26.4	41.7	39.0	17.0	2.3	0.10
33Ni/MgO	45.9	73.6	15.7	7.0	3.7	0.20
44Ni/MgO	79.3	88.1	4.8	3.3	3.7	0.32
55Ni/MgO	55.4	84.3	7.2	4.9	3.6	0.17
66Ni/MgO	30.9	72.5	13.6	10.4	3.6	0.068

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X-ray diffraction (XRD) patterns of MgO and Ni/MgO catalysts with different loadings were shown in Fig. 3. The fresh MgO gave the typical diffraction peaks at 2θ = 36.8°, 42.8°, 62.2°, 74.6°, 78.5° and 2θ = 18.5°, 37.9°, indicating the crystalline phase of most MgO (JCPDS card No. 77-2364) and a little Mg(OH)₂ (JCPDS card No. 44-1482), respectively. It showed that the sharp peaks for MgO turned much weaker and even partly disappeared, indicating the crystalline structure was severely destroyed in the process. Three diffraction peaks at 42.8°, 62.2° and 78.5° were assigned to the (200), (220) and (222) facets of MgO respectively. After loading Ni particles, another three peaks appeared at 2θ = 44.5°, 51.8° and 76.4°, with respect to (111), (200) and (220) facets of Ni (JCPDS card No. 70-0989) lattice reflection. The broad peak shape suggested the amorphous structure or small size of Ni particles. As the loadings of Ni increased, the peaks for MgO got weak, but got strong for Ni. This could be attributed to more Ni covering the surface of MgO matrix and destroying its crystalline structure. Using Scherrer's equation, the average crystalline sizes of Ni were estimated to be about 4.4 nm according to the 44Ni/MgO sample.

Fig. 3 XRD patterns of MgO and Ni/MgO catalysts with different loadings.

The TEM images of 44Ni/MgO were shown in Fig. 4a and b. Judging from the surface, Ni particles uniformly decorated on MgO support with less aggregation (Fig. 3). However, it seems difficult to definitely differentiate MgO and Ni possibly due to a strong interaction between metal and support. Furthermore, the image is magnified until a higher resolution in order to get much finer details. Based on Fig. 4b, the 0.20 nm and 0.18 nm lattice spacing could be assigned to (111) and (200) lattice planes in metallic Ni, which were also obtained in XRD patterns. For MgO support, the crystalline structure was partly destroyed during the preparation of catalysts, but the residual MgO crystal could also be observed by TEM images. The lattice distances at 0.21 nm and 0.24 nm were ascribed to (200) and (111) planes of MgO respectively, which were also obtained in the characterization of XRD. The good agreement between the results of TEM and XRD in crystally phase excludes the occurrence of new phase induced by interaction of the support and reduced metal.

Fig. 4 (a) TEM image of 44Ni/MgO catalyst. (b) TEM image of 44Ni/MgO catalyst.

For Ni/MgO catalysts, XRD analysis showed the presence of Ni metallic phase with a weak signal. It indicated that the Ni phase is spread on the surface of MgO support in an amorphous or highly dispersed form. The strong interaction between Ni and MgO might stabilize the small Ni particles and prevent their aggregation. On the other hand, as Ni content increasing, the LA hydrogenation to γ-hydroxyvaleric acid was accelerated but subsequent dehydration of γ-hydroxyvaleric acid was slowed down. Take two process into account, an appropriate Ni loading was necessary in this study. A detailed study on the mechanism of catalytic process is under way.

Effects of precursor salts

The precursor salts including NiSO₄, Ni(Ac)₂, NiCl₂ and Ni(NO₃)₂ were investigated for the preparation of Ni/MgO catalyst. Results in Table 4 indicated that the actual loadings were fairly similar according to ICP analysis (31.58–33.91%), but the catalytic performance was quite different from each other. It should be noted that the catalyst provides low activity and selectivity when using NiSO₄ or NiCl₂ as precursor salts. This may be due to the residual sulfur or chlorine occupying the active sites of Ni, leading to the catalyst poisoning.^59–63 In addition, when the Ni(Ac)₂ solution was used to impregnate into MgO, the mixture was too thick to be stirred, which might decrease the metal dispersion and the reaction activity. It appeared that the catalyst derived from Ni(NO₃)₂ precursor salt had the highest activity in the control experiments and the productivity was 0.29 mol_GVL g⁻¹ h⁻¹ with 93.3% selectivity to GVL.

Table 4 Effects of precursor salts for LA hydrogenation^a

Cat.	Actual loadings^b/%	Conv./%	Sel./%				Productivity/molGVL gmetal^−1 h^−1
			Ester	GVL	P-LA	Others
a Reaction conditions: 0.2302 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 2.0 MPa, 120 min, the nominal loading of each sample is 44 wt%.b Detected by ICP-AES.
Ni/MgO-S	33.06	17.7	44.6	17.5	36.2	1.1	0.0094
Ni/MgO-A	31.58	39.8	14.6	71.1	11.3	3.3	0.090
Ni/MgO-C	33.91	18.9	40.2	13.2	45.5	0.5	0.0074
Ni/MgO-N	31.80	100	1.5	93.3	0.8	4.4	0.29

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Effects of precipitants

To prepare Ni/MgO catalyst, MgO was firstly suspended in an aqueous Ni(NO₃)₂ solution. Then a precipitant being dissolved into H₂O was dropwise added into the mixture to transfer the Ni²⁺ ions in the liquid phase onto the surface of MgO. Generally, a moderate alkaline environment is preferential for the dispersion of Ni(OH)₂ precipitants. Herein, several frequently-used bases as a key factor were investigated for the precipitation process and results were summarized in Table 5.

Table 5 Hydrogenation of LA to GVL on Ni/MgO prepared with different precipitants^a

Entry	Precipitants^b	pH	Conv./%	Sel./%
				Ester	GVL	P-LA	Others
a Reaction conditions: 0.4604 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 1.0 MPa, 120 min, the nominal loading of each sample is 44 wt%.b n(precipitant) : n(Ni^2+) = 2.c n(precipitant) : n(Ni^2+) = 3.
1	NaOH	12.97	32.3	22.0	61.3	12.1	2.8
2	KOH	13.30	24.5	35.5	46.1	15.9	2.4
3	Na2CO3	11.26	30.1	25.9	58.1	13.3	2.7
4	NH3·H2O	8.87	53.8	8.7	80.5	7.1	3.5
5	(NH4)2CO3	9.20	79.3	4.8	88.1	3.3	3.7
6	CO(NH2)2^c	9.02	58.9	8.5	81.5	6.3	3.7

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The XRD patterns of Ni/MgO precursors prepared with different precipitants were shown in Fig. 5. The result with NaOH or KOH as a precipitant is similar with the patterns of Mg(OH)₂ (JCPDS card No. 86-0441). Comparatively, the peaks of Ni(OH)₂ (JCPDS card No. 73-1520) were overlapped with that of Mg(OH)₂. The peak at 2θ = 33.0° may contain two phases taking peak intensity into account. So Ni²⁺ ions could be completely converted into Ni(OH)₂ when using NaOH or KOH as precipitants (Entry 1 and 2). Obviously, it provided lower activity, which might be caused by the migration and aggregation of Ni particles when Ni(OH)₂ was reduced by H₂ flow at 500 °C, exceeding to the melting point of Ni(OH)₂.

Fig. 5 XRD patterns of Ni/MgO precursors with different precipitants, the nominal loading is 44 wt%.

When Na₂CO₃ was used as a precipitant, Ni(OH)₂ and NiCO₃ might be obtained due to the coexists of OH⁻ and CO₃²⁻ in the solution. Two diffraction peaks at 15.3° and 30.8° were assigned to the (011) and (310) facets of Mg₅(CO₃)₄(OH)₂·4H₂O (JCPDS card No. 25-0513). Two diffraction peaks at 33.5° and 41.1° were assigned to the (110) and (300) facets of 3Ni(OH)₂·2H₂O (JCPDS card No. 22-0444). There were no peaks of NiCO₃, which is consistent with other research.⁶⁴ According to reports, when pH was higher than 10, NiCO₃ turned into Ni(OH)₂. In this study, pH was 11.26 for Na₂CO₃ solution. In combination with the calculation results (listed in ESI†), we found a reaction for transformation of precipitation continuously happened: NiCO₃ + 2OH⁻ ⇌ Ni(OH)₂ + CO₃²⁻. So Ni(OH)₂ was obtained at last and it also provided low activity using Na₂CO₃ as a precipitant.

When NH₃·H₂O was added into the suspension containing MgO and Ni(NO₃)₂ solution, [Ni(NH₃)_x]²⁺ complex was firstly formed as an intermediate phase and it would impregnate the cavity of MgO.⁶⁵ With the decomposition of NH₃·H₂O upon heating, the precipitation of Ni(OH)₂ would be formed with more uniform dispersion. So it provided a higher activity with 53.8% conversion (Entry 4). Similar with NH₃·H₂O, it also showed good performance with CO(NH₂)₂ as a precipitant.

When CO(NH₂)₂ was regarded as a precipitant, NH₄⁺, CO₂ and OH⁻ generated by the hydrolysis of urea in neutral or basic solution.⁶⁶ There were some responding peaks of Mg₅(CO₃)₄(OH)₂·4H₂O [(110), (011) and (310)] and Mg(OH)₂ [(001), (011), (012) and (110)]. Other peaks were well consistent with the literature results for Ni₂(OH)₂CO₃·4H₂O (JCPDS card No. 38-0714). As a result, the basic nickel carbonate was obtained and it could effectively afford agglomeration of sheet-like or un-regular particles.⁶⁷

The XRD pattern of Ni/MgO[(NH₄)₂CO₃] showed that it also provided the basic nickel carbonate using (NH₄)₂CO₃ as a precipitant to react with Ni(NO₃)₂. After being reduced by H₂, Ni/MgO[(NH₄)₂CO₃] catalyst provided highest 79.3% conversion and 88.1% selectivity to GVL. Furthermore, the XRD patterns of Ni/MgO catalysts with different precipitants were shown in Fig. S7.† It indicated that the Ni particles are well dispersed on supports and particles size of Ni prepared by NH₃·H₂O, CO(NH₂)₂ and (NH₄)₂CO₃ are smaller than other sample. As a result, Ni/MgO with (NH₄)₂CO₃ precipitant showed highest activity and selectivity to GVL.

Optimization of reaction conditions and the reaction pathway

Employing Ni/MgO catalyst, effect of temperature, pressure and solvents were investigated (Fig. 6–8). The optimal reaction conditions were 150 °C, 1.0 MPa H₂, which were much milder comparing with the reported results^{33,34,36,37,68} and the catalyst's dosage was significantly reduced simultaneously.

Fig. 6 The effect of solvents with Ni/MgO (0.2302 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 2.0 MPa, 120 min).

Fig. 7 The effect of temperature with Ni/MgO (0.2302 g LA, 20 mL i-PrOH, 0.01 g catalyst, 2.0 MPa, 120 min).

Fig. 8 The effect of pressure with Ni/MgO (0.2302 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 120 min).

In this study, i-PrOH and 1,4-dioxane appeared to be more appropriate than MeOH, EtOH and H₂O as solvents. When there were acetyl group of LA, catalyst and i-PrOH, the Meerwein–Ponndorf–Verley (MPV) reaction might happen as reported.^69,70 According to the reports^71,72 MgO can be used as the catalyst for the MPV reaction. So a contrast test was designed with nitrogen replacing hydrogen. 23.7% conversion and 54.9% selectivity to GVL were obtained in Table S2.†

When H₂ is absence, isopropyl alcohol provides a hydrogen source and it is converted into acetone. Because acetone couldn't be removed out in the closed reactor, there is a reversible hydride transfer from alcoholate to carbonyl acceptor. It is not enough for i-PrOH to afford all the hydrogenation of LA to GVL. As a whole, i-PrOH acted as double roles not only solvent but also hydrogen donator. So when i-PrOH is solvent, we got the highest activity.

Under optimal reaction conditions, the reaction process was monitored by GC and the relationships of components concentration versus reaction time were shown in Fig. 9. Based on the above results and reports,^11,24,68,73 a reaction pathway was proposed in Scheme 2. Theoretically, GVL was obtained by the hydrogenation of LA to γ-hydroxyvaleric acid (1-b) and subsequent dehydration of 1-b. In this study, 1-b was not be detected according to the analysis method by Omar Ali Abdelrahman,⁷³ which might be attributed to the quick intramolecular esterification of 1-b under the reaction conditions. Pseudo-levulinic acid (1-d) was an intermediate generating from the cyclization of LA.

Fig. 9 The concentration–time plots for the hydrogenation of LA with Ni/MgO (0.4604 g LA, 20 mL i-PrOH, 0.01 g catalyst, 150 °C, 1.0 MPa, the nominal loading is 44 wt%).

Scheme 2 Proposed mechanisms for the hydrogenation of LA to GVL.

The reversible dehydration of 1-d and following hydrogenation would form the target molecule.¹¹ Moreover, isopropyl levulinate (1-e) was produced by the esterification of isopropanol and levulinic acid. 1-e was another intermediate and finally could be further converted into GVL.

Conclusions

In summary, several supported Ni catalysts were prepared according to the deposition–precipitation (DP) method. Various factors for catalyst preparation were systematically investigated to obtain the optimal preparation conditions with [Ni(NO₃)₂] as a precursor salt and [(NH₄)₂CO₃] as a precipitant. i-PrOH acted as double roles not only solvent but also hydrogen donator, which accelerating the reaction rate. As a result, 44Ni/MgO provided excellent 69.9% GVL yield under the optimal reaction conditions. The productivity reached 0.32 mol_GVL g_metal⁻¹ h⁻¹, which was highest compared with all reported Ni-based catalysts. However, the catalyst durability was still far from satisfaction and the improved work remains underway in our group.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21206015), Specialized Research Fund for the Doctoral Program of Higher Education (20120041120022), the Fundamental Research Funds for the Central Universities (DUT13LK29).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12548k

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