J. Phys. Chem. 1995, 99, 10590-10593
10590
Thermal Stability of Structural Aluminum in the Mesoporous Molecular Sieve MCM-41 Zhaohua Luan, Chi-Feng Cheng, Heyong He, and Jacek Klinowski* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 IEW, U.K. Received: December I S , 1994; In Final Form: March 21, 1995@
Just as in the parent Naf-MCM-41, all the aluminum in N&+-MCM-41 with structural SUA1 = 10-60 is in 4-coordination. Upon calcination in air, H+-MCM-41 is formed and a small part of aluminum is removed from the aluminosilicate structure. We conclude that structural aluminum in MCM-41 is thermally less stable than, for example, in zeolite Y,because the MCM-41 structure lacks strict crystallographic order at the atomic level and the very small ' H cations cannot satisfy the framework charge balance as efficiently as the Na+ cations.
Introduction The mesoporous molecular sieve MCM-41 has attracted much attention'%2because of its potential use in fluidized catalytic cracking (FCC) and the manufacture of fine chemicals. While several papers concerning the synthesis and structure of this novel material have been p ~ b l i s h e d , ~we- ~are not aware of any study of the catalytic properties of aluminosilicate MCM-41. This is probably caused by the uncertainty surrounding the chemical status of aluminum in the structure.6-8 We have shown9 that in MCM-41 prepared using Catapal alumina virtually all A1 is 6-coordinate, and we proposed a new synthetic route to MCM-41 containing only structural (4-coordinate) Al, which may have important catalytic applications. Since Bronsted acidity is created by heating the ammonium form, it is important to examine the properties of calcined N&+-MCM-41 before industrial catalysts of high activity, selectivity, and stability can be prepared. We have done so using 27Almagic-angle-spinning(MAS) NMR and demonstrate that, depending on the temperature of calcination, some A1 is expelled from the structure.
*'AI MAS NMR
- MCM - 41
'H equilibrated with r w m air
ppm
A
I\ .5
u) c
2$
fully hydrated
1
Experimental Section
The detailed synthesis procedure of Na+-MCM-41 with only structural aluminum has been described el~ewhere.~The samples are designated as MCM-41-X, where X = 10, 20, 30, 40, and 60 are the SdA1 ratios in the synthesis gel. NH4'MCM-41 was obtained by repeated ion exchange of NafMCM-41 with a 1 M aqueous solution of N&Cl. Calcination of N&+-MCM-41 at 400 or 550 OC for 12 h gave H+-MCM41. Table 1 shows the elemental composition of the MCM-41 samples as determined by X-ray fluorescence (XRF). The result revealed no loss of aluminum from MCM-41 upon ammonium exchange or thermal treatment. After fourfold ammonium exchange the Na content was below 0.1 wt %. X-ray diffraction (XRD) patterns were recorded using a Philips 1710 powder diffractometer with Cu K a radiation (40 kV, 40 mA), a 0.025' step size, and a 1 s step time. 27AlMAS NMR spectra were recorded on a Chemagnetics CMX-400 spectrometer at 104.3 MHz with 0.3 s recycle delays and corrected by subtracting the spectrum of the empty MAS rotor. Very short n120 radiofrequency pulsesI0 were used to ensure that they are quantitatively reliable. External Al(H20)63+was used as a reference. Prior to experiments all samples were fully hydrated" or further equilibrated with room air.
* Author
to whom correspondence should be addressed. Telephone: +(44)-01223-33 65 14. FAX: +(44)-01223-33 63 62. E-mail:
[email protected]. @Abstractpublished in Advance ACS Absrracts, June 1, 1995.
as prepared
150
50
100
0
-50
-100
-150
ppm from AI ( H ~ o ) ~ ~ +
Figure 1. *'A1 MAS NMR spectra on the absolute intensity scale: (a) H+-MCM-41-30 after calcination at 550 OC for 24 h; (b) fully hydrated; (c) equilibrated with room air.
TABLE 1: Elemental Composition of Aluminosilicate MCM-41 (bv X-rav Fluorescence) ~_______ ~ ~ _ _ _ _ _ ~
sample MCM-41- 10 MCM-4 1-20 MCM-41-30 MCM-4 1-40 MCM-41-60
~~
SUA1 ratio Na+form H+form 10.4 9.9 21.5 34.1 26.5 43.0 71.0 53.0
Na content of H+ form (wt %)
