Synthesis and characterization of niobium-containing MCM-41

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Synthesis and characterization of niobium-containing MCM-41

Maria Ziolek and Izabela Nowak Faculty of Chemist?, A. Mickiewicz University, Poznan, Poland

A mesoporous niobium-containing silicate of the MCM-41 type was prepared for the first time, according to our knowledge. Structure, mechanical stability, catalytic activity, and cation exchange properties were characterized and compared with those for aluminum-containing MCM-41 materials. 0 Elsevier Science Inc. 1997

Keywords: Nb-MCM-41; synthesis; structure; mechanical stability; catalytic activity

INTRODUCTION

Niobium compounds and niobium materials have recently been shown to enhance catalytic properties when used as a component of catalysts or when small amounts are added to catalysts. Depending on the catalyst composition and structure, they are used in the processes involving acidic active centers as well as redox character. The following reactions carried out on niobium-containing catalysts can be mentioned as exam- ples: dimerization and oligomerization of olefins,’ oxi- dative dehvdrogenation of ethane,2 alkylation of benzene,” dxidation of methanol,“,” NO reduction by NH,,” dehydration and dehydrogenation of alcohols,7,8 or the polycondensation reactions.”

A new family of mesoporous molecular sieves designated as MCX41, which was discovered in the laborato- ries of the Mobil Oil Company, lo. ‘I opened possible new applications of these materials, especially when they are synthesized in the presence of cations other than Si. A lot of elements have been already incorporated in the MCM41 lattice. Some of the metals that substitute Si and were described in the literature are as follows: Al, Ti, V, Mn, Ga, or Fe.“-” Most of these mesoporous solids exhibit interesting redox catalytic properties. We have prepared and characterized niobium<ontaining mesoporous silicates (Nb-MCM41) for the first time, according to our knowledge. Recently, Antonelli and Yang” have described a synthesis of a hexagonally packed mesoporous niobium oxide, NbTMSl.

EXPERIMENTAL

Mesoporous silica (MCM-41) and niobium-containing materials (Nb-MCM-41) were prepared according to the following procedure.

Address reprint requests to Prof. Ziolek at A. Mickiewicz Univer- sity, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, PO- land. Received 21 March 1997

Zeolites 18:356-360, 1997 0 Elsevier Science Inc. 1997 655 Avenue of the Americas, New York, NY 10010

MCM41

Fifty grams of distilled water was combined with 0.3 g of sulfuric acid (95%) with stirring, and 8.08 g of sodium silicate (27% SiO, in 14% NaOH) was added under stirring. Then 83.75 g of a template/water mixture (cetyltrimethylammonium chloride, 25 wt% solution in water) was added after 10 min. The formed gel was stirred for about 0.5 h before 20 g of distilled water was added. The gel was loaded into a stoppered PP bottle and was heated without stirring at 373 K for 24 h. The mixture was then cooled to room temperature, and the pH was adjusted to 11 by dropwise addition of sulfuric acid with vigorous stirring. This reaction mixture was heated again to 373 K for 24 h. This procedure for pH adjustment and subsequent heating was re- peated. The precipitated product was recovered by cen- trifugation, was extensively washed with distilled water, and was dried in air at ambient temperature.

Nb-MCM4 1 A quantity of 8.08 g of sodium silicate (27% SiO, in

14% NaOH) was added to 50 g of distilled water under stirring. A quantity of 83.75 g of a template/water mixture (cetyltrimethylammonium chloride, 25 wt% solution in water) was added after 10 min. Then 12.073 g or 6.037 g of niobium oxalate solution (1.207 g or 0.604 g of niobium oxalate in 10.866 g or 5.433 g of 0.1 LI oxalic acid, depending on the required Si/Nb ratio) was slowly added to achieve Si/Nb ratio of 16 or 32, respectively. The formed gel was stirred and next was prepared according to the procedure described above for silica MCM-41. The only difference was using oxalic acid for pH adjustment in the preparation of Nb-MCM- 41. Nb-MCM-41 with the Si/Nb ratios of 16 and 32 will be designated in this paper as Nb-MCM-41 (16) and Nb-MCM-41 (32), respectively.

The templates of the products: MCM-41 and Nb- MCM-41 were finally calcined at 773 K for 6 h in air or

0144-2449/97/$17.00 PII s0144-2449(97)00027-4

Niobium-containing MCM-41: M. i’iolek and 1. Nowak

Table 1 XRD characterization data of MCM-41 and Nb-MCM-41 materials

30- ---- Nb-MCM-41 (16)as-made

25- - NC-MCM-41 (16) calcined in air

5 20-

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cd I’

71:

0’ / 2 4 6 6 10

20, o

30

25

I I

2 4 6 6 10 20, o

25

2 ‘5 2 w 10

E -

d 5

I I O 2 4 6 6 10

2e, a

Figure 1 XRD patterns of Nb-MCM-41 (16) as-made and calcined followed by water saturation: (a) synthesized without pH adjustment; (b) synthesized with pH adjustment to 11 using oxalic acid; and (c) synthesized by repeating the pH adjustment.

in helium flow. Calcination in air led to a better orga- nized structure.

The characterization of the prepared materials has been accomplished using XRD and TEM techniques as well as the test reactions, the cation exchange, and the mechanical stability measurements. Isopropanol conversion was carried out using a pulse technique and hydrosulfurization of methanol (H,S + CHsOH) in a flow system.

RESULTS AND DISCUSSION

XRD patterns of as-made and air-calcined niobium- containing materials (Si/Nb = 16) are shown in Figure 1. The samples were saturated with water after calcination in air. The relatively well-defined pattern is typical of MCM-41 as described by Kresge et al.‘* The d,,, values and unit cell parameters of the Nb-containing MCM-41 and pure silica MCM41 are listed in Table 1. The low-angle XRD peak almost disappeared upon calcination of the as-made product when Nb-containing samples were synthesized without pH adjustment. The application of the pH adjustment to 11 caused the

Sample

Unit cell parameter d [IO01 (nm) a, (nm)”

As-made Calcined As-made Calcined

MCM-41 b 4.25 3.71 4.90 4.28 MCM-41’ 4.29 3.94 4.95 4.55 Nb-MCM-41 (16)b 4.17 3.16 4.81 3.64 Nb-MCM-41 (16)” 4.24 3.82 4.89 4.41

a Calculated using a, = 2d,,d3”2. *Synthesized without pH adjustment. ’ Synthesized with pH adjustment to 11 using sulfuric or oxalic acid.

increase of the intensity of this peak for the calcined sample. Upon calcination, the XRD [ 1001 lines in Figure 1 shifted to higher 20 values, indicating a significant lattice contraction, which has been reported for MCM-41 materials” and some cations containing materials such as Mn-MCM-41.‘” The calcination in air caused 13 and 8% lattice contraction for MCM-41 and 24 and 10% lattice contraction for Nb-MCM-41 (16) synthesized without and with pH adjustment, respectively, calculated by d,,, spacing (Table 1). This is in agreement with the earlier statement concerning the influence of pH adjustment on the preparation of highly polymerized and ordered materials.*O The XRD line width and intensities of the as-made samples were not inductively affected by the first pH adjustment during the synthesis. The decrease of the XRD signal intensity after calcination of Nb-MCM-41 (16) (Figure 1) is due to the saturation of the material with water. As was recently published*r~** sorbates that fill the pores of

70 - 60 - 4 - - - Nb-MCM-41 (16) as-made

-NWKM-~~ (16) calcined in air

j 50-

2 4 6 6 10

20, o

70 -

60 - - - - Nb-KM-41 (32) as-made

50 - - ~b.~Ct,4-41 (32) calcined in air

i

Figure 2 XRD patterns of Nb-MCM-41 synthesized with pH adjustment; as-made and calcined samples.

Zeolites 18:356-360, 1997 357

Niobium-containing MCM-47: M. Ziolek and I. Nowak

Figure 3 TEM micrographs of niobium-containing MCM-41 (16). A and B show various images. 1 cm = 15 nm.

MCM-41 materials and their nature influence the signal in XRD patterns.

Fipu-e 2 exhibits XRD patterns of Nb-containing mesoporous materials (with various Si/Nb ratios) obtained for samples that were not saturated with water after calcination. The increase of the XRD signal intensity after calcination is visible. Moreover, the intensity is influenced by the niobium content, i.e., the higher niobium conient (Si/Nb = 16) the higher XRD peak intensity. This behavior confirms the incorporation of niobium into the MCM-41 structure.

Transmission electron micrographs of Nb-MCh4-41 ( 16) are shown in F&wz 3. Parts A and B present various TEM images that were interpreted for pure siliceous MCM-41 by some authors2:’ as a view in the direction of pore axis (A) and in the direction perpendicular to the pore axis (B) . However, a recently published TEM mi- crograph of a B-MCM-41 sample”” seems to be similar to that of Nb-MCM-41 material presented in this paper. Thus, the interpretation proposed by the author? can be adapted to the image shown in F@wp 3. The arrangement of different parts of the Nb-MCM-41 sample is incoherent. Therefore, only some part of the material is well enough aligned to the electron beam in such a way that the regular hexagonal arrangement of the pores is visible. The other parts exhibit a pattern of parallel lines due to the resolution in one of the equivalent crystallographic directions.

The mechanical stability of MCM-41 materials was

tested by compression in a steel die of 20 mm diameter, using a hand operated press. Figuw 4 shows XRD patterns for Nb-MCM-41 material calcined in air, uncompressed and compressed using various pressures. The XRD patterns were done after release of pressure. Com- pressing at 50 MPa caused a decrease of the XRD signal intensity of about .SO% and did not change d,,,. The application of 200-MPa pressure led to the significant decrease of the XRD peak and d,,,,, value. Similar experiments carried out on aluminosilicate MCM-41 prepared using the same procedure and aluminum sulfate as a source of aluminum, indicated that -50 MPa pressure applied for compression caused a decrease of the XRD signal intensity of about 75% and a decrease in d 100 value. These results suggest that niobium-containing MCM-41 is more mechanically stable than alurnino- silicate materials. However, both are less stable than pure siliceous MCM-41.2” Similar experiments carried out on crystalline Na,SiO, indicated that a pressure of 200 MPa did not change the crystallinity of the sample (Fig-m 4).

To check whether the hexagonallv packed mesoporous niobium oxide was formed during the synthesis of niobium silicate MCM-41 material, as was recently described in the literature,‘” preparation of the material according to the described procedure using the same conditions with pH adjustment but without sodium silicate was performed. No crystal material was obtained.

358 Zeolites 18:356-360, 1997

Niobium-containing MCM-41: M. Ziolek and 1. Nowak

-Nb-KM-41 (16) uncompressed

25

C

--.-Nb-MCM-41 (16) compressed at 50 MPa

Nb-MM-41 (16) compressed at 200 MPa

20 n

:~, j -NarO: uncompressed ;

-.-.- Na SO compressed at 90 MPa

Na,S,O, compressed at 200 MPa

5 A z t-i 1.5

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s ..__.___.' _.._.__, :: :: :: :: :: . .._.... . . . . . .._. I _._._.__. I:...

g lo- (u E .

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0 10 15 20 25 30

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Figure 4 XRD patterns of Nb-MCM-41 (16) calcined in air and Na,SiO,.

The catalytic properties of calcined Nb-MCM-41 and MC11-41 were examined by carrying out the isopropa- no1 decomposition as a test reaction. The obtained conversion of isopropanol at 623 K on Nb-MCM-41 was high to compare with that for MCM-41 and was com- parable with that observed on aluminosilicate MC-M-41 (Table 2). The main reaction products were propene and diisopropyl ether, indicating the acidic activity of the samples. A small amount of acetone appeared from the third pulse. Hydrogen forms of niobium- and aluminum-containing MCM-41 both showed 100% conversion of isopropanol at 623 K. The application of a lower reaction temperature (573 K) allowed the differentia- tion of the activity of Al- and Nb-containing materials. showing the higher conversion of alcohol on aluminosilicate molecular sieves.

The activity of prepared materials in the hydrosulfurization of methanol was also studied. The main prob-

Table 2 lsopropanol conversion (the first pulse)

Catalyst

lsopropanol Selectivity (%I

conversion Diisoprop. (%) Propene ether Acetone

Table 3 Reaction between methanol and hydrogen sulfide at 623 K (results at the stationary state of the reaction)

Selectivity (%)

Catalyst WI,),0 CH,SH (CH,),S U-&S2

H,S/CH,OH = I:1

H,AI-MCM-41= 62 24 H,Nb-MCM-41 (16) 12 88

H,S/CH,OH = 2:l

H,AI-MCM-41 57 35 8 H,Nb-MCM-41 (16) 11 84 2 3 NaY 15 77 8 H,NaYb 3 60

a -14% selectivity to hydrocarbons. * 37% selectivity to hydrocarbons (see Ref. 27).

lem in this reaction is to obtain a high selectivity to one of the sulfur-containing products: methanethiol or dimethyl sulfide. 2’~‘L7 From the practical point of view there is great interest in the catalytic synthesis of methanethiol. As the results in TublP 3 show H,Nb-MCM-41 (16) exhibits high selectivity to CH,SH even if the H,S/CH,OH molar ratio 1:l is applied. At the same conditions dimethyl ether is the main reaction product on H,Al-MCM-41. In Tabb 3 one can compare these results with those obtained on NaY and H,NaY zeolites. The later one is very active in the dehydration and transformation of methanol, i.e., in the reactions that are competitive to the hydrosulfurization process. Both Y-type zeolites showed a very low selectivity to sulfur organic products when the H,S/CH,OH molar ratio 1:l was used. The sulfur compounds were produced when the excess of HI,S was applied (H,S/CH,OH = 2:l). Taking into account environmental restrictions, it is important not to use an excess of hydrogen sulfide.

The cation exchange in both materials, i.e., MCM-41 and Nb-MCM-41, was conducted using CU(NO,)~ solution and ammonium chloride. ,i\s expected, the cation exchange did not occur in the case of silica MCM-41. The exchange of sodium cations on Cu’+ and NHa+ cations in Nb-MCM-41 has been found, confirming the expected incorporation of niobium into the lattice. Assuming that the whole number of Nb used in the synthesis was introduced into the structure of MCM-41 and that each niobium terahedra is connected with one sodium cation (according to the scheme proposed in Ref. 28), the degree of NH,+ exchange was calculated as -80%.

CONCLUSIONS

MCM-41 Nb-MCM-41 (16) AI-MCM-41 (16) H,Nb-MCM-41 (16) H,AI-MCM-41 (16)

H,Nb-MCM-41 (16) H,AI-MCM-41 (16)

623 K X-ray diffraction and transmission electron microscopy

10 98 1 Traces 37 99 1 Traces

studies both confirm the ordered mesoporous structure

35 99.5 Traces Traces of Nb-MCM-41 powder prepared due to the procedure

100 100 Traces described in this work. This ordered structure can be 100 100 Traces altered by application of compression using an external

573 K pressure of -50 MPa. The niobium-containing

77 98 2 Traces MCM-41 structure is less mechanically stable than the

99 100 pure silica MCM41 but is more stable than silica-alu- mina materials.

Zeolites 18:356-360, 1997 359

Niobium-containing MCM-47: M. Ziolek and I. Nowak

The acidity of Al-MCM41 and Nb-MCM41 is similar. However, the hydrogen form of niobium-containing MCM41 is less acidic than H,Al-MCM-41 and, thanks to that, shows higher selectivity to thiols in the hydrosulfurization of methanol. Nb-MCM-41 materials seem to be better matrices than Al-MCM-41 molecular sieves for the preparation of water-stable solid-base catalysts 7&z

the modification with alkali metal elements.

ACKNOWLEDGMENT

This work was partially supported by Polish State Com- mittee for Scientific Research under grants 3 T09A 099 12 and 3 T09A 098 011.

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Synthesis and characterization of niobium-containing MCM-41