DOI: 10.1002/cctc.201100273 Selective Hydrodeoxygenation of Lignin-Derived Phenolic Monomers and Dimers to Cycloalkanes on Pd/C and HZSM-5 Catalysts Chen Zhao and Johannes A. Lercher* [a] Lignin is an abundant renewable resource with a high energy density, but it is considered to be difficult to process because of the high reactivity of its building blocks, that is, the substi- tuted phenol units, which tend to react even at small concen- trations and reaction temperatures. [1] It has been shown re- cently that this reactivity can be overcome by combining me- tallic (Pd) and acidic functions (H 3 PO 4 , CH 3 COOH, SO 3 H, and OH) in the appropriate concentrations in an aqueous phase or ionic liquids. [2] This has led to the successful hydrodeoxyge- nation of phenol derivatives and the synthesis of a pure cyclo- alkane product. Because of the different polarity of the substi- tuted phenols and the alkanes, a second hydrocarbon phase is formed during the process, which can be easily separated. Noble and base metals have been found to be active for hydrogenation in the aqueous phase. Replacing liquid mineral acids by a solid acid (Nafion/SiO 2 ) allowed for an increase in efficiency. [3] In addition to the hydrodeoxygenation of phenolic mono- mers, the selective cleavage of the aromatic carbon–oxygen (C O) bonds in aryl ethers is also challenging because of the strength and stability of these linkages. [4] This cleavage is very important for facilitating the depolymerization of oxygenrich lignin by breaking down the C O C linkages, and for the hydrodeoxygenation of lignin-derived phenolic dimer frag- ments to the deoxygenated biofuels. Here, we report on the use of a weaker solid acid, that is, a zeolite (HZSM-5), as a se- lective catalyst component for the quantitative hydrodeoxyge- nation of diversely substituted lignin-derived mono- and binu- clear phenols to cycloalkanes in combination with a noble metal (Pd) in aqueous solutions at a mild temperature (473 K). We have shown previously that phenol is converted to cy- clohexane in water through the sequential hydrogenation of phenol to cyclohexanone and cyclohexanol on metal sites (Pd or Ni), dehydration of cyclohexanol on acid sites (H 3 PO 4 , CH 3 COOH, or Nafion/SiO 2 ), and finally the hydrogenation of cy- clohexene to cyclohexane on metal sites. [1–3] To maximize the hydrodeoxygenation rate and selectivity under milder condi- tions (low reaction temperatures and pressures) in addition to the catalyst stability, various solid acids (acid-site densities and specific surface areas are listed in Table S1 in the Supporting Information) are explored in the presence of palladium Pd/C as hydrogenation catalyst. The characterization of the catalyst was achieved by determining the Brunauer–Emmett–Teller (BET) surface area and by using XRD, SEM, and TEM and is compiled in the Supporting Information. As a suitable solid acid should have a high acid-site density in combination with a sufficient stability in an aqueous phase above 473 K, the results (see Table S3 in the Supporting Information) for the conversion of 4-n-propylphenol show that solid Lewis acids, such as alumina, silica, and amorphous silica alumina, are not effective for oxygen removal (through dehydration of cycloal- cohol). Such catalysts produce less than 3 % cycloalkanes and more than 90 % cycloalcohols in the presence of Pd/C at 473 K and 5 MPa H 2 for 0.5 h. This shows that the acidic hydroxyl groups and Lewis acid sites on oxide surfaces are ineffective for alcohol dehydration in the presence of water because of competing water molecules, which reduces the effective acid strength. [5] Although alumina pillared clay leads to 50 % yields of cycloalkanes, the reaction rates are rather low because of diffusion limitations of the reactant. [6] In contrast, solid Brøn- sted acids with a sufficiently high acid density, such as sulfated zirconia, Amberlyst 15, Nafion/SiO 2 , and Cs 2.5 H 0.5 PW 12 O 40 , lead to 90 % yield of cycloalkanes. HZSM-5 with a Si/Al ratio of 45 and a Brønsted acid site (BAS) density of 0.278 mmol g 1 pro- duces yields of 93 % C 9 cycloalkanes, 2.5 % C 9 cycloalcohols, and 4.5 % ethers (formed by intermolecular dehydration of C 9 cycloalcohols). Hydrogenation of phenol on Pd/C leads to cy- clohexanol as the initial product. Its dehydration in water is catalyzed by BAS in the pores, whereas ethers are formed on Lewis-acidic sites on the external surface. [5] The dehydrated cy- cloalkene is immediately converted to the saturated cycloal- kane by accessible Pd atoms. Compared to other solid Brønsted acids, high yields of C 9 cycloalkanes can be produced from 4-n-propylphenol by using a combination of HZSM-5 and Pd/C at lower temperatures. Approximately 90 % cycloalkanes were produced in the pres- ence of HZSM-5 and Pd/C at 433 K after 0.5 h, whereas less than 20 % cycloalkanes, but more than 80 % cycloalcohols, were formed in the presence of sulfated zirconia, Amberlyst 15, Nafion/SiO 2 , and Cs 2.5 H 0.5 PW 12 O 40 under identical conditions (Figure 1). This points to a lower apparent activation energy and a higher reaction rate for the cycloalcohol dehydration on HZSM-5 compared to the other solid Brønsted acids for the overall hydrodeoxygenation. The dehydration rate of cycloalcohol (in situ produced by phenol hydrogenation) in water estimated from Figure 1 for the overall phenol hydrodeoxygenation reaction was approxi- mately 41 mol mol 1 BAS h 1 on HZSM-5 at 433 K, whereas these rates were approximately 0.3, 6.7, 11, and 4.3 molmol 1 BAS h 1 on sulfated zirconia, Amberlyst 15, Nafion/SiO 2 , and Cs 2.5 H 0.5 PW 12 O 40 , respectively. In our previous study, aqueous H 3 PO 4 also showed a low dehydration rate for cyclohexanol (turnover frequency: 15 mol mol 1 Hþ h 1 ) and a high activation [a] Dr. C. Zhao, Prof. Dr. J. A. Lercher Catalysis Research Center and Department of Chemistry Technische Universität München, Garching 85747 (Germany) Fax: (+ 49) 89-28913544 E-mail : johannes.lercher@ch.tum.de Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201100273. 64  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemCatChem 2012, 4, 64 – 68