Titanium-containing ZSM-48 is synthesized with silicon to titanium ratios of 26 or larger; changes in unit cell volume and IR data show that titanium is incorporated into framework positions

Davis, Mark E.

Li, Hong-Xin

Serrano, David P.

English

Caltech Authors - Main

J .  C H E M .  S O C . ,  C H E M .  C O M M U N . ,  1992 745 
Synthesis of Titanium-containing ZSM-48 
David P. Serrano, Hong-Xin Li and Mark E. Davis* 
Department of Chemical Engineering, California Institute of Technology, Pasadena, CA 91 125, USA 
Titanium-containing ZSM-48 is synthesized with silicon to titanium ratios of 26 or larger; changes in unit cell volume 
and IR data show that titanium is incorporated into framework positions. 
Titanium silicalite-1 (TS-1)' is a remarkable catalytic material 
capable of promoting the reaction of aqueous hydrogen 
peroxide with a broad spectrum of organic substrates, e.g. 
alkenes, alkanes or ar0matics~2.3 selectively to yield partial 
oxidation products. Currently, Enichem produces hydroquin- 
one and catechol by oxidizing phenol over TS-1 with aqueous 
H202.2 TS-1 is a medium-pore molecular sieve with the 
ZSM-5 topology. Although the synthesis of TS-1 is somewhat 
difficult, several methods can be used (all use tetrapropyl- 
ammonium hydroxide as a structure-directing agent). 194 
Recently, TS-2, the titanium-containing pure-silica ZSM-11 
molecular sieve, has also been synthesized .5,6 ZSM-5 and 
ZSM-11 are structurally very similar. In fact, pure-silica 
ZSM-5 and ZSM-11 can be synthesized using the same 
procedure except that tetrabutylammonium hydroxide is 
substituted for tetrapropylammonium hydroxide when 
synthesizing ZSM- 11. The relationships between the synthesis 
of TS-2 and TS-1 are analogous to those between ZSM-11 and 
ZSM-5. Additionally, the catalytic behaviour of TS-2 is similar 
to that of TS-1 as expected.6-10 
The physicochemical properties of TS-1 and TS-2 that are 
deemed necessary for the catalytic applications of these 
materials include (i) the absence of trivalent ions, e.g. AP+,  
that could lead to the formation of acid sites and (ii) 
framework incorporation of the titanium atoms. For many of 
the proposed applications of TS-1 or TS-2, the presence of 
acidity is detrimental to the ultimate catalyst selectivity.2 
Thus, these materials must be composed of silicon and 
titanium oxides. Framework titanium has been found to be 
critical to the catalytic performance of TS-1 or TS-2 alone. 
Bellussi and Fattorell have reviewd the preparation of 
titanium-containing zeolites. If substitution of titanium for 
silicon occurs the unit cell size of the molecular sieve should 
increase. Also, it has been observed that an IR absorption 
peak at 960 cm-1 is indicative of Ti4+ siting in a silicate 
framework (TS-1 or TS-2) and that the intensity of the band 
can be related to the amount of titanium present.11 Recently, 
Behrens et al.l2 showed from XANES and EXAFS investiga- 
tions of TS-1 that the titanium is octahedrally coordinated. On 
the other hand, Lopez et a1.13 found from EXAFS analyses 
that hydrated TS-1 contains octahedrally coordinated titanium 
and that dehydration transforms the titanium from octahedral 
to tetrahedral coordination. Both investigations conclude that 
titanium is incorporated into framework positions. 
Although TS-1 was synthesized almost a decade ago, 
Bellussi and Fattore11 conclude that there is no clear evidence 
that titanium atoms are inserted into the frameworks of any 
materials other than ZSM-5 (TS-1) and ZSM-11 (TS-2). Here 
we report the synthesis and partial characterization of 
Ti/ZSM-48. (ZSM-48 is a unidimensional, medium-pore, 
high-silica zeolite. 14) Ti/ZSM-48 is the first non-ZSM-5 or 
ZSM-11 pure-silica molecular sieve to be synthesized with 
titanium incorporated into framework positions. 
TS-1 has been synthesized by three different procedures. 
The first involves the use of silicon and titanium alkoxides, 
e.g. example 1 of ref. 1. The second includes a step for the 
formation of peroxytitanates by the reaction of aqueous H202 
with a second aqueous solution containing hydrolysed titan- 
746 J .  CHEM. SOC., CHEM. COMMUN.,  1992 
Table 1 Properties of titanium silicate ZSM-48 
n-Hexane 
Si/Ti Si/Ti adsorbedb Unit cell 
Sample in gel in product" (g/g) volumeC/A3 
ZSM-48 00 Not measured 0.035 2416 
Ti/ZSM-48( 100) 100 66.2 0.054 2426 
Ti/ZSM-48(50) 50 49.5 0.068 2433 
Ti/ZSM-48(30) 30 26.1 0.067 2438 
* Molar ratio obtained from bulk chemical analysis. b Measured with a 
McBain-Bakr balance; samples were calcined in air at 600°C 
(Ti/ZSM-48) and 630°C (ZSM-48); Phexane = 40 Torr; ambient temp. 
c Calculated from X-ray powder diffraction data. 
I 1 
5 10 15 20 25 30 35 40 45 50 
28 I 0 
Fig. 1 X-Ray powder diffraction patterns of (a )  pure-silica ZSM-48 
and ( b )  Ti/ZSM-48(50). Peaks marked are from silicon internal 
standard. Data collected on a Scintag XDS 2000 diffractometer 
equipped with a liquid nitrogen-cooled germanium solid-state detec- 
tor and using Cu-Ka radiation. 
ium alkoxides, e .g .  example 2 of ref. 1. The third method 
involves the wetness impregnation of a coprecipitated, amor- 
phous Ti02-SiO2 with an aqueous solution of tetrapropyl- 
ammonium hydroxide.4 Note that in all cases tetrapropyl- 
ammonium hydroxide is used as a structure-directing agent. 
Additionally, it is critical that no alkali cations be present in 
the synthesis mixtures,7 even at ppm levels. 11 Alkali cations 
are most commonly used to synthesize high-silica zeolites. 
However, ZSM-5 and ZSM-11 are two of the very limited 
number of structures that do not require alkali cations to be 
present in the synthesis mixture for their preparation. Thus, 
extensions from the TS-1 (and TS-2 which is synthesized by 
either methods 1 or 2 listed above) synthesis procedures to the 
crystallization of other titanium silicates are not straightfor- 
ward. 
Pure-silica ZSM-48 can be synthesized in the absence of 
alkali cations.15 Thus, we started our syntheses of new 
titanium silicate molecular sieves by synthesizing titanium- 
containing ZSM-48 (Ti/ZSM-48). A typical procedure is as 
follows. Titanium tetrabutoxide (0.34 g) was added to doubly 
distilled, deionized water (9.2 g) whereupon a white precipi- 
tate (hydrous titanium oxide) was formed. Aqueous H202 
(0.63 g; 30 wt% H202) was added to the stirred slurry and 
stirring continued at room temperature for 2 h. The hydrous 
titanium oxide dissolved on reaction with H202 to form 
soluble peroxytitanates. A second solution was prepared by 
adding of Cab-O-Sil (M5) fumed silica (3 g) to H20 (31.5 8). 
Solution 1 was added dropwise to well-stirred solution 2 and 
upon complete addition blended for an additional 3 h. Finally 
a solution containing diaminooctane (3.1 g) and water (17.5 g) 
a .  1 
Ti I ZSM-48(50) i 
was added dropwise to the foregoing mixture. The complete 
reaction mixture was stirred for 1 h, charged into Teflon-lined 
autoclaves and heated at 175 "C for 10 days. The autoclaves 
were rotated at 50 rev min-1. The product was collected by 
filtration. 
This sample is labelled Ti/ZSM-48(50) (%/Ti = 50). For 
Ti/ZSM-48( 100) and Ti/ZSM-48(30), only the amount of 
titanium tetrabutoxide was changed to give Si/Ti = 100 and 
Si/Ti = 30, respectively. The H202 : Ti ratios used in synthesiz- 
ing TS-1 and Ti/ZSM-48 are approximately 25 : 1 and 5 : 1, 
respectively. Thus, less H202 is employed for synthesizing 
Ti/ZSM-48. If an H202 to Ti ratio of 50 : 1 is used in attempting 
to synthesize Ti/ZSM-48, only an amorphous solid is obtained. 
In the preparation of TS-1, the final mixture is heated at 
70-80 "C for 6-7 h before heating in the autoclave. This step is 
not necessary when synthesizing Ti/ZSM-48. 
Table 1 shows the physicochemical properties of Ti/ZSM-48 
samples and of pure-silica ZSM-48 for comparison. Ti/ZSM- 
48 can be synthesized from gels containing Si/Ti 3 30 and the 
Si/Ti ratio in the product reflects the ratio in the precursor in 
most cases. For high Si/Ti ratios in the gel, the product tends to 
concentrate the titanium. TS-1 is typically synthesized for 
%/Ti = ca. 30. Thus, Ti/ZSM-48 can be synthesized at a bulk 
composition similar to TS-1. Fig. 1 shows the X-ray powder 
diffraction patterns of ZSM-48 and Ti/ZSM-48( 50); no peaks 
other than those ascribable to the ZSM-48 framework are 
observed, Additionally, the peaks for Ti/ZSM-48 are broader 
than those for ZSM-48, most likely owing to particle size 
effects and not a loss of crystallinity (see hexane adsorption 
results in Table 1). The Ti/ZSM-48 crystals are significantly 
smaller than those of pure-silica ZSM-48. As mentioned 
above, if titanium is located within the framework, the unit 
cell size should increase. The complete X-ray powder diffrac- 
tion patterns of ZSM-48 and Ti/ZSM-48 were indexed and the 
unit cell dimensions refined using an orthorhombic cell. The 
unit cell volumes of the samples studied here are listed in 
Table 1 and increase with decreasing Si/Ti. Additional 
evidence for framework siting of Ti is given in Fig. 2. TS-1 and 
Ti/ZSM-48 (all samples) reveal an IR band at around 960 cm-1 
while pure-silica ZSM-5 and ZSM-48 do not. Also, no 
indication of bulk Ti02 is observable by Raman spectroscopy. 
The Raman spectra of TS-1 and Ti/ZSM-48 show only one 
additional peak, at 960 cm-1, in the 1000-400 cm-l region 
compared with the spectra obtained from pure-silica ZSM-5 
and ZSM-48. The adsorption of n-hexane was used to test for 
the presence of intracrystalline amporphous material. The 
adsorption capacity for n-hexane on the Ti/ZSM-48(50) and 
Ti/ZSM-48(30) samples is that which is expected from the 
ZSM-48 structure.14 Thus, we believe that the titanium in the 
Ti/ZSM-48 samples is located within the framework. Note that 
the adsorption capacity of pure-silica ZSM-48 is low, owing to 
J .  C H E M .  S O C . ,  CHEM. COMMUN.,  1992 
the presence of residual carbon. Thermogravimetric analysis 
shows that Ti/ZSM-48 does not contain any mass remaining 
from the decomposition of the organic structure-directing 
agent at 600°C whereas some organic material remains in 
pure-silica ZSM-48 until 800 "C. Additionally the Ti/ZSM-48 
samples are white after calcination at 600 "C and the pure silica 
ZSM-48 is tan after calcination at 630 "C. Ti/ZSM-48( 100) 
most likely has a small amount of residual carbon as well (from 
thermogravimetric analysis) that accounts for the slightly 
lower adsorption capacity when compared to the other 
Ti/ZSM-48 samples. 
Ti/ZSM-48 is the first non-ZSM-5 or ZSM-11 titanium 
silicate to have titanium incorporated into framework posi- 
tions. Since the pore structure of ZSM-48 is significantly 
different from either ZSM-5 or ZSM-11 we expect that 
Ti/ZSM-48 will reveal shape-selective catalytic behaviour 
unlike that observed from TS-1 and TS-2. Catalytic testing of 
Ti/ZSM-48 is in progress. 
We thank Shell Oil Company for support of this work. 
D.  P. S. thanks the Universidad Complutense de Madrid for 
providing a Del Arno grant. 
Received, 5th February 1992; Corn. 2f00636G 
747 
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Synthesis of titanium-containing ZSM-48

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