An environmentally benign process for the efficient synthesis of cyclohexanone and 2-methylfuran

Abstract

A novel process involving the coupling of the dehydrogenation of cyclohexanol and the hydrogenation of furfural has been studied for the synthesis of cyclohexanone and 2-methylfuran over the same Cu–Zn–Al catalyst, realizing good energy efficiency, optimal hydrogen ultilization, and environmentally benign process.

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2-Methylfuran (2-MF) and cyclohexanone (CHN) are two important chemicals and versatile intermediates in fine chemical industrial practices. 2-MF is mainly used for the synthesis of crysanthemate pesticides, perfume intermediates and chloroquine lateral chains in medical intermediates.¹ CHN is mainly used for the production of caprolactum and adipic acid, and both of them are the major raw materials in producing polyamide fiber.² One of the important synthesis routes of 2-MF is the hydrogenation of furfural (FFA),^1,3 and that of CHN is the dehydrogenation of cyclohexanol (CHL).⁴ However, there are many deficiencies existing in the conventional individual hydrogenation of FFA [eqn (1)] and dehydrogenation of CHL [eqn (2)], such as hardly controlled reaction temperature, conversion constrained by thermodynamic equilibrium, poor hydrogen utilization. So we have developed a new coupling route to synthesis 2-MF and CHN simultaneously [eqn (3)], in which the hydrogenation of FFA and the dehydrogenation of CHL are combined at the same reactor.

As shown in eqn (1), the vapor-phase hydrogenation of FFA to 2-MF is exothermic by 142 kJ mol⁻¹,¹ and this strongly exothermic nature makes it extremely difficult to control the temperature over this process, which results in apparent hot spots, typically in an industrial tubular fixed-bed reactor, thus seriously lowering the yield of desired product. The vapor-phase dehydrogenation of CHL to CHN [eqn (2)] is an endothermic process (63.4 kJ mol⁻¹).⁵ Due to the endothermic properties of this reaction, the increase of the liquid hourly space velocity (LHSV) of CHL is relatively limited by low external heat supply in a practical reactor. In addition, the released hydrogen cannot be used effectively. Furthermore, the conventional dehydrogenation process is severely constrained by thermodynamic equilibrium, leading to lower conversion.⁶ The combined reaction in eqn (3) shows that producing 1 mol 2-MF and 2 mol CHN requires 1 mol FFA and 2 mol CHL, and is exothermic by 15.2 kJ mol⁻¹. The new catalytic process improves the selectivities of CHN and 2-MF, as well as leading to a better thermal balance and the effective use of hydrogen.

In industrial application of single FFA hydrogenation, Cr-containing catalysts are usually used, which are getting increasingly difficult due to their toxicity and pollution, while an environmentally friendly Cu–Zn–Al catalyst is used in this work. In addition, single FFA hydrogenation consumes a significant amount of hydrogen. Currently, hydrogen sources are dominantly manufactured with the carbonaceous substances, such as coal and natural gas. These processes result in many greenhouse gases and sulfur-compounds, such as CO₂, SO₂, H₂S, and cause serious environmental problems. The coupling process proposed in this work eliminates the separate hydrogen preparation procedures leading to a well arranged environmentally-friendly clean process.

Table 1 shows that almost complete conversion of FFA at 220–300 °C can be achieved over the Cu–Zn–Al catalyst. However, the selectivity of 2-MF varies substantially with operating temperature, with the best value of 87.0% at 250 °C.

Table 1 Influence of temperature on FFA hydrogenation to 2-MF^a

T/°C	Conv. (%)	Selectivity (%)
		2-MF	Others^b
a Reaction conditions: 1 atm, LHSV = 0.3 h^−1, n(H2) ∶ n(FFA) = 10 ∶ 1 (molar ratio). b Other products: mainly furfural alcohol, 2-pentanol, 2-pentanone, etc.
220	99.1	70.9	29.1
250	99.3	87.0	13.0
270	99.2	85.3	14.7
280	99.9	82.8	17.2
300	99.3	81.2	18.8

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In this work, the equilibrium constant for the dehydrogenation of CHL to CHN is defined as eqn (4).

The equilibrium constant K⁰ used in the present work is cited from reference.⁷K_p represents the equilibrium constant based on partial pressure. The typical reaction condition in this work is at atmospheric pressure, so P⁰ = P. Here χ_i is the mole fraction of component i, and ϕ_i is the fugacity coefficient. K_ϕ is the fugacity coefficient ratio. The values of ϕ_i and K_ϕ can be calculated on the basis of the Redlich–Kwong equation of state [eqn (5)].⁸

P = RT/(V − b) − a/T^0.5V(V + b) (5)

The a and b are all constants in the Redlich–Kwong equation, which can be obtained from their critical properties.

Fig. 1 gives the influence of different ratios of hydrogen to CHL on the conversion of CHL dehydrogenation to CHN. There are no significant effects on the conversion of CHL in a wide range of 15 to 100, while the conversion rapidly decreases with the H₂/CHL ratio in the range of 0 to 15. The results of CHL dehydrogenation at different temperatures (Table 2) indicate that the conversion of the main reaction for CHL dehydrogenation to CHN (Conv2) is lower than the equlibrium conversion by thermodynamic calculation (Conv1) from 220 to 300 °C. In addition, side reactions become serious over 280 °C.

Fig. 1 Influence of n(H₂)/n(CHL) (molar ratio) on the dehydrogenation of CHL to CHN. Reaction conditions: 1 atm, T = 270 °C, LHSV(CHL) = 0.6 h⁻¹. A = equilibrium conversion of CHL dehydrogenation to CHN by thermodynamic calculation. B = (moles of CHN produced by CHL)100/(total moles of CHL).

Table 2 Influence of temperature on CHL dehydrogenation to CHN^a

T/°C	Conv1^b (%)	Conv2^c (%)	Conv3^d (%)	Selectivity (%)
				CHN	Others^e
a Reaction conditions: 1 atm, LHSV = 0.6 h^−1, n(H2) ∶ n(CHL) = 2 ∶ 1 (molar ratio). b Conv1 = equilibrium conversion of CHL to CHN by thermodynamic calculation. c Conv2 = (only moles of CHN produced by CHL) × 100/(total moles of CHL). d Conv3 = (moles of all converted CHL) × 100/(total moles of CHL). e Other products: mainly phenol, benzene, cyclohexene, etc.
220	30.9	30.1	34.6	87.0	13.0
250	50.6	49.2	56.4	87.2	12.8
270	63.4	60.6	69.3	87.4	12.6
280	69.1	62.4	72.8	85.6	14.4
300	78.7	64.7	82.5	78.5	21.5

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The coupling reaction results (Table 3) display that the conversion of CHL to CHN is improved from 220–300 °C. Furthermore, the conversion of the main reaction for CHL dehydrogenation to CHN (Conv2 in Table 3) from 220 to 280 °C is higher than the equilibrium conversion (Conv1 in Table 2), namely, the thermodynamic conversion of CHL dehydrogenation is broken by adding FFA (a hydrogen acceptor).⁹ The selectivities of CHN and 2-MF are also increased compared with those in the individual processes (Table 1 and Table 2). At 270 °C, for example, the selectivity of CHN is raised by 8.7%, and that of 2-MF is increased by 7.5%.

Table 3 Influence of temperature on coupling of CHL and FFA to CHN and 2-MF^a

T/°C	Conv2^b (%)	Conv3 (%)		Selectivity (%)
		CHL^c	FFA^d	CHN^e	2-MF^f
a Reaction conditions: 1 atm, LHSV (CHL + FFA) = 0.9 h^−1, n(H2) ∶ n(CHL) = 2 ∶ 1(molar ratio), n(FFA) ∶ n(CHL) = 0.5 ∶ 1 (molar ratio). b Conv2 = (only moles of CHN produced by CHL) × 100/(total moles of CHL). c Conv3(CHL) = (moles of all converted CHL) × 100/(total moles of CHL). d Conv3(FFA) = (moles of all converted FFA) × 100/(total moles of FFA). e Selectivity(CHN) = (moles of CHN produced by CHL) × 100/(moles of converted CHL). f Selectivity(2-MF) = (moles of 2-MF produced by FFA) × 100/(moles of converted FFA).
220	40.7	42.5	99.0	95.8	77.5
250	58.2	61.0	99.6	96.0	89.5
270	67.2	70.0	99.6	96.1	92.8
280	70.6	76.7	99.4	94.4	89.4
300	76.9	81.5	99.8	92.1	83.8

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A 200 h stability test for the Cu–Zn–Al catalyst shows that the catalyst has relatively high activity and good stability (Fig. 2).

Fig. 2 Catalytic performance of Cu–Zn–Al in the coupling process of CHL dehydrogenation and FFA hydrogenation. Reaction conditions: 1 atm, T = 270 °C, LHSV(CHL + FFA) = 0.7 h⁻¹, n(H₂) ∶ n(CHL) = 2 ∶ 1 (molar ratio), n(FFA) ∶ n(CHL) = 0.3 ∶ 1 (molar ratio).

The boiling point (at 101.3 kPa) of 2-MF is about 63 °C and that of CHN is about 155 °C, leading to easy separation of the product mixture, meaning less additional costs in the coupling process for industrial applications. These advantages may be of great interest for industrialization of the coupling process to produce these two important products, 2-MF and CHN.

In conclusion, the coupling process proposed in this work has several advantages over the two conventional single processes, such as enhanced conversion, good energy efficiency, optimal hydrogen utilization and environmentally benign process. In order to explain the enhancement in the catalyst performance observed in the coupling process, we propose that the activated hydrogen species on the catalyst surface due to CHL dehydrogenation probably plays an important role in improving the selectivity of 2-MF in the hydrogenation of FFA.⁹ Furthermore, the consuming of the activated hydrogen species breaks the thermodynamic equilibrium of CHL dehydrogenation and facilitates the dehydrogenation of CHL to CHN. In an industrial process, other factors like an improved temperature profile along the reactor may further enhance the effect, suggesting the potential for coupling reactions in practical applications. In addition, the simplified technical procedure due to a “no-hydrogen-supply” operation shows improved technology. Further exploration is of both practical and theoretical importance.

Experimental

The Cu–Zn–Al catalysts were prepared via the continuous precipitation method. In this preparation reaction, a solution of mixed Cu(NO₃)₂, Zn(NO₃)₂ and Al(NO₃)₃ (1 M of total metal ions) was used as metal precursors, with a 1 M Na₂CO₃ solution added as the precipitating agent. Precipitation was performed at 65 °C, and the flow rates of the two solutions were adjusted to give a constant pH of about 7.5. After precipitation, the suspension was washed and filtered. The precipitate was dried at 110–120 °C for 24 h in an air atmosphere. The dried catalyst was calcined at 350 °C for 4 h.

The reactions were carried out in a tubular fixed-bed reactor (length of 500 mm and diameter of 12 mm). Before the reaction, 5.0 g of catalyst with a particle size of 20–40 mesh packed in the reactor was activated insitu at atmospheric pressure in a flow of H₂/N₂ (5 ∶ 95 v/v) stream, and the temperature was progressively increased from ambient temperature to 270 °C. After reduction, the gas flow was switched to a steam of reactant in hydrogen. The liquid products collected in the ice trap and gaseous products were all determined by a SP-2000 gas chromatograph (Ruihong Analyser Co., Shandong, P. R. China) with a flame ionization detector (FID).

Acknowledgements

This work was financed by the Natural Science Foundation of China (No. 20276077). The authors would like to thank Dr Bo-Tao Teng for helpful discussions.

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Footnote

† Electronic supplementary information (ESI) available: Thermodynamic calculations. See DOI: 10.1039/b513584b

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