The potential of zeolite membranes in hydroisomerization processes

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The potential of zeolite membranes in

hydroisomerization processes

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ISBN: 90-8559-169-4

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The potential of zeolite membranes in

hydroisomerization processes

Proefschrift

ter verkrijging van de graad van doctor aan de Technische Universiteit Delft,

op gezag van de Rector Magnificus prof. dr. ir. J.T. Fokkema, voorzitter van het College voor Promoties,

in het openbaar te verdedigen op dinsdag 25 april 2006 om 15:30 uur

door

Maikel Laurence MALONCY

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Prof. dr. J.C. Jansen

Samenstelling promotiecommissie:

Rector Magnificus voorzitter

Technische Universiteit Delft, Nederland, promotor Prof. dr. J.C. Jansen Universiteit van Stellenbosch, Zuid Afrika, promotor Prof. dr. ir. G. Baron Vrije Universiteit Brussel, België

Prof. dr. ir. H. van Bekkum Technische Universiteit Delft, Nederland Prof. dr. D. Cardoso Federal University of São Carlos, Brazilië Prof. dr. D. Cazorla Amorós University of Alicante, Spanje

Dr. L. Gora Polish Academy of Sciences, Polen, adviseur

Reservelid

Prof. dr. F. Kapteijn Technische Universiteit Delft, Nederland

Prof. dr. J.A. Moulijn

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4.2. Appraisal of the heptane isomerized product 94 4.3. Challenges in the heptane hydroisomerization process 95 5. Concluding remarks 96 Acknowledgement 96 References 96 Chapter 7. Technical and economical evaluation of a zeolite membrane based heptane hydroisomerization process Abstract 99 1. Introduction 100

2. Process concept 100

3. Process description and simulation 102

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1

General introduction and thesis outline

1. General

The oil we are burning in two centuries took hundreds of millions of year to form from primeval plants. The average light vehicle burns 100 x the car weight in prehistoric plants in the form of gasoline. Only 12.5 % of the fuel energy reaches the wheels, 7% is used in accelerating the car and less than 1% moves the driver [1]. The draconically inefficient use of our energy sources in car transportation is alerting the community in other areas as well. Gradually, a mind setting turns to focus on higher efficiencies. A typical example of the size of changes that should be made is given in the Innovative Roadmap Separation Technology, 2004 [2]. The authors recommend to reduce energy consumption in separation processes up to 75%. Such numbers cannot be realised by improving state-of-the-art systems but only by developing revolutionary, new technologies, based on new materials.

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2. Zeolites

Zeolites are crystalline, microporous, aluminotectosilicates. The tectosilicate configuration is based on so-called corner-connected tetrahedral units, see Figure 1.

(a) (b)

Figure 1. (a) tetrahedral unit comprising formally 1x silicon and 4 x 1/2 oxygen. (b) 2 tetrahedral units sharing 1 oxygen atom. The total charge of a ‘Si unit’ is zero. In the case of an’Al-unit’ the charge is –1. The corner-connected tetrahedral units result in a microporous well ordered topology, Figure 2, with remarkable specifications, see intermezzo 1. The model presented in Figure 2 for the MFI type zeolite shows the so-called straight channels while the so-called sinusoidal channels are running in the plane of the picture, and intersecting the straight channels, see also Figure 3b. Each pore direction has a unique pore diameter, providing an opportunity to separate mixtures of small molecules.

Figure 2. Typical presentation of the topology of the zeolite type MFI which can be found for other zeolites as well in the Atlas of Zeolite Structures [4]. Only the connections of the Si and Al centers is given. The oxygen

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General introduction and thesis outline _________________________________________________________________________________________

3

Intermezzo 1

Remarkable specifications of zeolites are the following:

The specific pore wall area can be about 500 – 1000 m2/g.

The external area, depending on the particle size can be 5% up to 50% of the pore wall area.

Each atom of the framework is part of the pore wall. Minor changes such as a defect or a modification in the framework can change the adsorption or catalytic properties. Silicalite-1 is an all silica lattice and therefore hydrophobic, see also Table 2. However, the material can show uptake of water. This can only be explained by defects in the framework generating hydrophilic centers. Even at a Si/Al ratio of 20000 Brønsted acid cracking catalytic activity can be still present and measurable in cracking of e.g. n-hexane.

The pore volume can occupy half of the crystal volume. The relative density can be between 1.4 and 1.9

Zeolites have ferro-elastic properties, implying that upon external stimuli the pore aperture can deform and adsorb unexpected molecule sizes. Molecules of naphthalene with a kinetic diameter of 0.72 x 0.38 nm were adsorbed in the pores of MFI, which are 0.51 x 0.55 nm. The passage of the guest molecule was realized through change of the pore shape [5].

Of the more than 150 types of zeolite each type differs in a particular property. In Table 1 the mnemonic code of the structure (topology) of frequently used zeolite types is given, together with the specific properties, which result(ed) in various applications.

Table 1. Mnemonic code, specific properties of zeolites as well as (potential) applications. Code a _Type _Si/Al _{Pore configuration (nm)} _{Channel system} _Applicationb

LTA NaA 1 0.41 3-D ion exchange, drying, separation

GIS NaP 1 0.31x0.45 0.28x0.48 2-D ion exchange FAU HY CaX ≥ 2 1-1.5 0.74 0.74 3-D 3-D catalysis catalysis, separation

MOR HMor ≥ 6 0.7x0.65 1-D catalysis

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As zeolites are crystalline materials the pore size distribution is unique per zeolite type and the pore configuration clearly identified in the crystal form, see Figure 3. The topology of the framework as well as the pore configuration was well established for most of the zeolite types based on single crystal analysis applying x-ray diffraction techniques.

In the case the zeolite contains Al ions in the framework the lattice is balanced by cations. Most favorable for catalysis applications is a proton, resulting in Brønsted catalytic sites [7]. Together with the size and the uniformity of the pores the zeolites unit two properties in one material; the molecular shape selectivity and the Brønsted acidity. Next to these attractive properties a third function, e.g. Pt or a coordination complex can be introduced in the zeolite pores.

Table 2 illustrates the change in properties upon the influence of Al isomorphously substituted for silicon in the framework.

(a)

(b)

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Table 2. General properties of zeolites as a function of the Si/Al ratio. (The direction of the arrows indicates an increase in a certain property)

Si/Al 1 5 10 103 ₁₀6 property Acid sites Acid strength Cations Hydrophobic Hydrophylic Thermal stab. Si/Al 1 5 10 103 ₁₀6 property Acid sites Acid strength Cations Hydrophobic Hydrophylic Thermal stab.

As indicated in Table 2 the number of acid sites is at its maximum at a Si/Al ratio of about 1.26. Acid strength increases from Si/Al 1.26 up to Si/Al ~ 10 and is further on more or less constant. Interesting for maximum ion exchange capacity is the lowest Si/Al ratio (1.0) while the material is simultaneously hydrophilic. Reducing the Al content provides gradually a more hydrophobic material. Thermal stability is controlled by the Si/Al ratio as well. From Si/Al ratio 1 up till infinity the thermal stability can range from 600 o_{C till 1100}o_C.

3. Zeolite membranes

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Figure 4. n/i-hexane separation with zeolite membrane as an example of industrial application. In the column denoted with Dp the kinetic diameters of n- and i-hexane are given together with the pore aperture of zeolite

A (actually CaNaA as not all the Na has been exchanged by Ca)

Separation based on affinity allows actually both or more types of molecules in a mixture to permeate through the micropores of the membrane. Two modes of component permeation can be identified. One of the mode is surface diffusion in which, e.g. hydrocarbon molecules, interact with different energies with the pore wall through a number of C-H---O interactions. The other mode is the activated gaseous diffusion. In this mode the molecules follow the gas rules thus increasing distance between molecules while displaying lesser interaction with the pore wall. Surface diffusion dominates at low temperature whereas activated gaseous diffusion prevails at elevated temperature. At intermediate temperature both modes occur. The two modes are illustrated in Figure 5 for single component permeation of C1 – C4 alkanes through a silicalite-1 membrane [9].

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7 The separation of mixture components smaller than the pore size occurs at relatively low temperatures where the component permeation is governed by surface diffusion. Thus, the molecules diffuse with different fluxes through the membrane due to the different interactions with the pore wall. At elevated temperatures the activated gaseous diffusion is dominant resulting in a loss in separation selectivity. The flux of permeating components is optimal at a temperature where the diffusion is just still described by surface diffusion and not by activated gaseous diffusion. This is typical for linear hydrocarbons diffusion through zeolites.

Almost absolute separation due to affinity with the pore wall can also occur as was demonstrated experimentally with n-butane/hydrogen [9], and olefin/hydrogen mixtures [10]. In both cases the pore aperture was larger than the kinetic diameters of the molecules. However, the interaction between the wall and the hydrocarbons excluded the transport of hydrogen through the pores. Only at elevated temperatures the transport was governed by activated gaseous diffusion thus allowing both molecules i.e. hydrocarbons and hydrogen to permeate.

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(a) (b)

Figure 6. (a) cross sections at low (upper part) and high magnification (lower part) and (b) top view of grains of oriented crystalline material of MFI type zeolite in a membrane configuration. The 2-D channel configuration is oriented as follows: straight channels perpendicular to the support provide optimal passage.

Sinusoidal channels run parallel to the support.

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Figure 7. (a) cross section and (b) top view of randomly oriented crystallites of zeolite NaA on alumina support. The 3-D geometry provides maximal transport through the membrane.

4. Zeolite membrane reactors

Zeolite membrane as part of a membrane reactor should at least meet the requirements regarding selectivity and flux at the reactor temperature. In the case of absolute separation this can be expected, however, in affinity separations surface diffusion might be changed in activated gaseous diffusion as a function of temperature resulting in an unfavorable selectivity.

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9 hydrogenation reaction studies [10]. The membrane reactor consists of a rutile support, an intermediate layer of Pt and a silicalite-1 top layer. The component of interest permeates through the silicalite-1 layer and reacts on the Pt layer. The advantage of using a silicalite-1 layer is that no Si is isomorphously substituted by Al, thus, no cations and/or Brønsted acid sites are present, neither on the extenal surface nor on the internal surface. Side reactions and pore blocking are therefore excluded. Furthermore, by coating the Pt layer with the zeolite no Pt is exposed to the open space and no side reactions will occur.

Figure 8. Parallel passage membrane reactor developed for hydrogenation studies [10]. The reactor comprises a support layer of rutile, a porous Pt sputtered coating of 3 nm and a continuous silicalite-1 film of 0.5 µm.

The concept of a micro membrane reactor consisting of a catalyst particle covered with a membrane layer [11] is depicted in Figure 9. The advantage of a system containing micro membrane reactors is that no continuous layer is needed and in the case of defects or leakage only one micro reactor fails. The membrane area per unit reactor volume is relatively large. There is simultaneously selectivity on the feed side as well as on the product side.

In e.g. hydrogenation reaction it is mandatory that the temperature in the parallel passage as well as in the micro reactor concept must be at least 275 o_{C as otherwise} hydrogen as a reactant is repelled from the catalyst sites by the preferred occupancy of the hydrocarbons.

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Figure 10. Tubular membrane reactor comprising a macroporous support coated with a zeolite layer and loaded with catalyst [12] for the hydroisomerization of mixtures of alkanes.

The tubular membrane reactor concept illustrated in Figure 10 is actually comprising a porous tube of alumina or stainless steel coated with zeolite and containing a catalyst bed [12]. The unit performs at one temperature which can be chosen as such that the performance of the membrane and the catalyst is optimal.

5. Hydroisomerization and its position in fuel supplies

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6. Outline of the thesis

The thesis describes studies performed on the synthesis and application of zeolite membranes in the separation of linear and branched light alkanes. This separation is of great importance in hydroisomerisation processes. These processes are also described in the thesis. Different applications of zeolite membranes in these processes are suggested.

In chapter 1, a general introduction is given. The importance of an alternative for energy intensive processes is stressed. Zeolitic materials are introduced and the application as membranes in separation and combined separation and reaction processes are described.

Chapter 2 and 3 present and discuss experiments in which the synthesis and performances of zeolite membranes in the separation of light alkanes mixture are evaluated. In chapter 2 different silicalite-1 membranes were synthesized and used in the separation of monobranched from linear C4, C6 and C7 alkanes. The use of zeolite beta membranes in separating dibranched from mono-branched hexane is described in chapter 3.

In chapter 4 and 5 hydroisomerisation processes of light alkanes are presented. Chapter 4 describes a conceptual design of a state of the art C5/C6 total hydroisomerization process. This design was used as a basis for comparison for a conceptual design of an industrial scale C5/C6 hydroisomerization processes that uses membrane technology to combine reaction and separation in one process unit. In chapter 5 preliminary experimental studies are presented in which zeolite membranes are used to combine separation and reaction functions in one process unit as an alternative for the state of the art C5/C6 hydroisomerization process.

In Chapter 6 and 7 the hydroisomerization of heptane is discussed. Chapter 6 gives an overview of the mechanistic aspects, the current state and the potential application of heptane hydroisomerization. A conceptual design of an industrial heptane hydroisomerization process is presented in chapter 7. This process uses zeolite membrane technology for the separation of heptane isomer mixtures.

Most of the chapters in this thesis are based on the author’s publications. Therefore, each chapter can be read independently and overlaps between chapters can occur.

References

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[2] Separation Technology Innovation Roadmap, Report 17 September 2004, Dutch Ministry of Economic Affairs, DG Innovation, The Hague, the Netherlands.

[3] H. van Bekkum, E, Flanigen, P.A.Jacobs and J.C.Jansen (eds.) Introduction to Zeolite Science and Practice, second edition, Elsevier, Amsterdam, 2001.

[4] Atlas of Zeolite Structure Types, http://www.iza-structure.org/databases (assessed 15/12/2005)

[5] H. van Koningsveld, F. Tuinstra, H. van Bekkum, J.C. Jansen, Acta Cryst. B45 (1989), 423-431.

[6] H. Robson, K.P. Lillerud, Verified syntheses of zeolitic materials, 2nd edition, Elsevier, Amsterdam, 2001.

[7] R.A. Sheldon, H. van Bekkum (eds.), Fine Chemicals through Heterogeneous Catalysis, First edition, Wiley-VCH, Weinheim, 2001.

[8] Y. Morigami, M. Kondo, J. Abe, H. Kita & K. Okamoto, The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane, Sep. Purif. Technol. 25 (2001) 251-260.

[9] F. Kapteijn, J.M. van de Graaf, J.A. Moulijn, J. Mol. Catal. A: Chemical 134 (1998) 201. [10] N. van der Puil, E.J. Creyghton, E.C. Rodenburg, S.T. Sie, H. van Bekkum, J.C. Jansen, J.

Chem. Soc. Faraday Trans. 92 (1996) 4609.

[11] N. Nishiyama, K. Ichioka, D-H. Park, Y. Egashira, K. Ueyama, L. Gora, W. Zhu, F. Kapteijn, J.A. Moulijn, Ind. Eng. Chem. Res. 43(5) (2004) 1211.

[12] M.L. Maloncy, L. Gora, E.E. McLeary, J.C. Jansen, Th. Maschmeyer. Catal. Commun. 5 (6) (2004) 297.

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_________________________________________________________________________________________ This chapter is based on: M.L. Maloncy, L. Gora, J.A. Moulijn, J.C. Jansen, J. Membr. Sci., submitted

2

The performance of silicalite-1 membranes in the

separation of linear and branched alkanes

Abstract

Different silicalite-1 membranes were synthesised on seeded and nonseeded Trumem tubular supports. The nonseeded membranes were prepared by the in-situ hydrothermal synthesis using a two-step temperature crystallization procedure. The seeded membranes were prepared by the secondary growth method. Two types of seeded membranes were prepared; membranes prepared with seeds of about 700 nm and membranes with seeds of about 220 nm. The largest thickness of the continuous phase of the zeolite layer was obtained on the nonseeded membranes. The thickness was about 15 µm against 10 µm and 5 µm obtained on the 700 nm and 220 nm seeds membranes, respectively. The performance of the

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1. Introduction

Membranes composed of the MFI type zeolite (silicalite-1, ZSM-5) have been often investigated for catalytic and separation applications. Zeolites with the MFI structure have a high SiO2/Al2O3 ratio inducing hydrophobic properties and relatively high thermal and chemical stability. The structural porosity of MFI zeolites consists of channels of about 0.55 nm, which makes this zeolite type interesting for application in size and shape selective chemical and physical processes [1,2]. The synthesis of a good quality zeolite membrane and its reproducibility is a rather difficult task, due to several factors involved. The work of Gora et al. [3] is one of the few publications on reproducibility. The membrane quality can be determined by intercrystalline porosity [4,5], crystal orientation [6], size of the crystal [7], thickness and uniformity of the zeolite layer, and the presence of defects [8]. A good quality membrane needs to fulfil two counteracting requirements; a thin layer, to achieve sufficiently high fluxes, and a defect free layer, to achieve high separation efficiencies [9]. A common method for the synthesis of zeolite membranes is the in-situ hydrothermal synthesis. In this method a support is put in direct contact with the synthesis solution or gel allowing the growth of a zeolite film on the surface of the support under hydrothermal conditions. A competitive crystallization on the support and in the reaction mixture occurs. Another method is the synthesis by secondary growth. Two steps are involved in this method. The surface of a support is first coated with zeolite seed crystals, followed by the growth of the seeds layer to a continuous zeolite film under hydrothermal synthesis conditions [10].

In this work we evaluate the separation performance of silicalite-1 membranes prepared by the in-situ hydrothermal synthesis method and by the secondary growth method. The separation of branched from linear C4-C7 alkanes was studied. This separation is of great importance in the oil industry, due to the higher octane number of the branched hydrocarbons. The product of catalytic hydroisomerization of linear alkanes is a mixture of linear and branched hydrocarbons, and it’s a necessity to separate the branched alkanes from the linear ones. Several studies showed that silicalite-1 (pore diameter around 0.55 nm) is a good candidate for this separation (kinetic diameter of the branched hydrocarbons is about 0.5 – 0.62 nm, and that of the linear isomers is about 0.43 nm) [11-15].

2. Experimental

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The performances of Silicalite-1 membrane… _________________________________________________________________________________________

15 a thickness of 0.015 mm and a mean pore size of 160 nm. The supports (outer diameter 10 mm) were welded at the ends to nonporous metal tubes with a Swagelok connection (12 mm) on one side for mounting in the permeation testing equipment, and the other side of the tube was closed (dead end). The porous part of the support available for permeation had a length of 0.1 m, giving a total membrane surface area of about 0.003 m2_{. The support is shown in} Figure 1.

Figure 1. Tubular Trumem membrane support.

The zeolite layer was synthesised on the Trumem supports according to two different synthesis approaches: an in-situ hydrothermal synthesis and a secondary growth synthesis. In the in-situ hydrothermal synthesis the membranes were prepared by two subsequent crystallizations under different conditions. This synthesis was already described elsewhere [17]. The condition for the first crystallization was 393 K for 114 h. The second crystallization condition was set at 453 K for 17 h. A synthesis mixture with molar composition 100 SiO2:59.3 TPABr:63.7 TPAOH:14200 H2O was used. From the synthesis mixture a zeolite layer was grown on the TiO2 side of the support, which was placed vertically in a Teflon-lined autoclave. After the first synthesis the membrane was transferred into a new Teflon-lined autoclave with fresh synthesis solution. The reactants used for the synthesis mixture were: tetrapropylammonium hydroxide (TPAOH) (Chemische Fabriek Zaltbommel CFZ B.V., 25% in water), tetrapropylammonium bromide (TPABr) (CFZ B.V.), tetraethyl orthosilicate (TEOS) (Aldrich) and deionized water. TPAOH, TPABR and deionised water were mixed until TPABr was completely dissolved. TEOS was added and the solution was stirred at room temperature for 6 h.

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Helium P Silicalite-1 Shell side Tube side MFC MFC P Hydrocarbon Oven Retentate Permeate Helium

Figure 2. Schematic representation of the experimental set-up. (MFC) mass flow controller, (P) pressure transducer.

Once the silicalite-1 seeds were formed after the required period they were separated from the mixture, washed repeatedly and put into suspension in deionised water.

The seeds were applied on the Trumem supports by dipping the supports into the suspension. The supports were then dried at 373 K and used in the second step of the synthesis. The synthesis mixture used in the second step was similar to that used in the in-situ hydrothermal synthesis approach. The supports were placed vertically in Teflon-lined autoclaves and the synthesis mixture was added. The condition set for crystallization was 453 K for a period of 17 h. This condition was set for the support containing the 700 nm seeds as for the support containing the 220 nm seeds. Each end of the tubular supports, equipped with the Swagelok connection, was wrapped with Teflon tape and plugged with a Teflon tap, so that the zeolite layer could only grow on the TiO2 side of the support. After crystallisation the membranes were washed with distilled water, dried overnight and calcined at 673 K for 16 h in air with heating and cooling rates of 1 K min-1_{. The surface and cross-section} morphology of the resulting membranes was analysed by scanning electron microscopy (SEM) using a Philips XL20 microscope.

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17 The experiments were performed at temperatures between 353 K and 473 K. The final set of experiments was carried out using a feed containing a 80/20 wt% n-heptane/2-methylhexane mixture with a feed flow rate of about 0.07 ml min-1_{. The separation was studied at} temperatures ranging from 393 K to 473 K. Helium was used as carrier and sweep gas at a flow rate of 50 ml min-1_{in the second and the last set of experiments. In all the experiments} the pressure set on both sides of the membrane was 1 bar. Feed, retentate and permeate were analysed online with a gas chromatograph (FID detector for the hydrocarbons measurements). A GC-column, CP-Sil PONA CB fused silica, WCOT, 100 m x 0.25 mm, 500 nm df, was used to determine the amounts of hydrocarbons quantitatively.

The separation performances were evaluated in terms of linear/branched alkane selectivity and the hydrocarbon flux through the membrane. The flux and the selectivity (S n/i)

were calculated as follows:

m F Flux = A (1) n i Permeate n/i n i Feed

X

S =

X

⎛ ⎞ ⎜ ⎟ ⎝ ⎠ ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ (2)

Where Fm, A, Xn and Xi are the component flow through the membrane (mol s-1), the

membrane area (m2_{), the mole fraction of the linear alkane and the mole fraction of branched} alkane, respectively.

3. Results and discussion

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(a)

(b)

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Figure 3. SEM images of the nonseeded membrane (a) and the membranes prepared with seeds of 700 nm (b) and 220 nm (c). The last image is 90 degrees rotated compared to the first two images, moreover another

magnification is used.

The different synthesis procedures resulted in membranes in which the supports where fully covered with silicalite-1 crystals. The silicalite-1 layer was firmly bounded to the surface of the titania layer of the supports (see Figure 3). Figure 3a and 3b show clearly the

Silicalite-1

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19 layer at the bottom and the titania layer in between. Figure 3c shows only the zeolite layer on top of the titania phase.

Although other morphology characteristics can determine a membrane quality the focus on the membrane characterization will be on the zeolite layer thickness. The thickness can be easily deduced from SEM images. The thickness influences significantly the membrane separation performances. A thin membrane can provide sufficiently high fluxes. The thickness of the monolithic part (almost continuous phase) of the silicalite-1 layer indicated by the double arrows in Figure 3 varied from 5 to 15 µm depending on the synthesis procedures followed. The smallest thickness was found for the membranes prepared with the seeds of 220 nm. These membranes had a thickness of about 5 µm, as deduced from Figure 3c. The membranes synthesised with 700 nm seeds, see Figure 3b, had a thickness of around 10 µm while the membranes prepared on the nonseeded membranes had the largest thickness, of about 15 µm, see Figure 3a.

Basically, each membrane was prepared by using two synthesis steps. The second synthesis step was similar for the three membranes. The preparation of the three membranes only differs in the first synthesis step. For the nonseeded membrane the first synthesis step consists of a crystallization process at low temperature and a longer crystallisation time aiming at nucleation. For the seeded membranes seeds of 220 nm and 700 nm were applied to the support from which they eventually should grow into a continuous layer in the second synthesis step. As the second synthesis step is similar the thickness of the continuous phase of zeolite layer is mainly influenced by the first synthesis step. The different membrane thicknesses obtained indicate that the use of a low temperature and longer crystallisation time leads to the formation of a thicker continuous layer while the use of seeds reduces the thickness. Moreover, smaller seeds further decrease the thickness. Probably, the different seeds grow with different rate at the same experimental conditions. The small seeds may not be fully crystallised. The growth of the small seeds may be a surface limited reaction.

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at low crystallisation temperature was about 114 h. However, due to the longer crystallisation time and especially the lower temperature in the first synthesis step a denser zeolite layer with the largest thickness was formed.

The separation performances of the membranes evaluated in the three sets of experiments are presented in Figures 4 and 5 in terms of the linear/branched alkane selectivities and fluxes of the various hydrocarbons as a function of temperature. In all cases there was an increase in hydrocarbon fluxes with temperature. The fluxes of the linear alkanes were always higher than those of the branched ones.

The highest hydrocarbon fluxes were obtained for the membranes prepared with the 220 nm seeds. The branched isomer fluxes were the lowest on the nonseeded membranes. At temperatures below 410 K the lowest linear alkane fluxes were obtained with the membranes prepared with 700 nm seeds. Above this temperature the lowest linear alkane fluxes were observed on the nonseeded membranes. This behaviour could be an indication of a better quality of the nonseeded membrane having less defects in the zeolite layer compared to the 700 nm seeded membrane. In zeolitic pores the linear alkanes, preferentially adsorbed from the mixture, permeate by surface diffusion at low temperatures thereby hindering the branched hydrocarbon permeation. The fluxes of the linear alkanes are thus higher than those of the branched ones. As the temperature increases the hydrocarbon molecules vibrate more vigorously and have reduced interaction with the zeolite pore wall and consequently will diffuse faster through the pores yielding generally higher fluxes. The surface diffusion becomes less dominant while the activated diffusion increases. The adsorption of the linear hydrocarbons reduces and simultaneously an increase in the branched hydrocarbons permeation occurs. The presence of the branched alkanes in the zeolitic pores can obstruct the permeation of the linear hydrocarbon, resulting in a small decrease in the linear hydrocarbon permeation. The combined effects result in a maximum for the linear hydrocarbon flux that is typically for linear hydrocarbons in zeolitic pores. As shown in Figure 4 a maximum linear hydrocarbon flux is observed at a temperature around 393 K for the nonseeded membrane. As the temperature increases the fluxes of the linear molecules initially decrease slightly but subsequently increase with temperature. This behaviour is only observed on the nonseeded membrane and not on the 700 nm seeded membrane. This could be an indication of the presence of nonzeolitic pores (defects) on the 700 nm seeded membrane contributing to the continuous increase of the linear hydrocarbon fluxes.

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The performances of Silicalite-1 membrane… _________________________________________________________________________________________ 21 0 2 4 6 8 10 12 300 340 380 420 460 500 Temperature ( K ) n-C 4 Flux (x 10 -3 mol m -2 s -1 ) _(a) 0 2 4 6 8 10 12 300 340 380 420 460 500 Temperature ( K ) i-C 4 Fl ux ( x 1 0 -3 m o l m -2 s -1 ) _(b) 0 2 4 6 8 10 12 14 340 380 420 460 500 Temperature ( K ) n-C 6 Flux (x 10 -4 m o l m -2 s -1 ) _(c) 0 1 2 3 4 340 380 420 460 500 Temperature ( K ) 2MP Flux (x 10 -4 m o l m -2 s -1 ) _(d) 0 2 4 6 8 10 12 380 420 460 500 Temperature ( K ) n-C7 Fl u x (x 10 -4 mol m -2 s -1 ) _(e) 0 1 2 3 380 420 460 500 Temperature ( K ) 2MHx F lux ( x 10 -4 mol m -2 s -1 ) (f)

Figure 4. Hydrocarbon fluxes through the different MFI membranes as a function of temperature. From the n/i-C4 mixture the n-butane (a) and iso-butane (b) fluxes are shown. From the n/i-C6 mixture the n-hexane (c) and 2-methylpentane (d) fluxes are given. From the n/i-C7 mixture the n-heptane (e) and 2-methylhexane

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0 10 20 30 40 300 340 380 420 460 500 Temperature ( K ) n/i-C 4 S elect iv it y (a) 0 10 20 30 40 50 60 340 380 420 460 500 Temperature ( K ) n-C6 /2M P Selec tivity (b) 0 2 4 6 8 10 380 420 460 500 Temperature ( K ) n-C 7 /2MHx Selectivity (c)

Figure 5. Linear/branched alkanes selectivities at different temperatures: (a) n-butane/iso-butane, (b) n-hexane/2-methylpentane, (c) n-heptane/2-methylhexane. Symbols: 220 nm seeds membrane (circles),

700 nm seeds membrane (triangles), nonseeded membrane (squares). Note that different flux and temperature scales are used.

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23 resulting in a decrease in selectivity. The contributions by adsorption and diffusion govern the permeation selectivity of alkanes through silicalite-1 membranes. The diffusion differences between individual components dominate the selectivity at higher temperatures.

The highest selectivities over the whole temperature ranged studied were observed for the nonseeded membranes. At the lowest temperatures studied using these membranes the selectivities towards the linear alkane for the n-butane/i-butane, n-hexane/2-methylpentane and n-heptane/2-methylhexane mixture were 37 (at 303 K), 48 (at 353 K) and 8 (at 393 K), respectively. Even at the highest temperature of 473 K the nonseeded membrane showed selectivity towards the linear component with selectivity values around 3, while the seeded membranes were not selective at all. The lowest selectivity values were obtained on the 220 nm seeded membrane.

The higher linear/branched alkane selectivities and the reasonable alkane fluxes for the nonseeded membrane suggest a better quality membrane compared to the seeded ones. Among the seeded membranes the 700 nm seeded membrane shows better separation performance than the 220 nm seeded membrane. Relating the fluxes and the selectivity with the membrane thickness it seems that a decrease in the zeolite layer thickness results in an increase in the hydrocarbon fluxes through the membrane. This is mainly observed at the high temperatures studied. The selectivity increases with an increase in the zeolite layer thickness, which is observed mainly at low temperatures.

Besides the zeolite layer thickness, a defect-free zeolite layer influences the membrane separation performances. High separation efficiency can be obtained on a membrane without defects. The lower separation performances of the seeded membranes, mainly the 220 nm seeded membrane could be due to the existence of defects in the zeolite layer. These defects may form during hydrothermal synthesis or calcinations. The preparation of the seeded membranes by the secondary growth method aimed at sealing the intercrystal voids. Most probably, the crystal growth was not sufficient to close the intercrystalline gaps, resulting in the presence of defects.

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the thickness alone doesn’t fully describe the resistance to calcinations, but it can be used as an indication.

The best separation performance of the nonseeded membrane is most probably due to the use of two subsequent crystallisation temperatures which not only influence the membrane thickness but also tighten the intercrystalline boundaries, resulting in a dense membrane with less defects.

The separation performance of the membranes can be compared with literature data. The selectivity values for the n-butane/i-butane mixture obtained are comparable with those found in literature on silicalite-1 membranes. Values in the range of 4.5-52 can be found [20-22]. Literature data on n-hexane/2-methylpentane and n-heptane/2-methylhexane selectivity are scarce, making direct comparison rather difficult. However, our results compare well with those of Flanders et al. [11] who studied the separation of a n-hexane/3-methylpentane mixture and reported similar n-hexane/3-MP selectivity ranging between 50 and 75 at 373 K.

Funke et al. [12] also observed similar selectivity behaviour. The selectivity decreases with increasing temperature from 24 (at 326 K) to 1 (at 443 K). The single component fluxes of n-hexane through MFI membranes reported in the literature are around 1 x 10-3 mol m-2 s-1 (at about 373 K), comparable to our results.

4. Conclusions

Silicalite-1 membranes were prepared successfully on tubular supports by two different methods. In-situ hydrothermal synthesis with a two-step crystallisation procedure was applied to prepare nonseeded membranes, while seeded membranes with seeds of 700 nm and 220 nm in size were prepared by the secondary growth method. The size of the seeds influences the zeolite layer thickness. The thinnest zeolite layer was observed for the membranes synthesised with the 220 nm seeds. The hydrothermal synthesis approach with a two-step crystallisation procedure provides the membrane with the thickest zeolite layer. This membrane showed the best separation performance with the highest selectivities at reasonable fluxes. Thus, good quality membranes can be obtained by applying the in-situ

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25

References

[1] L. Cot, A. Ayral, J. Durand, C. Guizard, N. Hovnanian, A. Julbe, A. Larbot, Solid state Sci. 2 (2000) 313.

[2] J. Caro, M. Noack, P. Kölsch, R. Schäfer, Microporous Mesoporous Mater. 38 (2000) 3. [3] L. Gora, N. Nishiyama, J.C. Jansen, F. Kapteijn, V. Teplyakov, Th. Maschmeyer, Sep.

Purif. Tech. 22-23 (2001) 223.

[4] E.R Geus, H. van Bekkum, W.J.W. Bakker, J.A. Moulijn, Microporous Mesoporous Mater. 1 (1993) 131.

[5] J.M. van de Graaf, E. van der Bijl, A. Stol, F. Kapteijn, J.A. Moulijn, Ind. Eng. Chem. Res. 37 (1998) 4071.

[6] J.C. Jansen, W. Nugroho, H. van Bekkum, in: J.B. Higgens, R. von Ballmoos, M.M.J. Treacy (Eds.), Proceedings of the ninth International Zeolite Conference, Butterworth-Heinemann, Montreal, 1993, 247.

[7] Z.A.E.P. Vroon, K. Keizer, M.J. Gilde, H. Verweij, A.J. Burggraaf, J. Membr. Sci. 113 (1996) 293.

[8] Z.A.E.P. Vroon, K. Keizer, A.J. Burggraaf, H. Verweij, J. Membr. Sci. 144 (1998) 65 [9] F. Kapteijn, J. van de Graaf, J.A. Moulijn, J. Mol. Catal. A: Chemical 134 (1998) 201. [10] Y.S. Lin, I. Kumakiri, B.N. Nair, H. Alsyouri, Sep. Purif. Methods 31 (2) (2002) 229. [11] C.L. Flanders, V.A. Tuan, R.D. Noble, J.L. Falconer, J. Membr. Sci.

176

(2000) 43. [12] H.H. Funke, A.M. Argo, J.L. Falconer, R.D. Noble, Ind. Eng. Chem. Res.

36

(1997) 137. [13] C.J. Gump, R.D. Noble, J.L. Falconer. Ind. Eng. Chem. Res. 38 (1999) 2775.

[14] J. Coronas, R.D. Noble, J.L. Falconer. Ind. Eng. Chem. Res. 37 (1998) 166.

[15] T. Matsufuji, K. Watanabe, N. Nishiyama, Y. Egashira, M. Matsukata and K. Ueyama. Ind. Eng. Chem. Res. 39 (2000) 2434.

[16] L. Trusov, Membr. Technol. 128 (2000) 10

[17] L. Gora, J.C. Jansen, J. of Catal. 230 (2005) 278-290

[18] A.E. Persson, B.J. Schoeman, J. Sterte, J.E. Otterstedt, Zeolites 14 (1994) 557. [19] M.L. Maloncy, L. Gora, E.E. McLeary, J.C. Jansen, Th. Maschmeyer. Catal. Commun. 5

(6) (2004) 297.

[20] S. Alfaro, M. Arruebo, J. Coronas, M. Menendez, J. Santamaria, Microporous Mesoporous Mater. 50 (2001) 195.

[21] K. Keizer, A.J. Burggraaf, Z.A.E.P. Vroon, H. Verweij, J. Membr. Sci. 147 (1998) 159. [22] J. Hedlund, J. Sterte, M. Anthonis, A.-J. Bons, B. Carstensen, N. Corcoran, D. Cox, H.

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_________________________________________________________________________________________

This chapter is based on: M.L. Maloncy, A.W.C. van den Berg, L. Gora, J.C. Jansen, Microporous Mesoporous Mater. 85(1-2) (2005) 96

3

Preparation of zeolite beta membranes and their

pervaporation performance in separating di- from

monobranched alkanes

Abstract

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1. Introduction

Zeolite beta is relevant to petrochemical industrial processes such as aromatic alkylation and the hydroisomerization of alkanes. Pt-loaded beta zeolite catalyst shows very high activity and selectivity in the hydroisomerization of C5-C7 alkanes [1,2]. Adsorption experiments using different zeolites [3,4] suggest a relatively good performance of zeolite beta in separating branched C5-C8 isomers. Denayer et al. [3] studied ZSM-5, ZSM-22, mordenite, NaY, NaUSY and beta type zeolites. The authors suggest that zeolite beta and ZSM-22 can be used to separate C5-C8 alkane isomers based on the degree of branching. Huddersman and Klimzcyk [4] studied the separation of 3-methylpentane and 2,3-dimethylbutane over zeolite beta, mordenite, silicalite, EU-1, ZSM-12 and SAPO-5. The beta type zeolite proved to be the most effective separator with stronger adsorption capacities for the monobranched compared to the dibranched isomer.

These findings encourage studies on the preparation of zeolite beta membranes for application in hydroisomerization processes. In these processes the membranes could be potentially applied as a separator for branched alkane isomers or even as a catalytic membrane reactor, combining separation and reaction functions. Literature data on zeolite beta membranes are very limited. Furthermore, to our knowledge there is no report on branched alkane mixture separation using this type of membrane. It is difficult to use zeolite beta membranes to separate mono- from dibranched isomers mainly because these molecules have similar diameters (0.56 - 0.62 nm) and they fit into the zeolite beta pores (0.75 nm). Therefore, it is not possible to achieve separation due to absolute molecular sieving. Preferential adsorption of one of the molecules could bring about the separation of mono- from dibranched isomers.

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Preparation of zeolite beta membranes… _________________________________________________________________________________________

29

2. Experimental

In this work commercial Trumem disks with a diameter of 25 mm were used as supports. These disks are composed of porous stainless steel coated with TiO2 [10]. The stainless steel layer has a thickness of 0.25 mm and a mean pore size of 2-5 µm. The TiO2 layer has a thickness of 0.015 mm and a mean pore size of 160 nm. Some of the supports were first seeded with beta particles. The size of the seeds was about 300-500 nm. The seeds were grown from a reaction mixture identical to that of method 2 (vide infra). The support was put into a flat holder with one open end and the other end attached to a vacuum pump. The seeds were placed on the titania side of the support by applying vacuum at the stainless steel side of the holder and by dipping the holder vertically in the zeolite beta seeds suspension. The seeds were than dried in air at 353 K overnight.

The membranes were prepared according to two different methods. In the first method a synthesis mixture with molar composition 1.00 SiO2:0.0026 Al2O3:0.269 (TEA)2O:15.5 H2O was used followed by crystallisation at 423 K for 96 h. The reactants used for the synthesis were: tetraethylammonium hydroxide (TEAOH) (35 wt%), silica (Degussa Aerosol 200), aluminium nitrate (Al(NO3)3.9H2O) and deionised water. The membranes from the second method were prepared from a mixture with composition 1.00 SiO2:0.02 Al2O3:0.25 (TEA)2O:15 H2O:0.0394 Na2O:0.02 K2O:0.058 HCl followed by crystallisation at 423 K for 40 h. The reactants used for the synthesis were: tetraethylammonium hydroxide (TEAOH) (35 wt%), sodium chloride, potassium chloride, sodium hydroxide, silica (Degussa Aerosol 200), sodium aluminate (56 wt% Al2O3, 37 wt% Na2O) and deionised water. The preparation of the synthesis mixtures was done by following the procedure of zeolite [Ti,Al]-beta (without use of the Ti-containing reagent) and zeolite Al-beta, both given in [11] for method one and two, respectively. Crystallisation in both methods was done with the support vertically placed inside a Teflon-lined autoclave. A Teflon holder protected the stainless steel side of the support so that the membrane could only grow on the TiO2 side.

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Table 1. Beta membranes synthesised.

Membrane Beta seeded Method No. of layers BEA1 yes 1 2 BEA2 yes 2 2 BEA3 no 1 2 BEA4 no 2 2

Zeolite beta particles formed after crystallisation were collected from the Teflon bottom, washed, dried and element analysis (Inductively Coupled Plasma – Optical Emission Spectroscopy, ICP-OES) was performed using a Perkin Elmer Optima 3000DV. The identification of the existing phases on the collected beta particles and on the membranes was done by X-ray diffraction analysis (XRD) with CoKα radiation (λ = 0.179026 nm). The surface and cross-section morphology of the membranes was analysed by scanning electron microscopy (SEM) using a Philips XL20 microscope.

To check the permeation performance and the existence of defects, N2 permeation experiments according to the batch method [13] were performed on the calcined membranes. In this method, pure N2 was introduced to the feed side (the zeolite side of the membrane) up to a pressure of 200 kPa. The feed stream was shut down and vacuum was applied at the permeate side. The decrease of pressure on the feed side was monitored with a pressure transducer to determine the flux trough the membrane. The experiments were performed at ambient temperature. As the volume of the compartment at the feed side is known, ideal gas law can estimate the amounts of moles present at a measured pressure. The N2 permeance over a certain time interval, ∆t (s) was determined as follows:

2

∆n N permeance =

∆t A ∆P⋅ ⋅ (1)

where _∆n, _∆P and A are the amount of N2 permeated (mol), the pressure difference across the membrane (Pa) and the membrane area exposed to permeation (m2_{), respectively.}

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Preparation of zeolite beta membranes… _________________________________________________________________________________________ 31 Feed solution Membrane in holder

Stop valves Vent

Vacuum Cold traps PI Pressure indicator Stir bar

Figure 1. Schematic view of the pervaporation set-up.

Membrane Closing piece (facing stainless steel side of membrane) Sealing ring Membrane holder Figure 2. Membrane holder used in the pervaporation set-up.

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during a period of 6 h. Before each experiment, the membranes were calcined overnight at 573 K in air with heating and cooling rates of 0.5 K/min to remove adsorbed species. The flux and the selectivity (S2MP/22DMB) were calculated from the following equations.

∆n Flux = ∆t A⋅ (2) 2MP 22DMB Permeate 2MP/22DMB 2MP 22DMB Feed

X

S

=

X

⎛ ⎞ ⎜ ⎟ ⎝ ⎠ ⎛ ⎞ ⎜ ⎟ ⎝ ⎠ (3)

Where _∆n, _∆t, A, X2MP and X22DMB are the permeate amount (mol), the permeation time (s),

the membrane area (m2_{) the mole fraction of 2MP and the mole fraction of 22DMB,} respectively.

Modification by post-treatment with trimethylchlorosilane was done on the non-seeded beta membrane prepared by method 1 (BEA3). The membrane was placed into 5 ml trimethylchlorosilane and kept there at room temperature for about 30 min. The membrane was then dried and calcined at 673 K for 4 h in N2 followed by 16 h in air with heating and cooling rates of 0.5 K/min. This membrane was used also in the pervaporation experiments.

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33 the alkanes and the zeolite beta O-atoms is described by a set of Lennard Jones equations [26].

3. Result and discussion

Figure 3 shows the XRD spectra of the membranes BEA1 and BEA2, confirming the existence of the zeolite beta phase on the supports. Peaks from the support (rutile, TiO2 and iron austenite) were detected as well. Membrane BEA1 had higher beta zeolite peak intensities compared to BEA2, suggesting higher crystallinity. Among the different variables distinguishing the two synthesis methods, the longer crystallization time of method one is most probably accounting for the higher crystallinity of BEA1. XRD analysis of the particles collected from the Teflon bottom also shows higher peak intensities of the zeolite beta prepared by the first method, as well as the existence of zeolite beta as the only zeolite phase. 5 15 25 35 45 2 Θ support support support support support support BEA2 BEA1

Figure 3. XRD pattern using CoKα radiation (λ = 0.179026 nm) of the zeolite membranes BEA1 and BEA2.

Table 2. Elemental analysis of the zeolite beta particles prepared by the two methods.

Si (%) Al (%) Na (%) Si/Al (mol/mol)

Method 1 37 0.9 - 40

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The results of the elemental analysis performed on the collected zeolite beta particles from the Teflon bottom is given in Table 2. The obtained Si/Al ratios are in agreement with the ratios given in [11].

SEM images of the cross sections of the different membranes are shown in Figure 4. The images clearly show the zeolite layers on top of the titania layer of the support. The zeolite and the titania layers are intergrown. The membrane surfaces also show well-intergrown zeolite beta crystals. SEM images from the membrane cross sections suggest a zeolite layer thickness around 1 µm for all the membranes. In principle, the use of seeds on a support gives rise to a dense layer when the system is exposed to further synthesis steps. The use of a seeded mixture was found essential for a dense coating [12]. Apparently, the use of seeds in this work does not influence significantly the membrane thickness as deduced from the SEM images.

Figure 4. SEM images of the different zeolite beta membranes (cross sections).

Results from the nitrogen permeation experiments are depicted in Figure 5. The permeance of N2 was plotted against the pressure difference over the membrane. For all membranes the permeance decreases with increasing pressure difference across the membrane. This behaviour indicates that the permeation of N2 is mainly governed by surface diffusion, that the permeation is basically through zeolitic pores and that there is an absence of large defects (non-zeolitic pores).

BEA1 BEA2

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Preparation of zeolite beta membranes… _________________________________________________________________________________________ 35 0 2 4 6 8 10 0 0.5 1 1.5 2 ∆P (x 105_{Pa )} N2 Perm eance (x 1 0 -7 mol m -2 s -1 Pa -1 ) viscous Knudsen / activ. gaseous surface BEA4 BEA2 BEA3 Silicalite-1 BEA1

Figure 5. N2 permeance through the different membranes according to the batch method [13]. (Note: The x-axis of the inset should be based on the feed pressure rather than the pressure difference in the case of

viscous flow)

The permeance of a component through a membrane could take place according to various transport modes, such as viscous flow, Knudsen diffusion, activated gaseous diffusion and surface diffusion. Nishiyama et al. [13] discuss these transport modes in zeolite membranes. For single component systems viscous flow and Knudsen diffusion occurs in non-zeolitic pores. Viscous flow occurs in the presence of an absolute pressure gradient. The permeance from viscous flow increases with increasing feed pressure. The permeances from Knudsen diffusion and activated gaseous diffusion are independent from the pressure difference across the membrane. The permeance from surface diffusion decreases with increasing pressure difference across the membrane. Surface diffusion and activated gaseous diffusion occur in zeolitic pores.

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The influence of the different synthesis methods on the N2 permeation can also be deduced from Figure 5. The membranes synthesized by method 2 have higher permeance values than those from method 1. This could be due to the higher crystallinity, which resulted from method 1 as previously mentioned in the XRD analysis. It is possible that BEA1 and BEA3 have a better intergrown membrane surface of zeolite beta particles than the membranes from method 2. The use of seeds does not influence significantly the N2 permeance of the membranes synthesized by method 2. The influence of the use of seeds is more pronounced for the membranes synthesized by method 1. The seeded BEA1 had permeances of roughly one order of magnitude lower than the unseeded BEA3. Although the use of seeds didn’t seem to influence the membrane thickness it surely affects the N2 permeation through the membranes synthesized by method 1. The seeds could account for a dense and compact layer of zeolite beta increasing the resistance to N2 permeation. The BEA1 membrane had permeance values even lower than those of the seeded silicalite-1 membrane, as can be seen in Figure 5. This gives an indication of the good quality of the membrane synthesized, because silicalite-1 membranes have zeolite pores of around 0.55 nm, which are smaller than the zeolite beta pores (0.75 nm). The performance of the silicalite-1 membrane on the permeation of N2 (decreasing slope) also suggests the absence of large defects. Overall it seems that synthesizing the beta membranes by method 1 and using seeds gives good quality membranes with hardly any defects.

The membranes BEA1 and BEA2 were used in pervaporation experiments at 303 K to separate 2MP and 22DMB. The results of the pervaporation experiments are given in Table 3. The membrane prepared by method one, BEA1, showed a better performance in separating 2MP from 22DMB with higher permeation flux of 2MP compared to 22DMB. BEA1 also showed a better performance than the silicalite-1 membrane. Comparison of the BEA1 membrane performance with that of a membrane reported in the scarce literature on branched alkane separation [29] shows a better separation performance of the BEA1 membrane. Funke et al. [29] used a silicalite membrane but were not able to separate a 50/50 wt% 3-methylpentane/22DMB mixture in vapour permeation experiments at 363 K. They obtained selectivity equal to unity.

Table 3. Pervaporation experiments at 303 K with a 50/50 wt% 2MP/22DMP mixture. Membrane Fluxes (mol s-1_m-2_{) S}

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37 In principle separation by molecular sieving could be achieved by using a defect-free silicalite-1 membrane, since one of the isomers (the monobranched hexane) has a diameter similar to the pore diameter of silicalite-1 and the other one (the dibranched hexane) a larger diameter. However, this possibility is not straightforward since studies showed that larger molecules could penetrate into zeolite MFI pores [30]. Moreover, one should be very cautious with the values of the isomers diameter, since in literature accurate estimation is lacking. As given in Table 3, both molecules 2MP and 22DMP, permeate with similar fluxes through the silicalite-1 membrane. This suggests that separating 2MP from 22DMB by molecular sieving using silicalite-1 is rather difficult. The fact that the larger pore zeolite BEA1 membrane could separate these isomers suggests that this separation is most probably due to preferential adsorption of the monobranched isomer on zeolite beta. Simulation studies discussed further on showed higher adsorption energy for the monobranched isomer. Another cause for the higher flux of 2MP could be that during its permeation through the zeolite pore it suffers less sterical hindrance than the more bulky 22DMB.

To evaluate the influence of post-treatment with a silane coupling reagent, BEA3 was post-treated with trimethylchlorosilane and used in the pervaporation experiments. The result is presented in Table 3. This membrane showed the best separation performance. An explanation for the better performance could be that the silane post-treatment changes the surface in a manner that increases the adsorption of the 2MP. This is in accordance with the higher fluxes of 2MP. During the post-treatment the zeolite external surface could be covered with the products of reactions between the silane reagent and the surface hydroxyl groups. Calcination with O2 removes the hydrocarbon residue and produces silica coated zeolites [7]. The external surface may become more apolar, thus increasing the interaction between the surface and the apolar 2MP isomer. The pore interior remains unchanged because trimethylchlorosilane is too large to penetrate into the pore. The post-treatment with possible formation of a silica coated surface could also cause a reduction in the zeolite pore openings. In combination with the increased interaction, the reduction in pore size could contribute to the larger difference in fluxes, thus increasing the selectivity. For a complete understanding of the surface modification and the enhanced pervaporation performance further investigation is needed.

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To gain more insight in the adsorption of the two isomers in zeolite beta molecular simulations were performed to estimate adsorption energies for 2MP and 22DMB in zeolite beta pores. Figure 6 gives a schematic representation of the adsorbed 2MP and 22DMB in the zeolite beta pore obtained by performing a full optimisation of this system (see experimental section). From the simulations adsorption energies of -48 and -42 kJ/mol were predicted for 2MP and 22DMB, respectively. This supports the assumption that the monobranched molecule is stronger and preferentially adsorbed in the zeolite beta pore, which could explain the selectivity towards the monobranched isomer. The larger interaction area most likely causes the preference for 2MP. 22DMB has a quaternary C-atom, which is largely shielded from the pore wall by its surrounding C-atoms, while all C-atoms of 2MP are exposed to the framework O-atoms (see also Figure 6). Furthermore, 2MP is more flexible so it can easier adapt to the shape of the pore. Other researchers have also observed the difference in adsorption energy between monobranched and dibranched hexane experimentally. Huddersman and Klimczyk [31] found a difference between 3-methylpentane and 2,3-dimethylbutane of -5.0 kJ/mol, comparable to the calculated value of -6.0 kJ/mol of the present work. Denayer et al [3] found a difference of about -7 kJ/mol between 2MP and 22DMB. It should be stressed that the energy values of the present work should be seen as a qualitative indication, because they are based on optimisation calculations. Monte Carlo simulations are required in order to obtain more accurate quantitative data.

4. Concluding remarks

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39 Figure 6. Schematic representation of an optimised all silica zeolite BEA structure with 2MP (above) and 22DMB (below) inside one of the pores (black sticks are oxygen, grey sticks are silicon). The CHx-groups (x = 0 - 3) of the alkanes are given as spheres in accordance with the united atom model as employed in the

calculations.

Acknowledgements

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119.

[3] J.F. Denayer, W. Souverijns, P.A. Jacobs, J.A. Martens, G.V. Baron, J. Phys. Chem. B, 102 (1998) 4588.

[4] K. Huddersman, M. Klimczyk, AIChE Journal, 42 (1996) 405.

[5] M. Namuro, T. Yamaguchi, S. Nakao, J. Membr. Sci. 144 (1998) 161. [6] T.C. Bowen, R.D. Noble, J.L. Falconer, J. Membr. Sci. 245 (2004) 1. [7] E.F. Vasant, P. Cool, Colloids Surf. A, 179 (2001) 145.

[8] N.R.E.N Impens, P.van der Voort, E.F. Vansant, Microporous Mesoporous Mater. 28 (1999) 217.

[9] T. Sano, M. Hasegawa, S. Ejiri, Y. Kawakami, H. Yanagishita, Microporous Mater. 5 (1995) 179.

[10] L. Trusov, Membr. Technol. 128 (2000) 10.

[11] H. Robson, K.P. Lillerud, Verified syntheses of zeolitic materials, 2nd edition,

Elsevier,

Amsterdam,

2001, pp. 115-120.

[12] L. Gora, G. Clet, J.C. Jansen, Th. Maschmeyer, in: A. Galarneau, F. Di Renzo, F. Fajula, J. Védrine (Eds), Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Studies in Surface Science and Catalysis, Vol. 135, Elsevier Science, Amsterdam, 2001, p. 3145.

[13] N. Nishiyama, L. Gora, V. Teplyakov, F. Kapteijn, J.A. Moulijn, Sep. Purif. Technol. 22-23 (2001) 295.

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[15] A. Banerjee, N. Adams, J. Simons, R. Shepard, J. Phys. Chem. 89 (1985) 52. [16] International Zeolite Association website (assessed June 2004):

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[18] M.J. Sanders, M. Leslie, C.R. A. Catlow, J. Chem. Soc., Chem. Commun. (1984) 1271. [19] N. J. Henson, A. K. Cheetham, J.D. Gale, J. Chem. Mater. 6 (1994) 1647.

[20] G.D. Price, I.G. Wood, D.E. Akporiaye, in: C.R.A. Catlow, C. R. A. (Ed.), Modelling of Structure and Reactivity in Zeolites, 1st edition, Academic Press Inc., San Diego, 1992, p. 38.

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41 [23]

S.J. Weiner, P.A. Kollman, D.A. Case, U.C. Singh, C. Ghio, G. Alagona, S. Profeta,

P. Weiner,

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Vlugt, R. Krishna, B. Smit, J. Phys. Chem. B, 103 (1999) 1102.

[25] S.K. Nath, R. Khare, J. Chem. Phys. 115 (2001) 10837.

[26] Ph. Ungerer, B. Tavitian, A. Boutin, Applications of Monte Carlo simulation in oil and gas technology, Technip, Paris, 2004.

[27] V.A. Tuan, S. Li, J.L. Falconer, R.D. Noble, Chem. Mater. 14 (2002) 489.

[28] V.A. Tuan, L.L. Weber, J.L. Falconer, R.D. Noble, Ind. Eng. Chem. Res. 42 (2003) 3019. [29] H.H. Funke, A.M. Argo, J.L. Falconer, R.D. Noble, Ind. Eng. Chem. Res. 36 (1997) 137. [30] H. van Koningsveld, J.C. Jansen, Microporous Mater. 6 (1996) 159.

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_______________________________________________________________________________________ This work was developed in cooperation with P.C. Perez, currently at the Laboratory for Process Equipment,

TUDelft

4

Design of a state of the art C

5

/C

6

hydroisomerization

process

Abstract

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1. Introduction

Rearranging the structure of hydrocarbons in order to achieve high octane number in gasoline fraction is becoming increasingly necessary because of environmental regulation. To increase the octane number n-pentane and n-hexane (the main component of the light strain run gasoline fraction) are hydroisomerized into isopentane and the dibranched hexanes [1,2]. A typical example of a hydroisomerization process is the Shell Hysomer process [3], first commercialized in 1970, and currently employed in over 70 plants worldwide. This process uses a bifunctional catalyst composed of a noble metal supported on a zeolite i.e. platinum supported on Mordenite. The catalyst is resistant to water and sulfur compounds up to quite high concentrations, eliminating the need for extra pre-treatment facilities. Different from the other hydroisomerization processes that use chlorinated catalysts the Hysomer process doesn’t require continuous addition of a chloride activator, removal of HCl from the effluent streams, and precautions against chloride corrosion [4]. The process operates at a temperature in the range of 240 – 280 o_{C and at pressures between 8 – 30 bar.}

Due to thermodynamic equilibrium limitations, complete conversions of linear alkanes into branched ones are not achieved by once-through operation. To obtain conversion to extinction the hydroisomerization process can be completed with a physical separation process that allows isolating and recycling of linear alkanes. An example of such a physical separation process is the IsoSiv process of Union Carbide Corporation [5,6] This process provides efficient branched/linear paraffin separation using a molecular sieve (zeolite CaA) unit instead of distillation columns. The mixture of pentanes and hexanes is a close boiling point mixture, which makes the separation by distillation difficult and energy consuming. The use of Zeolite CaA enables the selective adsorption of linear alkanes over branched ones. The branched alkanes have larger molecular diameter, inhibiting them to enter the zeolite CaA pores.

The Total Isomerization Process (TIP) is a combination of the Hysomer and the IsoSiv processes [7]. The TIP, commercialized since 1975 [8], is widely used and is considered a process for virtually complete hydroisomerization of the linear paraffins. In the TIP the application of molecular sieve separation and recycle of linear alkanes not only leads to a higher octane number of the product, but also decreases the effect of hydroisomerization temperature on product quality. Therefore, the disadvantage of zeolites with their higher operating temperatures than catalysts based on chlorinated alumina largely disappears.

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Design of a state of the art C5/C6 hydroisomerization process

_________________________________________________________________________

45 process economics. The design served as a base case for comparison for a new process that combines reaction and separation in one process unit. The new process described by McLeary et al. [9] uses membrane technology and aims at better performance in technological and economical aspects than the state of the art process.

2. Process description and simulation

A simplified scheme of the process is shown in Figure 1. The design was based mainly on the description reported in the patent of Holcombe [7]. The operation conditions for the reaction were obtained from literature data [3,4,10]. For the separation section information was retrieved from the works of Silva [11-13]. The feed streams going into the process were a make-up hydrogen feed and a hydrocarbon feed. The hydrogen purity in the make-up stream was about 85 mol%. The impurity consisted of light hydrocarbons (C4-). The hydrocarbon feed had a RON of 74 and contained essentially a mixture of linear and branched C5 and C6 hydrocarbons (about 90 wt%). The component distribution in the hydrocarbon feed was similar to that in a feed stream of a typical hydroisomerzation process [3]. The process feed is normally the result of refinery distillation operations, and thus contains small amounts of heptanes, aromatics and cycloparaffins. The hydrocarbon feed was about 1000 ton/day, which is in the range of industrial hydroisomerization process feed streams (600-1200 ton/day). Hydrogen Feed Reactor Adsorption/ Desorption section Product Purge

The potential of zeolite membranes in hydroisomerization processes

The potential of zeolite membranes in

hydroisomerization processes

ISBN: 90-8559-169-4

The potential of zeolite membranes in

hydroisomerization processes

Proefschrift

Contents

1

General introduction and thesis outline

2

The performance of silicalite-1 membranes in the

separation of linear and branched alkanes

X

X

S =

X

X

Silicalite-1

Silicalite-1

176

36

3

Preparation of zeolite beta membranes and their

pervaporation performance in separating di- from

monobranched alkanes

X

X

S

=

X

X

Elsevier,

Amsterdam,

S.J. Weiner, P.A. Kollman, D.A. Case, U.C. Singh, C. Ghio, G. Alagona, S. Profeta,

P. Weiner,

T. J. H.

Vlugt, R. Krishna, B. Smit, J. Phys. Chem. B, 103 (1999) 1102.

4

Design of a state of the art C

/C

hydroisomerization

process

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