Research Article pubs.acs.org/journal/ascecg
Enhanced Selectivity of Phenol Hydrogenation in Low-Pressure CO2 over Supported Pd Catalysts Tianzhu Liu, Hu Zhou, Bingbing Han, Yongbing Gu, Suiqin Li, Jian Zheng, Xing Zhong, Gui-Lin Zhuang, and Jian-guo Wang* Institute of Industrial Catalysis, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310032, China S Supporting Information *
ABSTRACT: Selective hydrogenation of phenol to cyclohexanone is an important process in both chemical industry and renewable feedstock processing. However, direct hydrogenation of phenol to cyclohexanone under mild conditions over catalysts with high reactivity, selectivity, and facile preparation is still a challenge. In the present study, we report that 99% conversion and 99% selectivity can be achieved over as-prepared Pd/γ-Al2O3 catalyst under the medium of low-pressure CO2 (0.05−0.2 MPa) and H2O at 373 K. According to experiment results, ab initio calculations and in situ high-pressure FTIR measurements indicated enhanced selectivity of cyclohexanone in low-pressure CO2; this result originated from the molecular interaction between cyclohexanone and CO2 and can prevent the further hydrogenation of cyclohexanone. Notably, enhancement of selectivity to cyclohexanone in low-pressure CO2 was also achieved using commercial Pd/γ-Al2O3 and Pd/C catalysts. KEYWORDS: Phenol hydrogenation, Low-pressure CO2, Cyclohexanone, Pd
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INTRODUCTION Cyclohexanone is an important precursor to caprolactam and adipic acid for manufacture of nylon-6 and nylon-66, respectively.1 Industrial production of cyclohexanone normally occurs by oxidation of cyclohexane2,3 or phenol hydrogenation.4−6 Hydrogenation of phenol is a desired strategy as the traditional route consumes more energy and yields low amounts of cyclohexanone (98%) did not drop significantly with the selectivity to cyclohexanone unchanged (98%) after five runs in low-pressure CO2 (Figure 8b). For commercial Pd/γAl2O3 and Pd/C catalysts, the presence of low-pressure CO2 compared with the absence of the same (Figures 8c and e) also presented high selectivity toward cyclohexanone even when catalytic activity gradually decreased (Figures 8d and f), respectively. In the present work, changes in morphology of the as-prepared Pd/γ-Al2O3 were observed by TEM after catalytic cycles. In the above reaction medium, average Pd NPs sizes (2.5 nm) did not change after recycling (Figure S6); this result agrees with that for fresh samples (2.4 nm) as shown in Figure 2. Thus, aggregation of Pd NPs could be neglected in both of reaction mediums. Since the leaching of active component of supported catalyst usually relates with deactivation. The possibility of Pd leaching during reactions in the presence and absence of low-pressure CO2 was also analyzed by ICP-MS. After the first to fifth runs of reaction, Pd was not detected in both of filtrate due to lower concentration than detection limit (0.1 ppb). This finding indicates that leaching of Pd was also not the main factor for the decreased catalytic selectivity in the absence of low-pressure CO2.
Whereafter, XPS was used to investigate electronic state of Pd on the surface of used Pd/γ-Al2O3 catalysts, and results are shown in Figure S9. According to corresponding characteristics of Pd0 and Pd2+ binding energy,52 atomic ratios of Pd2+/Pd0 reached 0.49 and 0.51 for fifth usage of Pd/γ-Al2O3 in the absence and presence of low-pressure CO2, respectively. Moreover, no significant change was observed in texture of used Pd catalysts (entries 3 and 4 in Table 1, Figure S8a). In conclusion, there are no distinct differences of used catalyst between H2 and H2−CO2 reaction medium. As for a significant drop in selectivity during recycling was conducted without CO2, it was speculated that slight change in the physical properties of Pd/γ-Al2O3 catalyst surface during recycling might lead cyclohexanone to leave difficultly from the catalyst surface, and then cyclohexanone was further hydrogenation. Thus, the presence of low-pressure CO2 should be main factor for remained selectivity of cyclohexanone after successive uses. For commercial Pd/C-com and Pd/γ-Al2O3-com catalysts, changes in morphology and structure of the catalysts were observed by TEM and BET test after catalytic cycles, the results were presented in Figures S7 and S8b and c, respectively. The Pd NPs of two commercial Pd catalysts obviously aggregated, and leads to average particle size has increased. After recycling, the BET surface areas of the Pd/C-com. and Pd/γ-Al2O3-com. are both decreased nearly 50%, it might result from the damage to the pore structure. The Pd catalysts after fifth run were analyzed by ICP-MS and proved that the leach of Pd was maximal (detailed data in Table S5). It can be concluded that an obvious drop in activity and selectivity for commercial Pd catalysts during recycling is attributed to the blockage of pores, aggregation, and leaching of Pd. Possible Mechanism. Based on the above experimental results and theoretical calculations, we summarized the most possible reaction mechanism for hydrogenation of phenol over Pd catalysts in Scheme 2. Hydrogen molecules were initially activated by Pd species. Simultaneously, phenol molecules were easily absorbed on γ-Al2O3 or carbon support by formation of H-bridge,19 and the aromatic ring of phenol favored adsorption 11634
DOI: 10.1021/acssuschemeng.7b02974 ACS Sustainable Chem. Eng. 2017, 5, 11628−11636
Research Article
ACS Sustainable Chemistry & Engineering through mixed σ−π interaction on the Pd surface, with hydrogen atoms tilting upward on the Pd surface (Scheme 2a).53,54 Then, adsorbed phenol was hydrogenated to cyclohexanone by the attack of two dissociated hydrogen atoms (this process also involves partial hydrogenation of phenol to cyclohexenol and its rapid isomerization to cyclohexanone).24 As cyclohexanone left the Pd surface, the LA−LB interaction occurred between CO2 and cyclohexanone carbonyl (Scheme 2b), and a cooperative C−H···O interaction transpired with hydrogen atoms attached to the α-carbon atom (this interaction must be weaker due to steric hindrance from the cyclohexanone ring).27 These two interactions promoted the cyclohexanone to leave rapidly from the Pd surface to avoid excess hydrogenation of cyclohexanone (Scheme 2c). Finally, cyclohexanone is replaced by new phenol molecules with higher adsorption capacity.55
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CONCLUSIONS In the present work, enhanced selectivity of cyclohexanone (99%) and conversion (99%) in a low-pressure CO2 and H2O medium was achieved over as-prepared Pd/γ-Al2O3 at mild conditions. The as-prepared Pd/γ-Al2O3 catalyst exhibited excellent stability and was used of five times without losing its activity and product selectivity in low-pressure CO2. Moreover, the enhancement of selectivity to cyclohexanone in low-pressure CO2 can be also achieved by using commercial Pd/γ-Al2O3 and Pd/C catalysts. The possible mechanism of enhanced selectivity to cyclohexanone is the cooperative C− H···O interaction along with LA−LB interaction between CO2 and cyclohexanone through a hydrogen atom attached to the αcarbon atom, this interaction can prevent further hydrogenation of cyclohexanone. Our present work may provide an effective strategy for practical production of fine chemical products in a low-pressure CO2 medium.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b02974. Table for showing component details, TEM images, XPS spectra, in situ FTIR spectra, particle-size distribution of the Pd catalysts, additional spectra of gas chromatogram, and H2 pulse chemisorption profiles (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +86-571-88871037. Tel: +86571-88871037. ORCID
Jian-guo Wang: 0000-0003-2391-4529 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (NSFC-21625604 and 21671172). REFERENCES
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