MATERIALS CHEMISTRY COMMUNICATIONS Mesostructured lamellar chromium oxide S. Ayyappan, N. Ulagappan and C. N. R. Rao* CSIR Centre of Excellence in Chemistry and Materials Research Centre, Indian Institute of Science, Bangalore 560 012, India and Jawaharlal Nehru Centre For Advanced Scientijic Research, Jakkur, Bangalore 560 064, India Mesostructured l?mellar chromium oxide with an interlayer separation of 29 A has been prepared by employing a cationic surfactant. The introduction of phosphat? groups in the oxide increases the interlayer separation to 32 A. Mesoporous silica is known to occur with hexagonal, cubic and lamellar structures, although much of the work in the last three years has been on hexagonal phases.' There have been some efforts, however, to synthesize and characterize other types of mesoporous phases, in particular lamellar ones.Thus, Huo et aL2 have shown that lamellar metal oxides can be prepared by employing sodium alkyl sulfates (C,H,,+,OSO,Na; n=12, 14, 16, 18) and alkyl phosphates such as C1,H2,0P0,H2 as the templates, while Ciesla et aL3 have synthesized mesostructured lamellar molybdenum oxide using alkylammonium chlorides (C,H2,+ ,NMe,Cl; n = 12, 14, 16, 18) as the templates. Oliver et aL4 have obtained unusual lamellar aluminophosphates exhibiting surface patterns mim- icking diatom and radiolarion microskeletons by using decyl- ammonium dihydrogenphosphate. Sayari et aL5 have recently prepared lamellar aluminophosphates using neutral dodecyl- amine as the surfactant.In this communication, we report the successful synthesis of lamellar mesophases of chromium oxide by using cetyltrimethylammonium bromide (CTAB) as the surfac tant. In order to prepare mesostructured chromium oxide, we started with the molar composition CrO, :0.27CTAB: 14H20: 6C2H,0H, based on our experience with silica mesophases. In a typical synthesis, a solution of CTAB in aqueous ethanol was added to an aqueous solution of Cr03 and stirred for 30 min. To this solution ammonia was added to bring the pH to 6. The precipitate obtained was aged for 2 days, filtered, washed several times with water and dried at 373 K in a hot air-oven. Thermogravimetry showed the molar ratio of the oxide to the template to be 1: 1 and the adduct contained only the elements of chromium oxide along with the surfactant and water.The IR spectrum showed bands characteristic of the CH2 groups in the surfactant, as well as other vibration modes. The as-synthesized compound was subjected to X-ray diffraction. The X-ray diffraction pattern shown in Fig. l(a) clearly demonstrates the lamellar nature of the product with d-spacings $ue to 001 and other 001 reflections. The dool value of 29.43 A in the mesostructured chromium oxide found hFre is larger than that of the lamellar mesophase of MOO,(24.4 A)., In order to confirm the lamellar nature of the chromium oxide mesophase, we recorded high-resolution transmission electron microscope images using a JEOL JEM3010 microscope oper- ated at 300 kV. A typical image, as presented in Fig.2, reveals an interlayer spacing of ca. 30 A. Removal of the template by both thermal and solution routes destroys the lamellar meso- structure. The diffuse reflectance spectrum of the mesophase was recorded to establish the nature of the chromium species. The spectrum [Fig. 3(a)] of the as-synthesized sample shows a prominant band at 380 nm due to Cr6+; the shoulder around 450nm could be due to Cr3+ specie^.^.^ The EPR spectrum shows a very weak signal at g= 1.94 due to Cr3+. In order to investigate whether the introduction of phosphate 0 0 5 10 15 20 2Bldegrees (Cu-Ka) Fig. 1 X-Ray diffraction patterns of mesostructured lamellar (a)chro-mium oxide and (b)chromium oxide with some phosphate Fig.2 TEM image of mesostructured lamellar chromium oxide J. Muter. Chern., 1996,6( lo), 1737-1738 1737 wavelengthhm Fig. 3 Diffuse reflectance spectra of mesostructured (a) chromium oxide and (b)chromium oxide with some phosphate groups affects the lamellar structure in any way, we prepared mesophases starting with the molar composition Na20 CrO, 0 5P20, 0 2CTAB 150H20 In a typical syn- thesis, phosphoric acid was added to CrO, in an aqueous solution, to which the CTAB solution was then added After stirring for 30 min, the material was transferred to an air-tight polypropylene bottle and heated at 363 K for 2 days The product obtained was filtered, washed several times with water and dried at 373 K in a hot air-oven Analysis showed the phosphorus content to be 4 mol% with respect to chromium Thermogravimetry showed the molar ratio of the ‘chromium phosphate’ to the template to be 3 1 The X-ray diffraction pattern of the phosphate derivative [Fig l(b)], gives a doe, value of 32 13 A, which is higher than the value found for Cr03 alone E1ec:ron microscopy images show an interlayer separation of 32 A Thermal as well as solution treatments to remove the template resulted in the collapse of the mesostruc- ture in this case also The diffuse reflectance spectrum [Fig 3(b)] is similar to that of the chromium oxide-based mesophase discussed earlier S A thanks the CSIR, India, for a fellowship References 1 J S Beck and J C Vartuli, Curr Opin Solid State Muter SCI,1996, 1,76 2 Q Huo, D I Margolese, U Ciesla, D G Demuth, P Feng, T E Gier, P Sieger, A Firouzi, B F Chmelka, F Schuth and G D Stucky, Chem Muter, 1994,6, 1176 3 U Ciesla, D Demuth, R Leon, P Petroff, G D Stucky, K Unger and F Schuth, J Chem SOC,Chem Commun , 1994,1387 4 S Oliver, A Kuperman, N Coombs, A Lough and G A Ozin, Nature (London), 1995,378,47 5 A Sayari,V R Karra, J S Reddy and1 L Moudrakovski, Chem Commun ,1996,411 6 J S T Mambrim, E J S Vichi, H 0 Pastore, C U Davanzo, H Vargas, E Silva and 0 Nakamura, J Chem SOC, Chem Commun, 1991,922 7 N Ulagappan and C N R Rao, Chem Commun, 1996,1047 Communication 6/04395J, Received 24th June, 1996 1738 J Muter Chem, 1996, 6(10), 1737-1738