Elsevier

Journal of Catalysis

Volume 357, January 2018, Pages 223-237
Journal of Catalysis

Hydrogenation of diesters on copper catalyst anchored on ordered hierarchical porous silica: Pore size effect

https://doi.org/10.1016/j.jcat.2017.11.006Get rights and content

Highlights

  • Cu/HPS catalysts have ordered mesoporous structure and high copper dispersion.

  • Anchoring effect of HPS’s micropores hindered the agglomeration of copper species.

  • Dimethyl oxalate hydrogenation on Cu/HPS was controlled by surface reaction.

  • Dimethyl adipate hydrogenation will be easily influenced by the pore diffusion.

  • Dimethyl adipate hydrogenation can be enhanced by tailoring the mesopore size.

Abstract

An ordered hierarchical porous silica (HPS) presenting both mesopores and micropores was fabricated and used for supporting copper catalysts (Cu/HPS). In the prepared Cu/HPS catalysts, high dispersion of both Cu0 and Cu+ species together with ordered porous structure were achieved. The anchoring effect of micropores hindered the agglomeration of copper species, while the formation of Cu-O-Si species, derived from the strong interaction between surface silica and copper precursor, was prompted by the ammonia evaporation approach. The Cu/HPS catalysts were tested in the hydrogenation of diesters to diols. It shows excellent activity on dimethyl adipate hydrogenation with a 1,6-hexanediol space-time yield of 0.72 g/(g∙h) under WHSV = 1.2 h−1. It was further investigated that the activity in dimethyl adipate hydrogenation, in contrast to that of dimethyl oxalate hydrogenation, would be readily limited by pore diffusion, due to the porous structure of HPS.

Introduction

Diols, such as ethylene glycol (EG), 1,6-hexanediol (HDO), are value-added intermediates in the chemical industry. They have been applied in the synthesis of several polymers and fine chemicals, such as elastomers of polyurethane, adhesives, polyesters, plasticizers and pharmaceuticals [1], [2], [3], [4], [5]. Currently, HDO is mainly produced by hydrogenating carboxylic acids or their esters on industrial scale. In order to avoid the use of carboxylic acids which causes severe corrosion of the reactor and consequent catalyst deactivation, hydrogenation of dimethyl adipate (DMA) towards HDO attracted increasing interests from both fundamental research and industrial application. In a typical scaled-up operation, DMA hydrogenation takes place in the liquid phase, catalyzed by noble metals (Ru, Pd, Pt and Rh), facilitating the activation of Cdouble bondO bond [1], [2], [3], [4]. In addition to noble-metal catalysts, it has been further demonstrated that Co and Sn promote the reduction of ruthenium oxide to Ru(0) as the active species. The as-prepared Ru-Sn-Co/AlO(OH) catalyst showed excellent performance in the hydrogenation of DMA with a conversion of 98% and the selectivity of HDO as high as 95% at 493 K under a hydrogen pressure of 5 MPa [6]. However, the high cost of noble metal catalysts, as well as the technical difficulties involved in liquid phase operation, limited the further development of above-mentioned process. Therefore, considerable efforts aiming for the substitution of noble metals with copper-based catalyst on catalytic long-chain alcohol formation have also been reported [7]. The two major bottlenecks are, (1) the severe conditions (T = 523–623 K and P (H2) = 10–20 MPa) and the low yield of alcohols; (2) the use of toxic Cr as a promoter in catalysis. Therefore, the development of highly active Cr-free catalysts in gas-phase DMA hydrogenation is of great significance. Yuan et al. [8] reported a Cu-Zn-Al catalyst for the hydrogenation of DMA and a 1,6-hexanediol yield higher than 95% was achieved at WHSV of 0.5 h−1. They also proposed that Cu/Zn ratio played an important role in maintaining high activity. However, the structure effect of the catalyst on the hydrogenation of DMA was not yet revealed and the catalytic activity deserved a further improvement from an industrial point of view.

Examining other reaction systems involving Cdouble bondO activation, the synthesis of EG via dimethyl oxalate (DMO) hydrogenation by using copper-based catalysts has attracted much attentions for the achievement of commercial scale [9], [10], [11], [12], [13], [14]. As depicted in Scheme 1, the hydrogenation of both DMO and DMA involves similar reaction sequences. The hydrogenation of DMO first generates methyl glycolate (MG), which can be subsequently hydrogenated to EG and ethanol via two-step hydrogenation. Similarly, the hydrogenation of DMA forms methyl 6-hydroxyhexanoate (MHH), HDO and 1-hexanol as the main products. Normally, it is proposed that there are two different active sites responsible for DMO hydrogenation, namely Cu0 and Cu+. The former mainly contributes to H2 activation, while the latter acts as Lewis acid site to activate Cdouble bondO bond in carbonyl group [15]. In our previous work on Cu-MCM-41 catalyzed DMO hydrogenation, Cusingle bondOsingle bondSi unit, generated from interaction between Cu and the support MCM-41, is demonstrated for its crucial roles on the formation and dispersion of Cu species, resulting in its high catalytic activity [16]. Copper phyllosilicate generated during the preparation of Cu/SiO2 was also reported to contribute to the formation of Cu+ species and improve the copper species dispersion [9], [17].

For the rational catalyst design for DMA hydrogenation, in order to simultaneously activate the carbonyl functional group and hydrogen, one should be interested in a delicate Cusingle bondOsingle bondSi unit or copper phyllosilicate to enhance the formation of Cu+ and high dispersion of Cu0 species. However, compared to DMO, the significant larger molecular size of DMA might lead to lower rate of reagents’ diffusion. Moreover, the preparation of copper catalyst supported on mesoporous silica by conventional impregnation approach resulted in rare interaction between SiO2 support and Cu species, leading to lack of accessible active sites.

Herein, series of well-ordered hierarchical porous silica (HPS) with tunable pore size, featured by the co-presence of both mesopores and micropores, were prepared. Further incorporation of Cu species by ammonia evaporation allows the formation of Cusingle bondOsingle bondSi unit, while the agglomeration of copper species is hindered by the micropores in HPS. Meanwhile, the mesoporous structure of HPS was also successfully maintained after loading the copper species. It was for the first time found that the activity in dimethyl adipate hydrogenation would be readily limited by pore diffusion while the dimethyl oxalate hydrogenation was controlled by the surface reaction. Therefore, the copper catalysts on hierarchical porous silica with larger mesopores presented excellent performance for the hydrogenation of diesters with relatively long carbon chain.

Section snippets

Chemicals

F127 (template agent, tri-block copolymers EO106PO70EO106, EO: poly ethylene oxide, PO: poly propylene oxide, molecular weight 12000, MEILUN biology), HCl (37 wt%, Kermel), 1,3,5-trimethylbenzene (TMB) (>99 wt%, Titan), tetraethyl orthosilicate (TEOS) (98 wt%, Kermel), KCl (99.5 wt%, Kermel), Cu(NO3)2⋅3H2O (99.0–102.0 wt%, GuangFu), aqueous ammonia solution (25 wt% NH3 basis, Kermel), colloidal silica (30 wt%, Qingdao Grand Chemical), dimethyl adipate (DMA) (99 wt%, J & K), dimethyl oxalate

Textural properties of HPS-y and 20Cu/HPS-y

Textural properties of the mesoporous HPS-y, 20Cu/HPS-y catalysts and 20Cu/SiO2 are characterized and the results are listed in Table 1 and Fig. 1, Fig. 2. As depicted in Fig. 1A, for each catalyst sample, only one sharp peak is observed in the BJH mesopore size distribution curve. As shown in Fig. 1B, the HPS-y materials exhibit isotherms with characteristics of type IV patterns, indicating the presence of mesopores. The shape of the HPS-y hysteresis loop is of type H1 [22], as a result of the

Structure effect of HPS on the formation of catalyst

On basis of the characterization results, it is demonstrated that a novel hierarchical porous silica material having both micropores (∼1 nm) and mesopores (∼ 12 to 22 nm) was successfully prepared. The mesopore size of HPS can be easily tuned by varying the hydrothermal treatment temperature. It is noteworthy that the structure of the as-synthesized HPS is different from the previous reported FDU material by Fan et al. [26], [40], [41], [42] FDU is known for its 3D ordered porous structure

Conclusion

An ordered hierarchical porous copper-based catalyst was prepared by ammonia evaporation technique for the gas-phase hydrogenation of diesters. The Cu/HPS catalysts with hierarchical porous structure show high copper dispersion and surface areas of copper species. These characters can be ascribed to the anchoring effect of the micropores, which can well disperse the copper species during the ammonia evaporation process. Because of these outstanding properties, a remarkable HDO yield of about

Acknowledgement

The authors are grateful to the financial support from the National Nature Science Foundation of China (21276186, 21325626, 91434127, U1510203), the Tianjin Nature Science Foundation (13JCZDJC33000) and the Chinese Scholarship Council. This work by Y. Zhao was also supported by the US Department of Energy, Office of Science and Office of Basic Energy Sciences. Pacific Northwest National Laboratory (PNNL) is operated by Battelle for the Department of Energy under Contract DE-AC05-76RL01830. We

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