12588
J. Phys. Chem. 1995, 99, 12588-12596
SiO-0 *HOSi Hydrogen Bonds in As-Synthesized High-Silica Zeolites Hubert Koller, Raul F. Lobo; Sandra L. Burkett, and Mark E. Davis* Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received: April 13, 1995; In Final Form: June 9, 1 9 9 9
The high-silica zeolites NON, DDR, MTW, AFI, and MFI synthesized with organic quaternary ammonium cations show in the as-synthesized form a line at 10.2 f 0.2 ppm in the 'H MAS NMR spectra that does not originate from the organic structure directing agents (SDA's). This signal is assigned to Si0-n .HOSi hydrogen bonds between defect sites with an 0 0 distance of 2.7 A. The presence of Q3 sites is confirmed by 29Si MAS NMR. The intensity of the line at 10.2 ppm in the 'H MAS NMR spectra decreases when the positive charge of the quaternary ammonium cations is balanced by negative framework charge by the incorporation of aluminum or boron into the zeolite structure or when the amount of defects is reduced by using fluoride ions as mineralizing agents in the synthesis. Diffuse reflectance IR spectroscopy of high-silica MFI reveals a broad band centered at 3200 cm-' for the 0-H stretching vibration of the SiO-***HOSihydrogen bonds and the 0 -0distance compares well to that obtained from 'HMAS NMR data. Heating the sample induces protonation of the siloxy groups of the hydrogen bonds due to the decomposition of the organic tetrapropylammonium cations followed by the disappearance of the SiO-0 *HOSi hydrogen bonds. For MFI prepared with deuterated tetrapropylammonium cations, it is found that approximately one to two hydrogenbonded protons in these defect sites exist per quaternary ammonium cation. Other 'H NMR lines are found at 6.5 and 4.5-5.5 ppm and are assigned to silanol groups forming weaker hydrogen bonds and water molecules associated with alkali-metal cations, respectively. A model of the defect sites in as-synthesized high-silica zeolites is proposed.
Introduction The amount and nature of defect sites in high-silica zeolites are known to influence their catalytic, ion exchange, and adsorption properties.' Numerous papers on the characterization of these defect sites (mainly involving studies on ZSM-5) have been published, and it is impossible to give a comprehensive overview here. (For more information see refs 2 and 3 and references therein.) The first 29SiNMR spectroscopic evidence for defect sites in ZSM-5 (from now on denoted MFI, the threeletter code recommended by the Intemational Zeolite Association4) was published by Nagy et aL5 and work on this topic has continued since then. It is generally believed that the number and nature of the defect sites depend on many factors, e.g., synthesis method, SUA1 ratio, trace elements, etc. Additionally, for samples containing aluminum, it is reported that the number of silanol groups decreases with increasing A1 content in the zeolite.6 Zeolitic defect sites can be classified as follows: (a) Si-0defects (siloxy groups): balance the charge of cations in the pores of the zeolites; (b) SiOH defects (silanol groups): from hydrolyzed Si-0-Si bridges; (c) SiOH groups generated by missing tetrahedral framework atoms (T vacancies); (d) SiOH groups due to stacking disorder; and (e) SiOH groups at the extemal surface of the crystals. All of these types of defect sites can exist in as-synthesized zeolites. While many investigations on defect sites in zeolites have involved samples that have been calcined or exposed to various other treatments, e.g., dealumination, much fewer studies exist on as-made materials, i.e., when the organic structure directing agent (SDA) has not yet been removed by decomposiAlso at the Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545. * To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, July 15, 1995. @
0022-3654/95/2099-12588$09.00/0
tion at elevated temperatures (calcination). Typically, the latter studies have focused on the characterization by 29SiMAS NMR spectro~copy.~~J Lobo et al.' have shown that the number of Q3 units (Q" stands for X4-,Si[OSi],, X = OH or is much lower in the clathrasil nonasil (NON) when it is synthesized with an organic amine instead of a charged quatemaq ammonium cation as the SDA. This result was rationalized by suggesting that Si-0- defect sites are present for charge compensation when using an organic cation as the SDA. Similar results have been reported for high-silica AF18 and MFL9 On the other hand, MFI synthesized in the presence of F- ions is nearly defect free despite the use of tetrapropylammonium (TPA) cations as SDA's.'O*" In this case, the fluoride ions balance the charge of the organic cation, and a highly connected inorganic framework results.I0 If d o x y groups counterbalance the charge of the SDA, then often the quantity of Q3 sites observed from the 29SiMAS NMR spectra is larger than the required number density for charge compensation and therefore would suggest that the Q3 sites are composed of siloxy and silanol groups. Additionally, the question arises as to how the siloxy groups are stabilized, Le., the oxygen atom in Si-0- is lacking of one valence. For crystalline silicate hydrates, it is known that hydrogen bonds exist between siloxy groups and silanol groups or water molecules. (See e.g. refs 12 and 13 and references therein.) Therefore, we suspect that the siloxy groups in as-synthesized high-silica zeolites can also function as hydrogen bond acceptors. It is well known that a correlation exists between 'H NMR chemical shifts and the d o . .O or d o . .HO distances in 0- -H-0 hydrogen b ~ n d s . ' ~ - ~Because O of this, we conducted a 'H MAS NMR study of as-made, high-silica zeolites in order to ascertain a greater understanding of the nature of the defect sites. Here, 0 1995 American Chemical Society
SiO-***HOSiH Bonds in High-Silica Zeolites
J. Phys. Chem., Vol. 99, No. 33, 1995 12589
TABLE 1: Synthesis Conditions for MFI Zeolites" no. sample ID gel composition mineralizing agent TK ?/days 1 [TPA][Si-MFI] 0.1TPAOH:SiOz:38H20 OH423 6 2 [Na,TPA][Si-MFI] O.lTPABr:Si02:25H20:0.2NaOH OH423 2 3 [Na,TPA][Si,AI-MFII-1 0.1TPABr:SiO~:36H~0:0.25NaOH:0.05 Al(N03)3 OH423 6 4 [Na,TPA][Si,AI-MFI1-2 O.lTPABr:SiO~:36H~0:0.34NaOH:0.025Na~OAl~0~~3H~0 OH423 11 5 [TPAFI[Si-MFI] O.lTPABr:Si02:20Hz0:0.5N&F F473 5 6 [Na,TPAF][Si-MFI] 0.1TPABr:Si02:30H20:0.3N&F:0.2NaF F448 7 7 [TPAF][Si,AI-MFII-1 0.1TPABr:SiO~:25H~0:0.5N&F:0.025AI(N0~)~ F448 7 8 [TPAF][Si,ALMFI]-2 0.1TPABr:Si0~:25H~0:0.5N&F:O.O67A1(NO~)~ F448 7 OH383 7 9 [TPA-&~I[sI-MFTI 0.1(TPA-d~8)0H:Si02:30HzO For the samples 1 and 9, tetraethyl orthosilicate (Aldrich) was used as the Si02 starting material. All other MFI zeolites were prepared with fumed silica CAB-0-SIL M5 (Cabot Corp.). Other starting materials are from Aldrich (TPABr, piperazine hexahydrate, NH4F),Eastman-Kodak (25% TPAOH in HzO), EM Science (Al(N03)39HzO),VWR Scientific (Na20*A1203*3H20),Mallinckrodt (50% NaOH in HzO), and Cambridge Isotope Laboratories (TPA-dzg bromide). TPA-dZ8 bromide was transformed into the hydroxide form using an ion exchange resin. TABLE 2: Characterization Data for the Samples Listed in Table 1 29Si MAS NMR elemental analysis no. sample ID Q3/% SUA1 SUA1 AVuc Na+/uc TPA+/uc TGA" TPA+/uc 1 [TPA][Si-MFI] 18 ndc nd nd 0.06 4.1 4.5 2 [Na,TPA][Si-MFI] 23 nd nd nd 2.7 3.8 4.2 3 [Na,TPA][Si,AI-MFII-1 11 22 21 4.4 3.1 3.1 3.3 4 [Na,TPA][Si,AI-MFI]-2 8 17 16 5.8 3.5 3.4 3.3 5 [TPAF][Si-MFI]
