Composite catalysts of supported zeolites

PREPARATION OF CATALYSTS VI Scientific Bases for the Preparation of Heterogeneous Catalysts G. Poncelet et al. (Editors) 9 1995 Elsevier Science B.V. All fights reserved.

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Composite Catalysts of Supported Zeolites N. van der Puila, E.W. Kuipers b, H. van Bekkum a and J.C. Jansen a aLaboratory of Organic Chemistry and Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands bKoninklijke/ShelI-Laboratorium, Amsterdam, Shell Research B.V., Badhuisweg 3, 1031 CM Amsterdam, The Netherlands ABSTRACT

Three-layer composite materials were synthesized consisting of lateraUy oriented silicalite-1 crystals, a catalytic phase and a [100] Si support. In particular chromium(Ill), manganese(Ill) and iron(lit) oxide particles were obtained by spin coating. HRSEM and AFM measurements proved that continuous layers of chromium, rnanganese and iron oxide are deposited, which are 0.5-2 nm thick. HRSEM observat~ns, the spin coating model and XPS analysis were in good agreement. TEM measurerner~ combined with X-ray elemental analysis prove~th e presence of Fe towards the interface ~ ~ 'the silicalite and the support. 11,. I N T R O D U C T I O N

The synthesis of supported zeolite crystals has been ,repc,'ted ipEeviously. Different supports such as stainless steel, aluminium foil, mullite and mica ,were combined with different types of zeolites [1-3]. Zeolite c o a t ~ can ~be .a~lied as membranes, catalysts and sensors [4]. In general, catalytic activity of the zeolite framework is obtained by ion exchange or isomorphous substitution of silicon by other elements. However, the !t~mited accessibility and bonding restrictions of the framework reduce the n,umb~ of modifications and hetero-atom stability. Supported zeolite systems can not only offer additional modifications, but can also improve process handling. In order to optimize the catalytic activity the supported crystals must obtain a specific orientation on the support, which is directly related to the particular crystal shape and the channel direction. Most catalytic sitesare present at the internal surface of the zeolite, thus the number of channel entrances at the crystal surface has to be maximized. An example of this configuration is an "end-of-pipe" de-NOx catalyst system, in which axially oriented Cu-exchanged crystals of MFI on a metal gauze are used [5]. In conventional catalyst systems zeolites are used in a fixed bed or fluid bed configuration. Advantages offered by the structured supported zeolite systems are i) low pressure drop, ii) attrition

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reduction, iii) dust free process handling and iv) heat transfer improvement. In the present study the catalytic site is introduced on the support at the interface between zeolite and support. The diffusion pathway through the framework to the catalytic site on the support must be minimized, while the channels must be oriented in the direction of the support. Advantages offered by these composites are i) combination of a catalytic site which can not be synthesized or stabilized in the zeolite lattice with framework shape selectivity and ii) bifunctional catalysis by addition of framework activity. A schematical drawing of the composite system design is given in Figure 1.

oriented zeolite layer catalytic phase

support

2 0 0 - 3 0 0 nm 1-2 nm

T

ram-size

1 Figure 1: Schematical representation of the composite system. In case of laterally oriented sificalite-1 the straight channels are perpendicular to the support.

The shape selective capacity of the composite materials is depending on the continuity of the zeolite layer, although the presence of minor amounts of pin-holes is not expected to exclude shape selectivity. The performance and compatibility of the composite systems depends on the thickness of the zeolite layer and the silicon to metal ratio. The thickness of the zeolite layer as well as the continuity are dependent on the flatness of the support surface. For example, in order to maintain the flatness of a Si-wafer support, the catalytic phase must be as thin as possible. In the optimized case a monolayer of catalytic sites forms the interface between zeolite and support. In case of an oxidic substrate, bonding of the catalytic sites takes place at hydroxyl groups. If a model Si/SiO 2 support is used which contains 4 hydroxyl groups per nm , a monolayer of monomeric metal species consists of 6.64.10- 0 moles of metal atoms per cm2..A zeolite layer thickness of 100 nm thus leads to a silicon to metal ratio of 450, although closer packing of the catalytic sites should be considered possible. In case of a continuous monolayer of metal oxide, such as Fe203, in which the Fe-O distances are 1.96 and 2.10 ~, a silicalite coating of 100 nm results in a Si/Fe ratio of 287. The order of magnitude of this silicon to catalytic site ratio is to be compared with ion exchanged and isomorphously substituted zeolite catalysts. Depending on the synthesis conditions for the in situ growth of MFI-type crystals on [100] Si wafers, both axial and lateral orientations can be obtained [6]. The laterally oriented crystals form a continuous layer of 200 nm in thickness. In this orientation diffusion towards the support is possible, since the b-direction is perpendicular to the

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support and the 2-dimensional system will allow adsorption of reactants and desorption of products (molecular traffic). In case of the in situ growth of mordenite crystals, stacks of the needle shaped crystals are obtained, which are parallel to the support [7]. Since the channels in the one dimensional structure are in the direction of the crystal length, diffusion to the support is impossible. TEM results of the supported MFI layers showed that the crystals are attached to the support by chemical bonding, taking place by hydrolysis of surface OH-groups and Si-OH groups in the synthesis gel [8]. By this bonding the crystal symmetry after calcination remains orthorhombic, which influences adsorption and diffusion properties. The adsorption capacity for p-xylene molecules decreases, since the flexibility of the framework is reduced by the presence of surrounding crystals [9]. In composite materials coatings of laterally oriented silicalite-1 crystals are assumed to be a (shape) selective component in catalysis. A catalytic membrane is obtained if a noble metal coating is applied after growth of the crystal layer, which is self-supporting or bonded to a meso-porous support. Also the catalytic site can be applied onto the support before the in situ growth of the zeolite layer. In this paper preliminary results of the synthesis and characterization of model composites, consisting of Si wafers covered with thin layers of chromium, manganese and iron oxide and a silicalite-1 coating are presented. 2. EXPERIMENTAL 2.1: Synthesis As a support Si [100] wafers from a silicon single crystal, cut to 10x10 mm platelets (0.7 mm thick) were used. The wafers were cleaned by a special procedure [6]. Metal oxide coverage was obtained by spin coating of a metal salt solution [10]. The samples were mounted on a disk connected to a stirring motor, rotating at 2000 rpm in a nitrogen atmosphere at room temperature. The metal precursors were Cr(NO3)3.9H20 (p.A., Aldrich), Mn(OCOCH3)2.4H20 (p.A., Janssen Chimica) and FeCla.6H20 (p.A., Janssen Chimica). Of each precursor a 0.1 wt% solution in dry ethanol (p.A., J.T.Baker) was made. In each experiment 1 ml of the solution was passed onto the rotating wafer through a 0.45 #m FP-Vericel membrane filter (Gelman Sciences). Assuming that the radial flow and the evaporation of the liquid determine the film height, the amount of precursor material deposited was calculated with the use of the evaporation time [11]. The decrease of the film height h as a function of time is given by: dh dt

2*p*~2*h 3 3,TI

(1)

in which p is the density, ~ the radial velocity, 77the viscosity and ~0is the mass flux by evaporation. The mass flux can be obtained by solving the equation with h = 0 at t = tvaD, tvap being the evaporation time, and h = ht =0, which is set at 100/~m. At the equilibriu~n film height h e, the evaporation becomes dominant and precursor loss by radial flow becomes negligible. The weight amount of metal precursor deposited, mp, ~s given by"

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mp=Co,A,he=Co,312*P *~23.r1.r

(2)

in which c o is the precursor concentration in the spin coating solution, and A is the support area. The precursor materials were converted to the metal oxides by calcination at 450~ for 3 hours. The silicalite-1 coating was grown on top of the metal oxide layers. The platelets were cleaned in boiling toluene for 2 minutes before zeolite synthesis. Chemicals used for the layer growth were tetraethyl orthosilicate (98%, Aldrich), tetrapropylammonium hydroxide (25%, CFZ) and deionized water. The molar oxide ratio in the gel was 6.5 SiO2: 1 TPA20 : 800 H20. The gel was aged at room temperature for 1 hour. Crystallization took place at 150~ for 3 hours in teflon-lined 35 ml stainless steel autoclaves under static conditions. The platelets were positioned in the upper part of the synthesis mixture by suspension from the lid of the teflon insert. After cooling of the autoclave, the wafers were washed with distilled water. The template was removed by calcination at 450~ for 6 hours.

2.2: Analysis Atomic Force Microscopy measurements were carried out on a Topometrix 2010 TMX Microscope under ambient conditions. High Resolution Scanning Electron Microscopy was done with a Jeol JSM-6000F scanning microscope. Transmission Electron Microscopy was performed on a Philips CM-30 FEG microscope to prove the presence of metal oxide after crystallization of the silicalite-1 layer. X-ray Photoelectron Spectroscopy took place on a Phi 5400 spectrometer. The bonding electron peaks of the metals were corrected relative to the 2p C peak at 286.4 keV. The intensity ratios IM/Isi were used to estimate the metal oxide layer thickness, with the following formulas [12]:

I,=1;

011 -exp(-

t

)]

(a)

Is1 Is1 ( 1 - 0 ) [ 1 - e x p ( - ~ t ) ] ~"St,M,Oy in which IM~176 and Isi~176 are the intensities of the reference materials (MxO v and SiO2), 0 is the surface coverage, ,k.M,MXUy .... is the mean free path of the metal in ttie metal oxide phase, ,~.~I,MXUy .... is the silicon mean free path in the metal oxide phase, and t ~s the layer thickness of the metal oxide. The surface coverage =s e calculated by: e: mMxo~ A,t,p

(4)

in which is m Mxuy . . . - is the mass of the metal oxide precipitate, obtained with the use of Equation (2) and p is the metal oxide density.

ll6?

Figure 2: Chromium oxide coating on Si-wafer. The average particle size is 20 nm. The particles are part of a continuous layer of metal oxide. The support surface is visible between the metal coating and the large dust particle.

:x&

....! ~ i ~ .......~

~

..... ~ , ~ T

~J"

.....::~.~ ii~:~iil;;=.~I~'~" ......

------

KSLA i 5

10KU 2.0A

XIO.

l ~ m

808

W

Figure 3: Iron oxide coating on Si-wafer. The presence of the metal oxide coating becomes visible by scratching the surface.

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3. RESULTS AND DISCUSSION

XPS analysis of the spin coated iron samples showed a 2p3/2 peak at 710.8 eV, indicating the presence of Fe203 particles. The Mn 2p3/2 peak is positioned at 641.6 eV, which is identified as Mn203. The (Fe/Fe + Si) and the (Mn/Mn + Si) signal area ratio were 0.32 and 0.19 respectively, thus indicating a high coverage of the SiO 2 surface. The amount of deposited material was calculated using Equation (2), after which a layer thickness was deduced by assuming e = 1. From the intensity ratios measured with XPS, a layer thickness can also be calculated using Equation (3). The results of both calculations are compared in Table 1. The mean free paths of Fe in Fe203 and of Si in Fe203 are estimated at 2.030 and 2.693 nm, respectively, and the values of the mean free paths of Mn in Mn203 and of Si in Mn203 are 2.238 nm and 2.828 nm.

Table 1. Mn203 and Fe203 layer thickness t and mass of oxide deposited m, obtained by the spin coating model and XPS Sample

m spin (#g)

Mn203 Fe203

0.276 0.257

t spin (nm) 0.61 0.49

t xp s (nm) 0.44 0.60

m xPS (#g) 0.199 0.313

These data show that the modelling of the spin coating procedure gives a reasonable estimate of the amount of deposited material. AFM measurements of the Cr203 samples showed particles of 15-30 nm on top of a corrugated layer. No particles were observed for the Fe203 samples, although based on the XPS results a high coverage of the SiO2 surface was expected. HRSEM pictures demonstrate the presence of 20 nm particles in the Cr203 samples, see Figure 2, which however appear to be part of a thin continuous layer of metal oxide, that is probably formed between the hemispheres during calcination. The transition between support and metal oxide layer is demonstrated by flaws on the surface. The average number of hemispheres on the surface is lower than in figure 2. The chromium oxide layer thickness is estimated at 1-2 nm. HRSEM measurements again showed that the iron oxide coating does not consist of particles, but forms a continuous flat layer. This was demonstrated by scratching the surface, see Figure 3. AFM measurements of the Mn203 samples showed that the metal oxide coating consists of 50 nm hemispheres, forming a continuous layer, see Figure 4. According to light microscopy the silicalite-1 coating extends over 1 cm 2. XRD reveals that the crystals are laterally oriented, thus with the straight channels of the pore system perpendicular to the support. Figures 5 and 6 show the AFM and HRSEM pictures of the silicalite layer on top of the Fe203 coating. The zeolite crystals form a rather continuous layer leaving a small amount of pin holes. The average size of the laterally oriented intergrown crystals is 0.6 p,m. On top of the lateral crystal coating partly axially oriented and twin forms are present, which however do not affect the layer concept. The same silicalite-1 coatings were grown on the Mn203 and Cr203 layers. The average silicalite-1 layer thickness is estimated at 200 nm.

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'"~~"..i!i~iii~!!:ii":'i'!$i ...... ~,~'{" ...~,'!;i~'ir

i"ii.....:!:!~!i~

~i!.',:~=~'i'i.i !!i!}ii~i !i}

....i~i

9

Figure 4: AFM picture of manganese oxide coating on Si-wafer. Image size is 550 x 550 nm.

Figure 5: AFM picture of laterally oriented silicalite-1 crystals on a Fe203 layer. Image size is 5 x 5 #m.

Figure 6: HRSEM photograph of silicafite-1 crystals grown on a continuous layer of Fe20 3.

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Transmission Electron Microscopy measurements were made of wedge shaped slices of the composites, which were obtained from a cut. This preparation method leads to a random orientation of the sample on the holder, yielding a limited number of succesfull measurements. The silicalite crystals of the Fe203 composite materials are tightly attached to the support XES measurements show that the surface and the bulk of the zeolite layer, as well as the Si wafer do not contain iron. The presence of iron becomes evident only at the interface between the silicalite layer and the wafer. The thickness and continuity of the iron layer after zeolite synthesis however, remains unknown. Apparently the metal oxide layer has not dissolved drastically by exposure to the zeolite synthesis mixture at high pH. This phenomenon is also observed during synthesis of thin silicalite-1 layers on Si/SiO 2 surfaces, where no etching of the wafer takes place at relatively low temperatures and short synthesis times [6]. It appears that the kinetics and/or the equilibria which cause etching at high pH are shifted by the presence of a Si source. TEM micrographs and XES analysis of all composite materials using a crosssection technique, as well as preliminary catalytic tests will be published in a forthcoming paper.

4. CONCLUSIONS

The synthesis of composite materials consisting of a thin metal oxide layer such as Cr203, Mn203 or Fe203, and a thin oriented layer of silicalite-1 appears to be possible. Well defined thin layers of metal oxide are prepared by spin coating. The synthesis of laterally oriented zeolite layers on these metal oxide has succeeded, and in case of the Fe203/silicalite-1 composite the presence of iron towards the support/silicalite layer interface is proven by TEM/XES. The composites are expected to exhibit a high shape selectivity in future catalytic experiments.

ACKNOWLEDGEMENT

The authors thank Ing. P. van Acker and E. Rodenburg for their help with the spin-coating procedure. We also thank Dr.A. Knoester and Ing. N. Groesbeek from the Royal/Dutch Shell Laboratory in Amsterdam for the HRSEM measurements. Dr. H. Zandbergen is acknowledged for the TEM measurements.

LITERATURE

[1] I.M. Lachman and M.D. Patil, US Patent 4,800,187 (1989). [2] S.P. Davis, E.V.R. Borgstedt and S.L. Suib, Chem. Mater., 2 (1990), 712. [3] T. Sano, M. Kawamura, F. Mizukami, H. Takaya, T. Mouri, W. Inaoka, Y. Toida, M. Watanabe and K. Toyoda, Zeolites, 11 (1991), 842. [4] H. van Bekkum, E.R. Geus and H.W. Kouwenhoven, Proc. Summerschool of the 10th Int. Zeolite Conf., Garmisch-Partenkirchen, Elsevier, (1994), in print. [5] H.P. Calis, A.M. Gerritsen, C.M. van den Bleek, C.H. Legein, J.C. Jansen and H. van Bekkum, Can. J. Chem. Eng., (1994), accepted.

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[6] J.C. Jansen, W. Nugroho and H. van Bekkum, In: R. von Ballmoos, J.B. Higgins and M.M.J. Treacy (eds.), Proc. 9th Int. Zeolite Conf., Montreal, Bulterwoilh, (1993), 247-254. [7] J.C. Jansen, D. Kashiev ar~ A. Erdem-Senetalar, Proc. Summerschool o f t l ~ 10th Int. Zeolite Conf., Garmisch-Partenkirchen, Elsevier, (1994), in print [8] J.H. Koegler, J.C. Jansen, H. van Bekkum, Proc. 10th Int. Zeolite Co{ff., Garmisch-Partenkir~, (1994), accepted. [9] N. van der Pull, J.C. Jansen and H. van Bekkum, unpublished _dat~_: [10] E.W. Kuipers, C. Laszlo and W. Wieldraaijer, Cat. LetL, 17 (1993), 71-79. [11] R.M. van Hardeveld, P.LJ. Gunier, L.J. van Llzendoom, E.W. I(uipels and J.W. Niemantsverdriet, Appl. Surf. Sci., (1994), to be p u b l i C . [12] H.P.C.E. Kuipers, H.C.E. van Leuven and W.M. Visser, Surf. Interf. Analysis, 8 (1986), 235-242.