Article pubs.acs.org/IECR
Carbon Nanofibers Grown on Anatase Washcoated Cordierite Monolith and Its Supported Palladium Catalyst for Cinnamaldehyde Hydrogenation Jie Zhu,†,‡ Yong Jia,† Mingshi Li,*,† Mohong Lu,† and Jianjun Zhu† †
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China State Key Lab of Chemical Resource Engineering, Beijing 100029, China
‡
S Supporting Information *
ABSTRACT: We report the synthesis of a promising carbon nanofiber−titania−cordierite monolith composite, i.e., CNF/ TiO2/monolith, and its application as catalyst support in selective hydrogenation of cinnamaldehyde (CAL) to hydrocinnamic aldehyde (HCAL). The composite was synthesized through TiO2 coating on the surface of the monolith and the following CNF growth on it. Synthesized CNF/TiO2/monolith was subsequently employed to prepare its supported palladium catalysts, Pd/ CNF/TiO2/monolith. Attachment strength and acid-resistant properties of the composite have been studied to evaluate its structural stability in some severe conditions. In addition, the effects of supported Pd particles, oxygen-containing surface groups, and internal diffusion limitation on catalytic performance over Pd/CNF/TiO2/monolith were further studied. Results revealed that the total BET surface area of the composite was 31 m2/g, and the macro- and mesopore structure dominated the pore space of the material (about 93%). Meanwhile, 94% of the carbon deposit on the surface of the composite was CNF. TiO2 and CNF coating was deemed to increase its textural and acid-resistant properties. Although there is a relatively slow reaction rate over asprepared Pd/CNF/TiO2/monolith-R due to its small BET surface area and low Pd dispersion (about 15%), the selectivity to HCAL reamined high (about 90%) over it at 95% CAL conversion, the same as that over powdered Pd/CNF (about 93%), being much higher than that over Pd/FAC (about 45%), Pd/MC, and Pd/CNF/TiO2/monolith-O (each about 82%). This was attributed to removal of acidic oxygen-containing surface groups and elimination of internal diffusion limitation on Pd/CNF/ TiO2/monolith-R.
1. INTRODUCTION Selective hydrogenation is one of the key processes in pharmaceutical and cosmetic industries. With the everincreasing demand for special chemicals, selective hydrogenation of α,β-unsaturated aldehydes has become one of the current challenges to be tackled both from fundamental and from industrial standpoints.1 Cinnamaldehyde (CAL), a typical α,βunsaturated aldehyde, possesses two sites of hydrogenation: a carbonyl group (CO) and a conjugate double bond (CC), which leads to a complex reaction network of CAL-selective hydrogenation involving two parallel reactions (Figure 1). Specifically, cinnamaldehyde can be initially hydrogenated to cinnamyl alcohol (COL) and hydrocinnamic aldeyde (HCAL), depending on whether the CC double bond is hydrogenated or the CO bond. Both intermediates are further hydrogenated to hydrocinnamic alcohol (HCOL), giving 1propylbenzene (1-PB) as the final product.2−4 The performance of the hydrogenation reactions, especially product selectivity, is significantly influenced by several factors including metal particle size,5−7 nature of the support,8−10 presence of promoters,11,12 and reaction solvent.13,14 For this reason, many attempts have been made to promote the selectivity to different intermediates by taking advantage of some novel catalysts or catalytic systems. For example, Galletti et al.15 synthesized Pd/γ-Al2O3 nanocatalysts using a simple and an environmentally friendly microwave-assisted process. These new supported palladium nanocatalysts have shown © 2012 American Chemical Society
interesting catalytic performances in terms of both activity and selectivity when employed in CAL hydrogenation to HCAL in decalin, reaching from high to complete substrate conversions and selectivities up to 97% in HCAL at complete substrate conversion. Recently, a new group of carbonaceous materials, carbon nanotube (CNT) and nanofiber (CNF), abbreviation as CNF(T), showed potential applications in various fields of materials research including catalysis owing to their exceptional physical and chemical properties.16−18 Use of CNF(T) as catalyst support is inspired by the unique properties of these materials including their unique structure-dependent optical properties, resistance to strong acids and bases, and high mechanical flexibility and strength as well as improved carbon− metal interactions that have been found to enhance catalytic performance.19,20 In addition, they can form aggregates with high surface areas and pore volumes without microporosity, which will prefer to improve product selectivity in selective hydrogenation. Zhang et al.20 reported a striking enhancement of catalytic performance of gold nanoparticles (NPs) supported on CNTs for CAL hydrogenation in toluene, and the results Received: Revised: Accepted: Published: 1224
September 4, 2012 December 21, 2012 December 28, 2012 December 28, 2012 dx.doi.org/10.1021/ie302374a | Ind. Eng. Chem. Res. 2013, 52, 1224−1233
Industrial & Engineering Chemistry Research
Article
Figure 1. Consecutive reaction routes of cinnamaldehyde hydrogenation.
of CNF. The as-prepared macrostructured composite was named CNF/TiO2/monolith. TiO2, not β-zeolite or γ-alumina, was chosen to cover the monolith in composite preparation because it possesses several prominent advantages including the high chemical resistance to make the TiO2-coated monolith in highly acidic or basic medium as well as its preferable electronic properties to help to improve product selectivity in reactions.35,36 The catalytic behavior of Pd catalyst supported on CNF/TiO2/monolith (Pd/CNF/TiO2/monolith) was studied in selective hydrogenation of CAL to HCAL. Results were compared with those supported on formed activated carbon (Pd/FAC), mesoporous carbon (Pd/MC), and powdered carbon nanofiber (Pd/CNF). The possible formation mechanism of hydrogenation products was also discussed.
showed that the selectivity to HCAL was up to 91% at 95% CAL conversions. Although owning some of the advantages mentioned above, CNF(T) in powder form suffers from some drawbacks for slurry phase operation. Obviously, agglomeration of the nanofibers and difficulty of filtration can adversely impact catalytic processes. In addition, CNF(T) powders are difficult to use directly in fixed-bed reactors due to a high pressure drop. Therefore, many studies on macrostructured CNF(T) and their prepared catalysts have been launched21 for the purpose of resolving the disadvantages of the powdered catalysts employed in catalytic reactors, especially in fixed-bed ones. Some formed supports, including graphite felt,22 metallic filters, and foams,23−25 can be employed to immobilize CNF(T) to develop the macrostructured CNF(T) supports. Ceramic monolith is another promising candidate as well. Cordierite monolith, a formed industrial ceramic with honeycomb shape (Figure S1a, Supporting Information), is widely used in new reactor applications such as chemical processing, refining industries, and catalytic combustion.26−28 It has some distinct advantages including better mechanical strength, lower heat transfer capacity, and lower thermal expansion coefficients.29 Macroporous structure, another feature of the monolith, will be in favor of selective hydrogenation by eliminating internal diffusion limitation in the reaction. However, both the low specific surface area of cordierite monolith (
