Pseudomorphic Synthesis of Large-Particle Co-MCM-41 Sangyun Lim, Alpana Ranade, Guoan Du, Lisa D. Pfefferle, and Gary L. Haller* Department of Chemical Engineering, Yale UniVersity, P.O. Box 208286, New HaVen, Connecticut 06520-8286 ReceiVed June 8, 2006. ReVised Manuscript ReceiVed September 4, 2006 Large-particle (15 and 40 μm) Co-MCM-41 was synthesized using a pseudomorphic transformation. To maintain the spherical shape of the parent silica particles, overcondensation of silanol groups has to be avoided under a moderately basic synthesis condition. After 4 days of autoclaving at 100 °C with the Co-MCM-41 synthesis solution of which the initial pH was adjusted to 12.0, nonruptured spherical Co-MCM-41 particles, having one-dimensional pores in which Co ions are highly dispersed, were successfully synthesized. The reduction stability of this catalyst was affected by Co ion location controlled by pH adjustment and hydrogen spillover from residual cobalt oxide on the surface, which had not been incorporated into the silica matrix. Introduction Metal ion incorporated MCM-41 is a useful and effective catalyst for various catalytic reactions. The flexible frame- work (noncrystalline) structure of MCM-41 enables relatively facile introduction of a broad range of metal ions without structural collapse. Isomorphous substitution of Si by various metal ions of substantially improved physicochemical stabil- ity of active components has resulted in improved catalysts for several reactions. 1-6 However, small particle size with extremely high porosity, which results from the initial silica sources, may present unexpected challenges. For example, the low bulk density of Co-MCM-41 is a major barrier for use in a fluidized-bed reactor. A fluidized-bed reactor is considered as a promising approach for the catalytic growth of carbon nanotubes on a large scale with a uniform bed temperature distribution. For templated growth of a nano- structure, i.e., BMg and GaN, fewer defects and longer length are required to obtain the expected electrochemical proper- ties. To satisfy these requirements, metal ion incorporated MCM-41 with large spherical particles consisting of pores all the way through the particle with uniform distribution of metal ions is required. The concept of pseudomorphism may be a synthesis approach for large-particle MCM-41 catalyst applications. A pseudomorph, in mineralogy, is a crystal or other body consisting of one mineral but having the form or shape of another, a consequence of having been formed by substitu- tion, or by chemical or physical alteration. In this study, MCM-41 particles can be considered as pseudomorphs of the silica gel grains used in the synthesis. Each silica grain can behave like a microreactor in which silica may be dissolved by the alkaline solution, and silica species interact with surfactant to form the MCM-41 pore structure, as in the usual synthesis procedure. Pseudomorphic synthesis of pure siliceous MCM-41 using large silica particles, 5-15 μm in diameter, was first introduced by Martin et al. in 2002, 7 applied to a separation medium, 8 and expanded to MCM- 48. 9 These processes are for pure siliceous mesoporous materials and utilize NaOH. However, for the application to MCM-41 as a catalytic material, metal ion incorporated MCM-41, sodium has a negative effect in the substitution of metal ions and catalytic reaction, as well as stability. Uniform distribution of metal ions through the pore surface is a key property in the metal ion incorporated MCM-41 compared to pure siliceous MCM-41. In this study, therefore, a non-sodium process is introduced for the pseudomorphic synthesis of Co-MCM-41, which may be applied to a wide range of metal ions. The physical and chemical properties were investigated by temperature programmed reduction (TPR), nitrogen physisorption, and scanning electron micrograph (SEM) to suggest a set of preferred synthesis conditions for obtaining a successful Co- MCM-41 pseudomorph. Experimental Section A non-sodium process was used to synthesize Co-MCM-41 following the detailed synthesis procedure described elsewhere. 10,11 * To whom correspondence should be addressed. E-mail: gary.haller@yale.edu. (1) Du, G. A.; Lim, S. Y.; Yang, Y. H.; Wang, C.; Pfefferle, L.; Haller, G. L. Appl. Catal., A 2006, 302, 48. (2) Yang, Y.; Du, G.; Lim, S.; Haller, G. L. J. Catal. 2005, 234, 318. (3) Chen, Y.; Ciuparu, D.; Yang, Y.; Lim, S.; Wang, C.; Haller, G. L.; Pfefferle, L. Nanotechnology 2005, 16, S476. (4) Chen, Y.; Ciuparu, D.; Lim, S. Y.; Yang, Y. H.; Haller, G. L.; Pfefferle, L. J. Catal. 2004, 225, 453. (5) Chen, Y.; Ciuparu, D.; Lim, S.; Yang, Y. H.; Haller, G. L.; Pfefferle, L. J. Catal. 2004, 226, 351. (6) Lim, S.; Ciuparu, D.; Pak, C.; Dobek, F.; Chen, Y.; Harding, D.; Pfefferle, L.; Haller, G. J. Catal. B 2003, 107, 11048. (7) Martin, T.; Galarneau, A.; Di Renzo, F.; Fajula, F.; Plee, D. Angew. Chem., Int. Ed. 2002, 41, 2590. (8) Martin, T.; Galarneau, A.; Di Renzo, F.; Brunel, D.; Fajula, F.; Heinisch, S.; Cretier, G.; Rocca, J. L. Chem. Mater. 2004, 16, 1725. (9) Petitto, C.; Galarneau, A.; Driole, M. F.; Chiche, B.; Alonso, B.; Di Renzo, F.; Fajula, F. Chem. Mater. 2005, 17, 2120. (10) Lim, S.; Yang, Y. H.; Ciuparu, D.; Wang, C.; Chen, Y.; Pfefferle, L.; Haller, G. L. Top. Catal. 2005, 34, 31. (11) Lim, S.; Ciuparu, D.; Chen, Y.; Yang, Y. H.; Pfefferle, L.; Haller, G. L. J. Phys. Chem. B 2005, 109, 2285. 5584 Chem. Mater. 2006, 18, 5584-5590 10.1021/cm061342s CCC: $33.50 © 2006 American Chemical Society Published on Web 10/24/2006