ELSEVIER

Synthesis of MFI titanosilicates from methylamine-TPABr media

Masashi Shihata Facultks Universitaires de Namur, D@atiement de Chimie, Namur, Belgium

Zelimir Gabelica Universite’ de Haute Alsace, ENSCMu, 3, Rue A. l44rne~, F-68093 M&house, Cedex, France

A new synthesis route for MFI-type titanosilicates (TS-1) using methylamine along with tetra- propylammonium bromide (TPABr), silica or silicon tetrachloride and titanium tetrachloride, thus avoiding the use of costly TPAOH and Ti-alkoxide, has been investigated. The presence of titanium ions in the reaction gel slowed down the crystallization of MFI zeolites, while addition of hydrogen fluoride as a co-mineralizer into the gel dramatically improved the crystallization kinetics. Ti was readily incorporated into the zeolite framework, as confirmed by IR, XRD (unit cell expansion), and UV-vis spectroscopy. Reduced initial amounts of methylamine in the gel led to an increase of the apparent crystallization rate of the zeolite and to a remarkable decrease of the amount of framework Ti, suggesting that high concentration of methylamine is needed for the Ti incorporation into the growing MFI crystallites. 0 Elsevier Science Inc. 1997

Keywords: Titanosilicate; TS-1; synthesis; methylamine; hydrogen fluoride; TPABr; silica; titanium tetrachloride

INTRODUCTION

MFI-type titanosilicate materials, also designated TS-1, have received considerable interest since their first syn- thesis was reported in 1983.’ Such materials show ex- cellent performances in the epoxidation and hydroxy- lation of olefins with hydrogen peroxide.2s

As the presence of Na+ cations in the precursor gel favors the formation of extra-framework titanium spe- cies, microporous titanosilicates have always been syn- thesized in alkali-free conditions.’ In the original patent,’ tetraethyl orthotitanate was used as the Ti source, and a large quantity of TPAOH was necessary to bring the pH of the reaction gel to basic values needed for rapid crystallization. Kumar et al. suggested the use of tetrabutyl orthotitanate and acetylacetone that readily form stable complexes with titanium ions, to prepare well-crystalline TS-1 materials involving high Ti concentrations4 However, the major drawback of all these conventional syntheses still consists in using costly alkali-free TPAOH templates.

To avoid using TPAOH, Miiller et a1.j proposed the use of ammonia to fix the pH to basic values favorable for zeolite syntheses. They reported that TS-1 crystals were obtained from the mixture of titanium tetraiso-

Address reprint requests to Dr. Shibata at KAO Corp., Tokyo Research Laboratories, l-3, Burka P-Chome, Sumidaku, Tokyo 131, Japan. Received 10 March 1997; revised 2 June 1997; accepted 5 June 1997

Zeolites 19:246-252, 1997 0 Elsevier Science Inc. 1997 655 Avenue of the Americas, New York, NY 10010

propoxide, alkali-free TPAbr, H,O,, ammonia and wa- ter; however, the actual role of ammonia in such TS-1 syntheses was not investigated more in depth.

The synthesis method for MFI zeolites in neutral or acidic media using fluoride ions as mineralizing agents along with alkali-free TPABr6 has been successfully ap- plied for TS-1 syntheses’-“; however, the catalytic per- formancles of such materials were reported to be infe- rior to those prepared in basic media with TPAOH.” It is possibly because of the relative small framework Ti content and/or the presence of extra-framework by- products, (TiO,) .

Recently, we reported that methylamine had a poten- tial mobilizing and complexing ability toward not only silica species but also many metallic ions such as boron, aluminum, or galliun~.‘O-ls For example, alkali-free MFI alurninosilicate, gallosilicate, and alumino-gallo- bimetallosilicate could be synthesized using both alkali- free TPABr as a template and methylamine as a miner- alizing agent. In these syntheses, methylamine was shown tc) form medium-strong complexes with both Al’+ and Gas+, and thus proved an excellent mobiliz- ing agent toward these species. In addition, a slight excess of methylamine was used to bring the final pH of mixture to basic values, which does not require the use of TPAOH. lo

The purpose of this work was to examine the poten- tial of methylamine to mobilize titanium species for TS-1 synthesis in alkali-free media without using TPAOH. The possibility of using SiCl, as a Si source,

0144.-2449/97/$17.00 PI SOl44-2:449(97)00078-X

Synthesis of MN titanosilicates: M. Shibata and Z. Gabelica

which formed Si-O-B gel complexes more readily than SiO, in MFI borosilicate synthesis,13 was also examined.

EXPERIMENTAL

Synthesis of MFI-titanosilicates

In all the syntheses, alkali-free tetrapropylammonium bromide (TPABr, from Janssen Chimica) was used as a template for the MFI structure. Titanium tetrachloride (Merck) was the main titanium source. Either solid silica (Aerosil 200 from Degussa) or silicon tetrachlo- ride (Merck) was used as a Si source. Methylamine (40% aq. solution from Fluka) was the mineralizer (mcr bilizing). Some other ingredients (hydrogen fluoride, hydrogen chloride, oxalic acid, acetylacetone, or hydro- gen peroxide) known to make complexes with titanium were optionally added to the system as co-mineralizers. Four different procedures were used to prepare the reaction gels.

Procedure I: Preparation of TS-1 from SiO, and TX& TiCl, was added to an aqueous solution of TPABr

pre-cooled at 5°C. A complexing agent (hydrogen flu- oride, hydrogen chloride, or oxalic acid) was optionally added to that solution, then solid SiO, was added. The final mixture was stirred for at least 3 11 at 20°C. The resulting homogeneous gel was admixed with methyl- amine, stirred for 1 h, and then transferred to a Teflon autoclave. The gel molar composition was:

(96x)SiO, - x TiCI_, - y CHsNH,

- z complexing agent - 25 TPABr

- 3500 H,O

Procedure II: Preparation of TS-1 from SiO, a,nd Ti- acac complex

SiO, was added to the aqueous solution containing TPABr and Ti acetylacetonate (Ti-acac from Merck). The gel was stirred for 1 hr then admixed with meth- ylamine and stirred for another 1 h before being trans- ferred to the autoclave. The gel preparation was en- tirely achieved at about 20°C. The gel molar compo- sition was:

92 SiO,, - 4(Ti-acac) - 300 CHsNHs

- 25 TPABr - 3500 H,O

Procedure III: Preparation of TS-1 from SiO, and Tit& in the presence of H202

TiCl, was added to an aqueous solution of H,O, pre-cooled at 5°C. The resulting pale-red solution was added to the cold methylamine solution until the for- mation of a pale-yellow suspension. The latter was then admixed with SiO, and stirred for 1 h at 20°C before being transferred to the autoclave. The gel molar com- position was:

92 SiOs - 4 TiCI4 - 300 CHSNHS - 4 H,O,

- 25 TPABr - 3500 H,O

Procedure IV.. Preparation of TS-1 from SiCl, and TiCl,

SiCl, was added to an aqueous solution containing TPABr and, optionally, hydrogen fluoride pre-cooled at 5°C. The resulting gel was also cooled to 5°C before the addition of TIC],, then admixed with methylamine and stirred for another 1 h before being transferred to the autoclave. The gel molar composition was:

92 Sic& - 4 Tic& - 600 CHsNH, - x HF

- 50 TPABr - 3500 H,O

The autoclaves were heated at 185’C for various pe- riods of time under static conditions. The final solids were filtered, washed thoroughly with cold water, and dried at 100°C. The crystals were further submitted to an ultrasonic cleaning to remove the unreacted gel from products not being 100% crystalline, prior to IR, EDX, UV-vis, or XPS measurements.

All the products were calcined under nitrogen (heat- ing rate: lO”C/min) from 20°C to 550°C; the samples freed from TPA+ were maintained at 550°C for 6 h in flowing air to burn off the residual coke stemming from the nonoxidative TPA thermal decomposition.

Characterization All products were checked for their nature and purity

by SEM (Philips XL 20 microscope). Spot Energy Dis- persive X-ray analyses (EDX) of Si and Ti on selective areas (core and edge) across the individual crystallite were performed using an EDAX P.V. 9800 Phillips an- alyzer coupled with SEM. TS-1 samples with known Si/Ti ratios were used as standards. X-ray powder dif- fraction (XRD) patterns were recorded on a Phillips P.W. 1349/30 diffractometer (Cu-Kcw radiation) using a-alumina as internal standard. ET-IR spectra were ob- tained using a Bio-Rad FTS-6OA spectrometer with KBr pellets containing 1 wt% sample. The UV-vis spectra were recorded on Shimadzu UV-3100PC spectrometer. XPS (X-ray photoelectron spectroscopy) spectra were recorded on a JEOL JPSSO instrument using an Mg Kol X-ray source. The binding energy of 133.3 eV for Si(2p) was chosen as an internal reference. Chemical analyses of the products were carried out by inductively coupled plasma spectroscopy (ICP) on a Shimadzu ICPS-1OOOOIV instrument.

RESULTS AND DISCUSSION

Crystallization behavior Fi,pre 1 shows the crystallization curves of TS-1 and

silicalite-1 (metal-free MFI silicate) samples synthesized from methylamine-TPABr media, as coded in Tabb 1. The crystallinity of products was estimated by compar- ing the intensities of eight characteristic diffraction peaks of MFI zeolites. Pure silicalite-1 crystals were ob- tained after 2-d heating at 18O”C,‘” and this sample was arbitrarily considered as being 100% crystalline.

In contrast to the crystallization of silicalite-1, no crystalline phase was detected in the reaction gel after 14-d heating in the presence of TiCl, (sample 1). This shows that, as in the case of Al”+ and Gas’ ions,iO the

Zeolites 19:246-252, 1997 247

Synthesis of off tjtanosiiicates: M. S~j4ata and 2. Gabelica

80- ‘;i ?L .G GO- r .- = a

6 4o

2 4 6 8 10 12

Reaction Time (days)

J

Figure 1 Crystallization curves of TS-7 and silicate-l in meth- ylamine-TPABr media. (0) Sample ?, 2, 7, 9 and 10; (A) sample 4; (A) sample 6; (El) sample 8; (mf sample 11; (0) silicalite-1.

presence of Ti4+ impedes the growth of MFI zeolites. The absence of any XRD peaks corresponding to TiO, (anatase) indicates that the amorphous phase is still composed of stable Ti-0-Si oligomers.14 Indeed, we have checked that CHsNH, and TiCl, admixtures heated under similar conditions but without silica yielded pure anatase.

The effect of some potential Ti-complexing agents on the crystallization of TSl in methylamine-TPABr was examined. Adding either HCl, oxalic acid, or H,O, did not improve the crystallization kinetics, the reac- tion mixtures being still amorphous after at least 10-d heating (samples $7, and 9). When Ti-acac was used as the Ti source, low-crystalline (32%) MFI zeolite was obtained (sample 8).

In contrast, the presence of hydrogen fluoricle in the reaction gels markedly increased the kinetics of crystal- lization. A highly crystalline (84%) MFI product was obtained after 10-d heating (sample 4). Fluoride ions are excellent solubilizing agents toward silica, and sili- calite-1 can be synthesized using HF or NH,F even in neutral or acidic media.” Fluoride ions probably in-

Table 1 TS-1 sample synthesized in CH,NH,-TPABr media

crease the solubility of both the Si species and the Ti species and, hence, improve the crystallization kinetics of TS-1. Conversely, in methylamine alone, the stable “K-O-Si oligomeric species may not readily transform into crystalline TS-1, in contrast to Ti-free silicate spe- cies. Indeed, we have demonstrated elsewhere%* that once such T&-O--Si oligomeric species are formed in methylamine media but in the absence of fluoride ions, TS1 crystals were difficult to grow even in the presence of seed crystals.

SiCl, was expected to co-hydrolyze more readily along with TiCl, and make Si-O-Ti oligomers than SiO,, as shown in the case of the borosilicate synthe- sis.13 The reaction gel in which both SiCl, and TiCl, were co-hydrolyzed did not form any MFI crystals after 14-d heating (sample lo), in contrast to the borosilicate synthesis in which 100% crystallinity was attained in 5 d under the identical conditions.13 Adding HF again ac- celerates the crystallization (sample ll), as in the case of the synthesis from SiO, (sample 4).

The Ti concentration in the reaction gel also drasti- cally affects the crystallization kinetics, fiere 2 shows the crystallization curves for various Ti-containing gels in the presence of hydrogen fluoride. Clearly, the larger the titanium content in the gel, th:e slower the crystallization, indicating that Ti4’- ions tend to impede the nucleation of TS1. Under these synthesis condi- tions, the c~~llini~ of high Ti4+-containing sample (4 or 6 Ti/96 T atoms; T = Si + Ti) did not reach 100% crystallinity after 10-d heating. SEM and E<DX analyses showed that appreciable amounts of unreacted gel remained around the TS-1 crystals, and a higher Ti concentration was detected in the gel (about 5 Ti/96 T in the case of sample 4) than in the crystals (about 2 Ti/96 T).

The overall methylamine quantity in the reaction gel also affected the crystallization. When the methylamine content was reduced from 300 mo1/96 T to 100 (sam- ples 4 and 6, respectively), the pH value of the reaction gel was d.ecreased from 12.5 to 11.0. The crystallization was dramatically accelerated; 90% crystallinity was at- rained in 3 d in the case of sample 6, compared to 10% after 4 d for sample 4. As shown below such reduction of the methylamine quantity is not necessalrily desirable

Sample name (number) Procedure

1 I 2 I 3 I 4 I 5 I 6 I 7 I 8 II 9 III 10 IV 11 IV

OX: oxalic acid; acac: acetylacetonate.

CH,NH, Complexing (mot) agent (mol)

300 - 300 300 HCI 300 100 l-IF 300 100 I-IF 300 100 HF 100 100 HF 300 20 ox 300 (acac) 300 4.0 H,O, 600 - 600 50 HF

Si source

sio SiOz SiO, SiO, SiO, SiO SiOz SiO, SiOz SiCI, SiCI,

Ti source (mol)

4.0 TiCI, 4.0 TiCI, 2.0 TiCI, 4.0 TiCI, 6.0 TiCI, 4.0 TiCI, 4.0 TiCI, 4.0 Ti-acac 4.0 TiCI, 4.0 TiCI, 4.0 TiCI,

248 Zeolites 1!3:246-252, 1997

Synthesis of MFI titanosilicates: M. Shibata and 2. Gabetica

80 .z? C

z yj 60

40

2 4 6 8 10

Reaction Time (days)

Figure2 Crystallization curves of TS-1 grown from the reaction gels containing variable Ti concentration. (A) Silicalite-I (0 TV96 7); (Cl) sample 3 (2 Ti/% T); (A,) sample 4 (4 Ti196 T); (0) sample 5 (6 TV96 T).

for ?-S-l crystallization, because it leads to the straigbt- fonvard formation of silicalite-1 crystals (not TS-1).

Characterization of titanosilicates

T&de 2 describes some physiochemical properties of MFI crystals synthesized in methylamine-TPABr me- dia. All the crystallites exhibited a homogeneous size and shape (Qure J), as often observed in various MFI m~tallosilicates obtained under the identical condi- tions.“‘-‘9

TSl crystals synthesized in the presence of hydrogen fluoride as a co-mineralizer (samples 3, 4, and 5) ex- hibited a prism-like morphology (Z+$~W 3); the overall crystallite size decreased as the amount of titanium in the gel increased. Although samples 4 and 5 contained unreacted gel around the crystals, the gel phase could be easily separated and removed from the crystals through the ultrasonic cleaning.

F‘~Pw 4 shows the IR spectra of some products. Sam- ple 4, which was TSl synthesized in the presence of HF, showed a relatively well-resolved LR band at 960 ctC ’ that is not detected in silicafite-I synthesized under the same conditions. This IR band has been a good proof (fingerprint) of the existence of framework metallic ions, and particularly of framework Ti in TS1.l.’

The unit cell volume that was calculated from XIU) data provides important information on the metallic ions existing in the metallosilciate framework. An in- corporation of M I’+ ion into the zeolite framework would cause a unit cell volume variation. It would ex- pandicont~act depending on the relative size of the M “+ with respect to Si”+. The Ti-0 bond being longer than Si-0, the unit cell volume of TS-1 would be larger than that of silicalite-1.”

The unit cell volumes of a series of samples, as calculated from XRD data, are presented in T/;a& 2. Silicalite-1 synthesized from methylamine-TPABr media had a unit cell volume nearly identical to the one synthesized in basic media.’ The unit cell volume

Figure 3 Scanning electron mi~rographs of (A) sample 4; (Bf sample 6; and (Cl sample 11.

of sample 4 showed a marked increase (5368 A”) with respect to that. of silicalite-1 (5339 A”). Samples 3 and 5, which crystallized from a different Ti-contain- ing gel under the same synthesis conditions with sam- ple 4, also showed the expansion of the unit cell volnme. These results strongly support the IR data,

Zeoiites 19246-252, 1997 249

Synthesis of MN titanosilicates: M. Shibata and Z. Gabelica

Table 2 Physiocochemical properties of some MFI zeolites prepared from CH,NH,-TPABr media

Initial Ti Average Sample name content Reaction crystal size

Ti content (atom/96T) Unit cell Framework Til

(number) (atom/96 T) time (d) (pm) Symmetry” volume (A3) Total (ICP) Frameworkb total Ti ratio

3 2.0 6 20 x 8 0 5362 1.7 1.3 0.76 4 4.0 10 16 x 8 0 5368 2.1 1.6 0.76 5 6.0 10 16 x 8 0 5370 4.9 1.7 0.35 6 4.0 3 60 x 12 M 5340 2.0 0.1 0.05 11 4.0 10 60x 16 M 5342 1.0 0.2 0.20 silicalite-I 0.0 2 48 x 8 M 5339 - - -

aM: monoclinic; 0: orthorhombic. bCalculated from the unit cell volume.

which suggested that titanium ions were indeed incorporated into the framework of TS-1 synthe- sized in methylamine-TPABr media in the presence of HF.

A relationship between the unit cell volume of TSl and the quantity of framework Ti has been proposed.‘” Using this relation, we have evaluated that the quanti- ties of the framework Ti of samples 4 and 5, which, respectively, involve 4 Ti/96 T and 6 Ti/96 T in the initial gels, were very close. This suggests that the amount of lattice titanium seems to saturate to a value

I I I I I 1

1400 1200 1000 800 600 400

Wave number (cm-l)

250 300 350’ 400

Wave length (nm)

Figure 4 IR spectra of (a) silicalite-1; (b) sample 4; (c) sample 6; Figure 5 Diffuse-reflectance UV-vis spectra of (a) sample 3 and and (d) sample 11. (b) sampl,e 5.

corresponding to 1.7 Ti/96 T under these synthesis conditions. This value is actually smaller than the one obtained. using the conventional method involving Ti and Si alkoxides and TPAOH (namely 2.5 Ti/96 T in the original patent).’

Although samples 4 and 5 were estimated to contain almost the same amount of framework Ti (calculated from the unit cell volume), ICP data showed that larger amount of Ti existed in sample 5 than in sample 4 (Table 2). This suggests that some extra-framework tita- nium could exist in sample 5.

Diffusle-reflectance UV-vis is an efficient technique to evaluate qualitatively the presence omf framework and extra-framework Ti in TS-1. As shown in Figure 5, samples 3 and 5 showed a neat LN band at about 230 nm, corresponding to isolated framework titanium in tetrahedral coordination.17 The presence of this band could be considered as another proof for Ti incorpo- ration into the zeolite framework. Sample 5 showed, in addition to the 230-nm band, another band at 300 to 350 nm that was not detected in sample 3. This

250 Zeolites 19:246-252, 1997

Table 3 Surface Ti quantities of some TS-1 synthesized in the presence of HF

Sample name Total Ti content (EDX) Framework Ti/total (number) (atom196 T) Ti ratio (XPS)

3 1.0 0.77 4 2.0 0.75 5 2.2 0.41

broad band has been attributed to anatase.” This suggests that any excess of Ti in the reaction gel, with respect to the amount likely to be incorporated in TS-1 framework, would yield some Ti-bearing extra- framework phase.

The state (coordination) of titanium on the surface of crystals was semiquantitatively examined by XPS. The Ti 2ps,, peak was separated into two peaks (mixture of Lorentzian and Gaussian curves), centered at BE values of 459.8 ? 0.2 eV and 458.3 t 0.2 eV, respectively. The former peak has been assigned to framework Ti in tetra- hedral coordination. The latter has been attributed to an extra-framework Ti phase. l8 The ratio of framework Ti to the total Ti (framework + extra-framework) on the sur- face of TS1, calculated from the intensity of those two peaks, are reported in l’ubk 3. The ratios of samples 3 and 4 were almost equal (0.77 and 0.75, respectively) and that of sample 5 was fairly lower (0.41). These findings confirmed the results of UV-vis, ICP, and XRD. Accord- ingly, it was concluded that the maximum value of frame- work Ti was about 1.7 Ti/96 T and that a too large amount of titanium (probably more than 4 Ti/96 T) ‘in the initial gel leads to the formation of the extra-frame- work Ti.

Sample 6, whose initial methylamine quantity was l/3 that of sample 4, did not clearly show the IR band at 960 cm-’ (Figure 4). Moreover, its unit cell volume was almost the same as that of silicalite-1, suggesting that the amount of framework Ti was very small (if any). These findings indicate that Ti atoms are diffi- cult to incorporate into the zeolite framework when the methylamine quantity is not sufficient. Note that the initial quantities of HF in samples 6 and 4 were the same. It was, therefore, considered that HF may not play a dominant role for the Ti incorporation, al- though it was shown to accelerate the crystal growth (Figure 1). It can be assumed that the respective “mo- bilizing (mineralizing) )I roles of methylamine and flu- oride ions seem fundamentally different. The former probably plays a major role in forming the appropri- ate Si-0-Ti oligomeric species needed for the ‘Ti incorporation into the growing crystallite. The latter would solubilize them to accelerate the crystallization. These crystallization mechanisms will be discussed more in depth elsewhere.14

Sample 11, which was synthesized from SiCl, in the presence of HF, had a large needle-like shape similar to that of silicalite-1 prepared from SiO,, and the untreated gel phase was also observed on the surface of the crystals (@me 3). In contrast to sample

Synthesis of MN titanosiiicates: M. Shibata and Z. Gabeiica

4, this gel could not be removed even by ultrasonic washing.

Sarnple 11 showed the IR band at 960 cm-’ (Fipre 4); however, its intensity was weaker than in sample 4, suggesting that only a small amount of Ti was incorpo- rated into the framework. Indeed, from the unit cell volume increase, this amount was estimated to be about 0.2 Ti/96 T.

It was considered that the synthesis involving SiCl, leads to the formation of an inactive intermediate (such as p-titanic acid or highly polymerized Ti-0-Si spe- cies), which are not readily “solubilized” or “com- plexed” by methylamine through the hydrothermal treatment. This suggests that SiCl, is not an appropriate Si source for TS-1 synthesis, in contrast to t.he borosili- cate synthesis.‘”

CONCLUSION

TS-1 was synthesized by a new procedure using meth- ylamine and TPABr. Silica and titanium tetrachloride could be used as the Si source and the Ti source, respectively. The advantage of this procedure over the conventional method is that high-crystalline TS-1 can be obtained without using costly TPAOH, Si-alkoxide, and Ti-alkoxide.

Although the presence of Ti ions in the reaction gel slowed down the crystallization of MFI zeolites, the crystallization rate could be increased by further addi- tion of F- ions as a co-mineralizer.

In the case of the samples synthesized in the dual HF-methylomine media, IR and UV-vis, along with the lattice constant expansion, confirmed the incorpora- tion of titanium into the MFI framework.

The use of SiCl,, which efficiently forms StO-B gel complex in borosilicate synthesis, leads to a slow crys- tallization and low-framework Ti incorporation, sug- gesting that SiCl, is not an appropriate Si source for TS-1 synthesis.

Decreasing the quantity of methylamine in the gel improved the apparent crystallization rate; however, the amount of lattice titaniurn in the crystals dramati- cally decreased. These findings indicated that a larger quantity of methylamine is required to generate the appropriate Ti-0-Si oligomers that would potentially form the TS-1 framework; when the amount of meth- ylamine is not sufficient, only silicalite-1 would crystal- lize.

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Synthesis of MN titanosilicates: M. Shibata and Z. Gabelica

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