Effect of using polyvinyl alcohol and polyvinyl pyrrolidone in the synthesis of octahedral molecular sieves

Microporous and Mesoporous Materials 63 (2003) 11–20 www.elsevier.com/locate/micromeso

Effect of using polyvinyl alcohol and polyvinyl pyrrolidone in the synthesis of octahedral molecular sieves L.J. Garces a

a,*

, B. Hincapie a, V.D. Makwana b, K. Laubernds b, A. Sacco c, S.L. Suib a,b,d,*

Institute of Materials Science, University of Connecticut, 55 N. Eagleville Rd. U-3060, Storrs, CT 06269-3060, USA b Department of Chemistry, U-60, University of Connecticut, Storrs, CT 06269-3060, USA c Department of Chemical Engineering, Northeastern University, 342 Snell Engineering, Boston, MA 02115, USA d Department of Chemical Engineering, University of Connecticut, Storrs, CT 06269-3060, USA Received 21 April 2003; received in revised form 13 May 2003; accepted 22 May 2003

Abstract Octahedral molecular sieves (OMS-2) have been reported as catalysts for oxidation reactions. Interest exists in improving the properties of these materials. Effects of using polyvinyl alcohol (PVA) and polyvinyl pyrrolidone (PVP) in the synthesis of OMS-2 have been studied. The structure of OMS-2 was kept when PVA or PVP were used as indicated by XRD and Fourier transform infrared spectra data. The use of PVA or PVP promoted an increase in surface area (measured by Brunauer–Emmett–Teller) and a decrease in particle size (measured by XRD). Besides increasing surface area and decreasing particle size of OMS-2, PVA and PVP were useful to improve the film hardness of OMS-2 samples applied on glass surfaces as measured by the pencil hardness test, and Knoop microhardness test. Film hardness is an important property for possible applications of OMS-2 materials in continuous flow reactor systems. By using PVA or PVP as non-chelating agents, an increase in surface area from 59 to 114 (m2 /g), a decrease in particle size, from 29.8 to 12.1 nm, and a hardness value of 4H using the pencil hardness test, and 17.73 H K by Knoops micro hardness tests for OMS-2 prepared with PVA were observed. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Molecular sieves; Film hardness; OMS-2; Polyvinyl alcohol; Polyvinyl pyrrolidone

1. Introduction *

Corresponding authors. Address: Institute of Materials Science, University of Connecticut, 55 N. Eagleville Rd. U3060, Storrs, CT 06269-3060, USA. Tel.: +1-860-486-6565; fax: +1-860-486-2981 (L.J. Garces), Tel.: +1-860-486-2797; fax: +1860-486-2981 (S.L. Suib). E-mail addresses: [email protected] (L.J. Garces), [email protected] (S.L. Suib).

Octahedral molecular sieve (OMS-2) materials consist of edge-and corner-shared octahedral forming a tunnel structure [1]. The structure of OMS-2 is formed by the bonding of two octahedral on each side of a tunnel [1]. The interest in these materials is due to their potential use as

1387-1811/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S1387-1811(03)00424-4

12

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

catalysts in industrial processes such as the low temperature oxidation of carbon monoxide [2] as well as its possible use as cathodes in batteries [3]. Other reports have also suggested the possible use of these materials as sensors [4]. Recently, K-OMS-2 and H-OMS-2 have been reported as catalysts for the oxidation of alcohols into carbonyl compounds [5]. High surface area is desirable for catalysts. The rate of a reaction taking place on the surface area of a catalyst increases proportionally to the surface of the catalyst. Porous materials have high surface areas. Another method to increase the surface area is to use very small particles. This option is considered impractical because fine particles may cause a very high-pressure drop across a fixed bed reactor, or the particles can be mixed with the product and may be difficult to separate them [6]. These problems may be overcome if hard films with good adhesion to surfaces like ceramics or glass can be prepared with suspensions of the particles. Extensive research has been done regarding the synthesis and properties of cryptomelane also known as OMS-2 materials [1,3,4,7,8]. The methods reported to synthesize OMS-2 materials cover a broad range of synthetic procedures such as sol–gel processes [3], microwave heating [7], and a reflux method [9], which was also used to prepare the samples studied in this work. However, there are no previous reports on the preparation and characterization of hard OMS-2 films. Polyvinyl alcohol (PVA) has been previously used in a sol–gel preparation of LiMn2 O4 materials [10]. The role that has been assigned to PVA in these syntheses is to act as a colloidal protector or non-chelating agent useful to cover the cations in order to avoid precipitation [11,12]. We report here the synthesis of OMS-2 in the presence of PVA and polyvinyl pyrrolidone (PVP) (OMS-2 PVA and OMS-2 PVP, respectively), and the effect of the polymers on the types of phases formed, crystallinity, particle sizes, and surface areas of the synthesized OMS-2. Films of OMS-2 PVA and OMS-2 PVP were prepared and their hardness were evaluated and compared against films of standard OMS-2.

2. Experimental 2.1. Sample preparation Standard OMS-2 were prepared following a method described in Ref. [9]. According to this procedure a solution (A) was prepared by dissolving 11.00 g manganese acetate (MnAc2
4H2 O) in 40 ml double de-ionized water (DDW). A solution containing 5 ml of acetic acid plus 5.00 g potassium acetate plus 40 ml DDW was added to solution (A). 6.50 g KMnO4 were dissolved in 150 ml DDW and added drop wise to this resulting solution. The resulting solution was refluxed for 24 h with continuous stirring. Then the product obtained after reflux was filtered and washed several times with DDW. The OMS-2 prepared this way is called OMS-2 blank. To prepare OMS-2 PVA, solution (A) was prepared by dissolving different amounts of (PVA––100% hydrolyzed, MW: 115 000) in 40 ml DDW. After the dissolution of the PVA, 11.00 g of MnAc2
4H2 O were added to this solution. The procedure continues in the same way as the preparation procedure for sample labeled OMS-2 blank. The product obtained with this procedure is called OMS-2 PVA (when 2 g of PVA was used) or OMS-2 PVA-1 (1 g of PVA). The wet brown material obtained was dried at 80 °C for 15 h to remove water and then dried at 120 °C for 2 h. The same preparation method was done using (PVP, MW 10 000) instead of PVA. This sample was labeled OMS-2 PVP (2 g PVP) or OMS-2 PVP-1 (1 g PVP). The samples were calcined after preparation at 400 °C for 5 h. Films of OMS-2 blank, OMS-2 PVA and OMS2 PVP were prepared using the wet brown product obtained after washing. Wet OMS-2 material was applied over a glass slide using a metallic applicator that allows formation of films with the same thickness which were then let to dry at room temperature for 2 days. After this time the film hardness was measured. The films were calcined at 400 °C for 5 h. 2.2. Characterization of samples The XRD studies were carried out with a Scintag XDS-200 powder diffractometer on finely

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

powdered samples using Cu(Ka) radiation and 45 kV and 40 mA. The step size was 0.02° (2h) per minute with a step scan rate of 10 s/step. The XRD patterns were recorded for 2hÕs between 5° and 70° and the phases were identified using a JCPDS database. The particle size of the prepared materials was determined by using the Scherrer equation. The particle size reported is the average of the values obtained by applying ScherrerÕs equation to lines at 2h: 49.8 (411) and 60.2 (521). The instrumental line broadening was measured using a LaB6 standard. Field emission scanning electron microscopy (FESEM) experiments for the determination of particle size and morphology were performed on a Zeiss DSM 982 Gemini FESEM with a Schottky Emitter at an accelerating voltage of 2 kV with a beam current of about 1 lA. Fiber length distribution of some samples was measured by transmission electron microscopy (TEM). Samples for analysis by TEM were ground with a mortar and pestle and sonicated in n-butanol prior to dropping on a 300 mesh holey carbon coated copper grid (SPI). TEM measurements were performed on a JEOL 2010 FasTEM 200 kV instrument equipped with an EDAX energy dispersive spectrometer and a Gatan EELS/GIF system. The performance of OMS-2 materials as catalysts may be affected by their average oxidation state and their carbon content. The carbon content of prepared samples was determined with a CHN element analyzer Perkin–Elmer model 2400. The K/Mn molar ratio was determined by ICP, the percentage of water in the samples was determined by TGA (951 TGA DuPont instruments), and the average oxidation state (AOS) was calculated following the method described in Ref. [13]. Surface area (BET) was measured on a Micromeritics ASAP 2010 equipped with an ASAP 2010 V4 software. The sample (0.1 g) was loaded into the glass sample bulb, and degassed at 150 °C for 4 h. The surface areas of the samples were obtained using the Brunauer–Emmett–Teller (BET) model. Fourier transform infrared spectra (FTIR) experiments were done on a Nicolet Magna IR system 750 FT-IR spectrometer with a resolution of 4 cm1 . Potassium bromide (KBr) pellets of the

13

samples were analyzed in the range 4000–400 cm1 . Method ASTM D 3363-92a was used to measure film hardness. Pencils of known hardness (6H, 5H, 4H, 3H, 2H, H, F, HB, B, 2B, 3B, 4B, 5B, 6B, where 6H is the hardest and 6B is the softest) were used to measure film hardness. The value for hardness reported corresponds to the hardest pencil that leaves the film uncut or unscratched for a stroke length of at least 3 mm. Knoop microhardness of the films was also measured using a LECO DM-400 FT hardness tester, equipped with an optical microscope (objective lens magnification 60), and calibrated with standard lead. The load used in the analysis of OMS-2 films was 10 gf.

3. Results 3.1. Properties of prepared OMS-2 materials The X-ray diffraction patterns of samples prepared before calcination are presented in Fig. 1. All the samples show the same crystalline phase, which corresponds to OMS-2 (cryptomelane). No additional phases are detected. Differences in the widths and intensities of the OMS-2 peaks were observed. The X-ray pattern of OMS-2 PVA has low intensity and very broad peaks. The sample OMS-2 blank has the highest intensity and narrow peaks. As the amount of PVA (sample OMS-2 PVA-1), is decreased, the intensity of the peaks increases. Samples prepared with PVP also present some peak broadening, associated with a decrease in particle size [14]. In Fig. 2, the X-ray diffraction patterns for calcined samples are presented. OMS2 (cryptomelane) is the only crystalline phase detected. The width of the peaks decreases and the intensity of the peaks for all the samples increases after calcination. FTIR spectra of OMS-2 samples prepared with and without polymer are shown in Fig. 3. All the samples present the vibrations typical of OMS-2 [15] in the region from 800–400 cm1 . For samples prepared with PVA or PVP additional low intensity bands around 1600 cm1 were present (next to the band characteristic for water there are two other bands, at 1560 and 1420 cm1 ). These bands

14

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

Fig. 3. FTIR spectra: (a) OMS-2 blank, (b) OMS-2 PVP, (c) OMS-2 PVP-1, (d) OMS-2 PVA, (e) OMS-2 PVA-1. Fig. 1. X-ray diffraction patterns of samples after reflux: (1) OMS-2 blank, (2) OMS-2 PVA, (3) OMS-2 PVA-1, (4) OMS-2 PVP, (5) OMS-2 PVP-1.

Fig. 2. X-ray diffraction patterns of samples after calcination at 400 °C, 5 h: (1c) OMS-2 blank calcined, (2c) OMS-2 PVA calcined, (3c) OMS-2 PVA-1 calcined, (4c) OMS-2 PVP calcined, (5c) OMS-2 PVP-1.

may be due to the presence of residual organic material. Table 1 presents the results of carbon content (wt.%) and average oxidation state. The sample labeled OMS-2 PVA has a very high content of carbon that may come from some polymer left after preparation. All the samples show some amount of carbon that may be due to acetate ions

from the raw materials. Samples OMS-2 PVA-1, OMS-2 PVP, and OMS-2 PVP-1 have approx. One percentage of carbon, less amount of polymer was used to prepare these materials and the polymer can be removed during the washing process. Carbon left in the materials was removed by calcination at 400 °C for 5 h without causing any damage to the structure, as demonstrated by X-ray diffraction of the samples (Fig. 2). The average oxidation state of OMS-2 materials is very important for their application as catalysts. The average oxidation state for most of the samples is around 3.7–3.8. Sample OMS-2 PVA has the lowest oxidation number (3.47), this may be due to the presence of residual organic material that can interfere in the analysis method. For all the samples there is a slight increase in oxidation number after calcination. The molar ratio K/Mn for the prepared samples is in the range 0.14–0.20, and the water content is around 1.9%. These results are in agreement with those previously reported for cryptomelane [3,9]. The structure of OMS-2 synthesized in the presence of PVA or PVP is kept, there is some organic material left (polymer), but can be safely removed by calcination without phase transformation. Differences in crystal size are revealed by peak broadening in XRD patterns. The results of particle size calculated from line broadening of XRD peaks with ScherrerÕs equation [14] are reported in Table 2. Samples prepared

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

15

Table 1 Carbon content and average oxidation state of prepared OMS-2 materials Sample OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2

blank blank calcined PVA PVA-1 PVA calcined PVA-1 calcined PVP PVP-1 PVP calcined PVP-1 calcined

Carbon content (wt.%) by CHN element analyzer

Average oxidation state

0.17 0.11 5.88 1.04 0.11 0.14 1.34 1.51 0.10 0.08

3.70 3.73 3.47 3.65 3.77 3.76 3.78 3.74 3.79 3.77

Calcination conditions: 400 °C, 5 h; OMS-2 PVA: 2 g polyvinyl alcohol; OMS-2 PVA-1: 1 g polyvinyl alcohol; OMS-2 PVP: 2 g polyvinyl pyrrolidone; OMS-2 PVP-1: 1 g polyvinyl pyrrolidone.

Table 2 Textural properties of prepared OMS-2 materials Sample OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2 OMS-2

blank blank calcined PVA PVA-1 PVA calcined PVA-1 calcined PVP PVP-1 PVP calcined PVP-1 calcined

Surface area (m2 /g)

Particle size (nm)a

Average fiber diameter (nm)b

Average fiber length (nm)b

Film hardnessc

Film hardnessd (H K)

59 50 115 81 98 79 79 72 75 53

29.8 51.8 12.1 10.3 21.9 17.3 18.1 24.5 36.9 39.4

23.6 – 7.8 – 16.3 – 12.9 – 21.0 –

217.1 – 48.9 – 57.3 – 139.9 – 308.1 –

3B 3B 4H – 4H – 2H – 2H –

NA – 17.73 – – – 4.33 – – –

(–) Property was not measured for these samples. NA: The hardness for sample ‘‘blank’’ could not be measured, the surface was too rough. a By line broadening XRD. b Measured by TEM, sample size 100 fibers. c Pencils of known hardness (6H, 5H, 4H, 3H, 2H, H, F, HB, B, 2B, 3B, 4B, 5B, 6B, where 6H is the hardest and 6B is the softest) were used to measured film hardness (ASTM D 3363-92a). The value for hardness reported corresponds to the hardest pencil that leaves the film uncut or unscratched for a stroke length of at least 3 mm. d Measured by Knoop micro hardness method.

without any polymer have the largest particle size. As the amount of polymer (PVA or PVP) is increased the particle size decreases and the decrease is higher when PVA is used instead of PVP. Particle size of all the samples increased after calcination. Particle size determination by line broadening in Xray diffraction is consistent with the particle size observed by SEM. SEM micrographs (Fig. 4) revealed that the morphology of OMS-2 is not affected by the presence of PVA or PVP in the synthesis mixture. Fibers or needle-like particles are observed in all

cases, but the particle size changes. The largest fibers are obtained when no polymer was used in the synthesis procedure and the smallest particles were obtained when PVA was used in the synthesis. An intermediate particle size was obtained with PVP. The particle size of OMS-2 decreases by increasing the amount of polymer. A slight increase in particle size was observed after calcination of samples. The fiber length distributions for samples OMS-2 PVA, OMS-2 PVA calcined, OMS-2 PVP, and OMS-2 PVP calcined were determined by TEM analyses. The results are presented in Fig. 5. OMS-2

16

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

25 20 15 10 5 0

% of Fibers

% of Fibers

Fig. 4. FESEM micrographs: (1) OMS-2 blank, (2) OMS-2 PVA, (3) OMS-2 PVA-1, (4) OMS-2 PVP, (5) OMS-2 PVP-1.

0

20

(a)

40 60 80 Fiber Length (nm)

100

20 15 10 5 0

0

20

(b)

40 60 80 100 Fiber Length (nm)

120

(c)

% of Fibers

% of Fibers

25 40 30 20 10 0

0

100

200 300 400 Fiber Length (nm)

20 15 10 5 0

500

(d)

0

200

400 600 800 1000 1200 1400 Fiber Length (nm)

Fig. 5. Fiber length distribution measured by TEM (sample size 100 fibers): (a) OMS-2 PVA, (b) OMS-2 PVA calcined, (c) OMS-2 PVP, (d) OMS-2 PVP calcined.

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

PVA has very short fibers (around 50 nm), after calcination the number of long fiber increases, but they are still in the range 20–120 nm. OMS-2 PVP has larger fibers, around 100 nm. Upon calcination the fiber length increases to around 250 nm. The particle sizes obtained by XRD for these materials are similar to the diameters of the particles obtained from TEM (Table 2). Polymers affect the diameters and the lengths of OMS-2 fibers. The results of BET surface area are presented in Table 2. An increase in surface area is obtained when PVA or PVP polymers are used to synthesize OMS-2. The surface area for all the samples decreases after calcination. OMS-2 PVA with the smallest particle size also has the largest surface area. The sample labeled OMS-2 blank calcined, with the largest particle size has the lowest surface area. Surface area is inversely proportional to particle size [16]. The pore size of the prepared samples was determined by the Horvath-Kawazoe method. The range of values obtained was from , close to the value reported for OMS-2 4.7 to 5.1 A ) [1]. These results suggest that the (4.6  4.6 A increase in surface area is due to a decrease in crystal size of the samples and not to changes in pore size. 3.2. Film hardness Fig. 6 presents the SEM micrographs for the films prepared with OMS-2 PVA, OMS-2 PVP, and OMS-2 blank. Smooth films were prepared with OMS-2 PVA. Films prepared with OMS-2 PVP were less smooth than those prepared with OMS-2 PVA. A very rough film was obtained with OMS-2 blank. Films made with OMS-2 materials synthesized with polymers PVA or PVP are harder than films of OMS-2 with no polymer (Table 2).The film prepared with the blank sample can be cut by one of the softest pencils (3B). Using PVP the hardness of the film increases to 2H (this film is six levels harder that the blank film) and increases to 4H when PVA is used in the synthesis of OMS-2 (this film is eight levels harder than the blank film and two levels harder than the film of OMS-2-PVP). The hardness of the film was not affected by calcination. Knoop microhardness was measured for

17

samples OMS-2 PVA and OMS-2 PVP and the results are in agreement with those obtained with the pencil hardness test. OMS-2 PVA film is much harder than OMS-2 PVP film (17.73 vs. 4.33 H K). Knoop microhardness was not applicable to measure the hardness of OMS-2 blank film because the area was not uniform.

4. Discussion 4.1. Effect of addition of polymer in the synthesis of OMS-2 materials The use of polymers to stabilize dispersion of solids in liquids to avoid flocculation is well known. The adsorbed polymer is considered to reside partially at surface sites and partially in loops or tails in the solution. With a distribution of the strength of interactions between the polymer and the solid and the polymer and the solvent [16]. Variation of polymer content can have an effect on flocculation. At very low polymer concentration the solute can induce flocculation in a dispersion. At somewhat higher concentrations the polymer can stabilize the dispersion against flocculation [16]. PVA and PVP are water soluble polymers that have been previously used in the stabilization of particles [17–21]. PVA is considered as a linear polymer with side chains of secondary alcohol groups with the ability to form hydrogen bonds and complexes with transition metals [22]. PVP has a more complex structure. PVP has an amphiphilic character derived from the presence of a polar amide group and non-polar methylene and methine groups [23]. Highly polarized amide groups make the carbonyl active for the formation of complexes with transition metallic cations [24]. In the case of PVA used as a colloidal protector, the following relationship has been found: the higher the viscosity of the solution, the smaller the particle size and better stability of the colloid [22]. The use of polymers to avoid aggregation of metal atoms in solution has been previously documented [19]. Reduction takes place either before or after interaction of metal and polymer. When a complex between metal and polymer is formed

18

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

Fig. 6. SEM micrographs of films: (a) OMS-2 PVA, (b) OMS-2 PVP, (c) OMS-2 blank.

before reduction, better control of the metal growth by the polymer occurs. The opposite happens when reduction takes place without formation of any complex. In this case the protection effect by the polymer happens only after the particle has been formed [19]. When adding manganese acetate to aqueous solutions of PVA or PVP, a complex of Mn2þ with the polymers may be formed. Potassium permanganate is added slowly to the solution of polymer-

complexed Mn2þ and starts oxidizing Mn2þ forming manganese oxide particles (OMS-2). The growth of particles of manganese oxide is limited by the protecting effect of the polymers. Increasing the amount of polymer, leads to an increase in the viscosity of the medium [23] and smaller particles of OMS-2 are obtained. There is a limit to the maximum amount of PVA to be used. A different phase was detected by XRD (Mn3 O4 ) when 4 g of PVA was used. Potassium permanganate can oxi-

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

dize PVA. Potassium permanganate does not oxidize PVP as easily as PVA. Using PVP the particle size of OMS-2 obtained is smaller than the blank but larger than when PVA is used. The surface area is also higher than the blank. The molecular weight of the PVP used is much less than that of the PVA used (PVP, MW: 10 000 vs. PVA, MW: 115 000). PVA has a larger possibility of forming hydrogen bonds than PVP, due to the high concentration of hydroxyl groups. The combination of these properties will produce water solutions of PVP with less viscosity than water solutions of PVA (at the same concentration of polymer). The ability of the polymer solution to avoid particle aggregation decreases as the viscosity of the solution decreases. OMS-2 particles will aggregate easier in the PVP solution than when PVA is used. Studies using different crystallization times as well as polymers with different molecular weights are required to understand the effect of these parameters in crystal size and shape of OMS-2 materials. Calcination of OMS-2 materials leads to a slight growth of the particles. This behavior has been reported before [25]. The conditions used for calcination were sufficient to remove any remaining polymer and avoid a phase transformation.

19

When two materials of different degrees of hardness or rigidity are forced against each other, one of the materials either yields or crumbles, while the other is unaffected. Thus a scale of relative hardness can be established on the basis of the ability of one material to scratch or deform the other [30]. Mixing finely powered graphite with finely ground clay particles and shaping and baking the mixture by controlling the ratio of clay to graphite can lead to varying degrees of hardness [31]. Film hardness by the pencil hardness test is based on this principle. When OMS-2 PVA is used hard films with good adhesion can be obtained. OMS-2 PVP films are much harder than those obtained with OMS-2 blank sample but less than when OMS-2 PVA was used. PVA and PVP are film forming polymers [23]. Remaining amounts of PVA and PVP after washing the samples may help in the process of film formation. Using PVA in the synthesis of OMS-2 produced the smallest particles. During film formation more interaction and better packing of these small particles may occur. In the case of OMS-2 PVP, intermediate particle sizes and intermediate hardness were obtained compared to films prepared with OMS-2 PVA and OMS-2 blank.

5. Conclusions 4.2. Film formation with OMS-2 materials Film formation can be regarded as a process where a dense packing of particles is formed, followed by an interdiffusion of adjacent particles with mechanical strength developed during this last step [26]. When a stable colloidal dispersion is applied to a surface, water starts to evaporate, and the particles become more concentrated reaching maximum packing and starting to clump together and flocculate forming a film [16,27]. Hardness is a measure of how readily a large number of dislocations are generated and able to move through the material in response to the shear stresses produced by indentation. The strength of chemical bonds and the rigidity of the bonding network contribute to the hardness of the material [28]. Several authors have shown that hardening increases when particle size is decreased [29].

OMS-2 was prepared under the conditions reported here. Properties like particle size, surface area and film hardness were modified by the use of PVA and PVP. When 2 g of polymer (PVP) was used, very small particles (18.1 nm) of OMS-2 were obtained. Decreasing the amount of polymer, the particles increased their size (24.5 nm). Polymers may help to better disperse Mn2þ ions that are going to be oxidized by KMnO4 to produce OMS-2. The addition of polymers may avoid the diffusion and growth of particles of OMS-2. Film hardness of OMS-2 materials using both polymers is six to eight levels harder than when no polymer was used, as measured by the pencil hardness test. This property is important for the application of OMS-2 materials as catalyst in continuous flow reactor systems.

20

L.J. Garces et al. / Microporous and Mesoporous Materials 63 (2003) 11–20

Acknowledgements We acknowledge support from the Geosciences and Bioscience Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy and NASA, CAMMP. We also thank Dr. Francis S. Galasso for helpful discussions.

References [1] S.L. Suib, in: H. Chon, S.I. Woo, S.E. Park (Eds.), Recent Advances and New Horizons in Zeolite Science and Technology, Studies in Surface Science and Catalysis, vol. 102, Elsevier, Amsterdam, 1996, p. 47. [2] G.G. Xia, Y.G. Yin, W.S. Willis, J.Y. Wang, S.L. Suib, J. Catal. 185 (1999) 91. [3] S. Ching, J.L. Roark, N. Duan, S.L. Suib, Chem. Mater. 9 (1997) 750. [4] Q. Zhang, J. Luo, E. Vileno, S.L. Suib, Chem. Mater. 9 (1997) 2090. [5] Y.C. Son, V.D. Makwana, A.R. Howell, S.L. Suib, Angew. Chem. Int. Ed. 40 (2001) 4280. [6] B.C. Gates, Catalytic Chemistry, John Wiley and Sons, New York, 1992, p. 317. [7] J. Cai, J. Liu, W.S. Willis, S.L. Suib, Chem. Mater. 13 (2001) 2413. [8] R.N. DeGuzman, A. Awaluddin, Y.F. Shen, Z.R. Tian, S.L. Suib, S. Ching, C.L. OÕYoung, Chem. Mater. 7 (1995) 1286. [9] J. Luo, Q. Zhang, A. Huang, S.L. Suib, Micropor. Mesopor. Mater. 35–36 (2000) 209. [10] C.H. Lu, S. Kumar, J. Sol-Gel. Sci. Technol. 20 (2001) 27. [11] M.A. Gulgun, M.H. Nguyen, S.J. Lee, W.M. Kriven, J. Am. Ceram. Soc. 82 (1999) 556. [12] M.H. Nguyen, S.L. Lee, W.M. Kriven, J. Mater. Res. 14 (1999) 3417.

[13] Y.F Shen, R.P. Zerger, R.N. DeGuzman, S.L. Suib, L. McCurdy, D.I. Potter, C.L. OÕYoung, Science 260 (1993) 511. [14] D.M.A. Guerin, A.G. Alvarez, Crystallogr. Rev. 4 (1995) 261. [15] R.M. Potter, G.R. Rossman, Am. Mineral. 64 (1979) 1199. [16] P.C. Hiemenz, Principles of Colloid and Surface Chemistry, second ed., Marcel Dekker, New York, 1986. [17] X. Yang, H. Liu, H. Zhong, J. Catal. A: Chem. 147 (1999) 55. [18] K.S. Chou, K.C. Huang, J. Nanoparticle Res. 3 (2001) 127. [19] I.P. Santos, L.M. Liz-Marzan, Langmuir 18 (2002) 2888. [20] C.H. Lu, S. Kumar, Mater. Sci. Eng. B 79 (2001) 247. [21] F. Bertoux, E. Monflier, Y. Castanet, A. Mortreux, J. Mol. Catal. A: Chem. 143 (1993) 23. [22] C.A. Finch, Polyvinyl Alcohol: Properties and Applications, John Wiley and Sons, London, 1973, p. 183. [23] P. Molyneux, in: Water-Soluble Synthetic Polymers: Properties and Behavior, vol. I, CRC Press, Boca Raton, 1987, p. 119. [24] J.C. Hu, Y. Cao, P. Yang, J.F. Deng, K.N. Fan, J. Mol. Catal. A: Chem. 185 (2002) 1–9. [25] Q.H. Zhang, Ph.D. Thesis, University of Connecticut, 2001. [26] Y.J. Park, J.H. Kim, Colloids Surf. A: Physiochem. Eng. Aspects 153 (1999) 583. [27] S. Carra, D. Pinoci, S. Carra, Macromol. Symp. 187 (2002) 585. [28] M. Hebbache, Solid State Commun. 113 (2000) 427. [29] D. Robinson, in: A.S. Edelstein, R.C. Cammarata (Eds.), Nanomaterials Synthesis Properties and Applications, Institute of Physics Publishing, Philadelphia, 1996 (Chapter 12). [30] Available from . [31] D. Martin, Pencil hardness/softness rating or grading pencils, 1997. Available from .