of
of
Light Olefin/Paraffin Mixtures
Dispersion of CuCl in Faujasite Zeolites
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 11 september om 12:30 uur
door
Arjen VAN MILTENBURG
Prof. dr. J.A. Moulijn
Samenstelling promotiecommissie:
Rector Magnificus Technische Universiteit Delft, voorzitter Prof. dr. F. Kapteijn Technische Universiteit Delft, promotor Prof. dr. J.A. Moulijn Technische Universiteit Delft, promotor
Prof. dr. J.C. Jansen Technische Universiteit Delft/Universiteit van Stellenbosch Prof. dr. ir. H. van Bekkum em. hgl. Technische Universiteit Delft
Prof. dr. ir. G.V. Baron Vrije Universiteit Brussel Prof. dr. A.E. Rodrigues University of Porto
Prof. dr. ir. P.J.A.M. Kerkhof Technische Universiteit Eindhoven
Prof. dr. W. Zhu has provided substantial guidance and support in the research presented in this thesis.
Dit onderzoek is uitgevoerd in de sectie Catalysis Engineering, DelftChemTech, Faculteit Technische Natuurwetenschappen, Technische Universiteit Delft met financiële steun van de Technische Universiteit Delft, de International Research Training Group (IRTG) “Diffusion in Porous Materials” (NWO-DFG), en het Network of Excellence “Inside Pores” (EU).
Proefschrift, Technische Universiteit Delft
Met samenvatting in het Nederlands / with summary in Dutch ISBN: 978-90-6464-153-4
Chapter 1 Introduction 1
1.1 General introduction . . . 2
1.2 Production of light olefins . . . 2
1.3 Light olefin/paraffin separation processes . . . 5
1.3.1 Absorption . . . 5
1.3.2 Physical adsorption . . . 6
1.3.3 Supported ʌ-complex adsorbents . . . 7
1.3.4 Membranes . . . 8
1.4 Multi-component adsorption . . . 8
1.5 Outline of the thesis . . . 11
1.6 List of symbols . . . 11
1.7 References . . . 12
Chapter 2 Synthesis and characterization of CuCl/Faujasite for selective 17
olefin adsorption 2.1 Introduction . . . 18
2.1.1 Zeolites . . . 18
2.1.2 Cu+-ion dispersion in zeolites . . . 19
2.1.2.1 Ion-exchange . . . 19
2.1.2.2 Monolayer dispersion . . . 20
2.2 Experimental . . . 21
2.2.1 Synthesis of zeolite NaX . . . 21
2.2.2 Synthesis of CuCl/Faujasite . . . 22
2.2.3 Adsorption of light olefins and paraffins studied with FTIR . . . 23
2.2.4 Gases . . . 25
2.3 Results . . . 25
2.3.1 Synthesis of zeolite NaX . . . 25
2.3.2 Synthesis of CuCl/Faujasite . . . 28
2.3.3 Adsorption of light olefins and paraffins studied with FTIR . . . 34
2.4 Discussion . . . 37
2.4.1 Synthesis of zeolite NaX . . . 37
2.4.2 Synthesis of CuCl/Faujasite . . . 39
2.4.3 Adsorption of light olefins and paraffins studied with FTIR . . . 41
2.5 Conclusions . . . 42
3.2 Experimental . . . 46 3.3 Results . . . 48 3.4 Discussion . . . 54 3.5 Conclusions . . . 57 3.6 Acknowledgements . . . 57 3.7 References . . . 57
Chapter 4 Adsorption of olefins and paraffins on NaX and CuCl modified NaX 59 4.1 Introduction . . . 60 4.2 Theory . . . 61 4.3 Experimental . . . 63 4.3.1 Adsorbents . . . 63 4.3.2 Volumetric method . . . 63 4.3.3 Adsorptives . . . 64 4.4 Results . . . 64 4.4.1 Isotherms . . . 64 4.4.2 Thermodynamics . . . 67
4.4.3 Ideal adsorption selectivity . . . 70
4.5 Discussion . . . 71
4.5.1 Adsorption isotherms . . . 71
4.5.2 Thermodynamics . . . 73
4.5.3 Ideal adsorption selectivity . . . 74
4.5.4 Mixture selectivity . . . 75
4.6 Conclusions . . . 77
4.7 Acknowledgements . . . 78
4.8 List of symbols . . . 78
4.9 References . . . 79
Chapter 5 Binary adsorption of ethylene/ethane and propylene/propane 83
mixtures on NaX and CuCl/NaX 5.1 Introduction . . . 84
5.2 Mass balances . . . 86
5.3 Experimental . . . 89
5.3.1 Breakthrough setup . . . 89
5.3.2 Analysis . . . 92
5.3.3 Breakthrough column fillings . . . 94
5.4.2 36 wt% CuCl/NaX columns . . . 102
5.4.3 NaX-column . . . 107
5.4.4 Single component breakthrough . . . 109
5.4.5 Adsorption capacities and selectivities . . . 110
5.5 Discussion . . . 116
5.5.1 36 wt% CuCl/NaX adsorbent . . . 117
5.5.2 NaX adsorbent . . . 119
5.5.3 Single component breakthrough . . . 120
5.5.4 Adsorption capacities and selectivities . . . 120
5.6 Conclusions . . . 123
5.7 List of symbols . . . 124
5.7.1 Variables . . . 124
5.7.2 Abbreviations . . . 125
5.7.3 Flow sheet abbreviations . . . 125
5.8 References . . . 125
Chapter 6 Light olefin/paraffin separation: Summary, process options and 129 evaluation 6.1 Summary . . . 130
6.2 Evaluation and industrial application . . . 132
6.3 Conclusions . . . 137
6.4 References . . . 138
Samenvatting 139
List of publications and presentations 143
Dankwoord 147
1
1.1 General introduction
Light olefins, like ethylene and propylene are important feedstocks in the chemical industry. In the last decade worldwide production of ethylene and propylene has increased by ~50% to a capacity of more than 75 million tons of ethylene and over 47 million tons of propylene per annum (Table 1.1). In the coming future the production and demands are expected to continue to increase further at a rate of 3-5% per year (The Association of Petrochemical Producers in Europe (Appe) (2006)).
Table 1.1: 2004 worldwide production of light olefins.
Geographical location Ethylene
[Mt a-1] Propylene [Mt a-1] Asia 18 13 Western Europe 21 15 North America 32 17 South America 4 2 Total 75 47
(The Association of Petrochemical Producers in Europe (Appe) (2006))
More than 50% of the monomeric olefins are used directly in polymers (e.g. HDPE, LDPE or PP). The remaining is first converted to other base chemicals like vinyl chloride, styrene, ethylene oxide, propylene oxide, cumene, acrylonitrile and alcohols (The Association of Petrochemical Producers in Europe (Appe) (2006)). These chemicals are then further processed into polymers or chemical products. The use of the light olefins in polymer production sets strict demands to the purities of the olefins. High purities (> 99 wt%) are required and the presence of acetylene should be limited, because it would deactivate the polymerization catalyst and because of safety aspects. Chemical grade olefins allow a lower purity (92-94 wt%) (Kroschwitz (1995)).
1.2 Production of light olefins
unavoidable. A forthcoming process to produce the olefins is the methanol to olefin (MTO) process. Quench Water system Dilution steam generation Acid gas removal Drying Acetylene hydrogenation Fuel gas Ethane Butane LPG Naphtha Cracking furnace Oil fractionation Deethaniser 56 trays 2.5 MPa Depropaniser 55 trays 2 MPa Debutaniser C3 Splitter 180 trays 2 MPa Demethaniser & C2 Splitter
120 trays 2 MPa C5+ C4 Propane Propylene Ethane Ethylene Hydrogen, methane C2 -C3+ C4+ C3 fraction C2 fraction 243 K Fuel oil 321 K 321 K 260 K Quench Water system Dilution steam generation Acid gas removal Drying Acetylene hydrogenation Fuel gas Ethane Butane LPG Naphtha Cracking furnace Oil fractionation Deethaniser 56 trays 2.5 MPa Depropaniser 55 trays 2 MPa Debutaniser C3 Splitter 180 trays 2 MPa Demethaniser & C2 Splitter
120 trays 2 MPa C5+ C4 Propane Propylene Ethane Ethylene Hydrogen, methane C2 -C3+ C4+ C3 fraction C2 fraction 243 K Fuel oil 321 K 321 K 260 K
A typical process flow scheme for the production of ethylene and/or propylene via steam cracking is shown in Fig. 1.1. The feed of a cracker furnace consist of either naphtha, liquefied petroleum gas (LPG) or ethane and butane. In the cracker the larger molecules are cracked and dehydrogenated to methane, ethylene, propylene, C4-gases and other products. A naphtha feed would yield a diverse spectrum of products, while the gases result in larger yields of ethylene and propylene.
After cracking the heavy liquid components are removed first and the cracked gas is further cooled with quench water, followed by compression and scrubbing with caustic soda to remove the acidic gases (H2S and CO2). Before entering the deethaniser to split off the C3+ -fraction, the gas is dried to remove water. After the deethaniser the highly undesired acetylene is removed from the shorter hydrocarbons (C2--fraction) by a selective hydrogenation. Acetylene would seriously deactivate the catalyst that is used in the production of polymers. The C2--fraction is then further cooled and distilled to remove hydrogen and methane. Finally ethylene is obtained in the C2-splitter from the remaining ethane/ethylene mixture. Ethane can be reused in the cracker as a recycle stream.
The C3+-fraction is fed to a depropaniser to obtain a propane/propylene mixture. In the C3-splitter propylene will be obtained. From the heavier gases a fraction, containing C4-olefins, can be obtained in the debutanizer. Propane and these heavier gases can either be used for gasoline or other products or they can be recycled to the cracker.
Table 1.2: US energy demand for distillation.* (Total US energy demand ~ 100,000 PJ a-1.)
Feed Typical components Estimated US
energy demand [PJ a-1]
Petroleum Gasoline/naphtha 520
Crude Oil Light naphtha/heavy
naphtha/light distillate 440
Liquefied petroleum gas (LPG) Ethane/propane/butane 230
Olefins Ethylene/ethane,
propylene/propane 130
Miscellaneous hydrocarbons Cumene/phenol,
acetone/acrylonitrile 110
Water-oxygenated hydrocarbons Methanol/water, water/acetic
acid 110 Aromatics Ethylbenzene/styrene, benzene/toluene 80 Water-inorganics Ammonia/water 60 Air Nitrogen/Oxygen 20 Other 320
Total for distillation 2,000
*
E.g. the first row gives the energy demand needed to separate a petroleum feed into a gasoline and naphtha fraction via distillation.
Currently the separation of light olefin/paraffin mixtures is performed at high pressure in large and tall cryogenic distillation columns containing over one hundred trays (Fig. 1.1). The small divergence in relative volatility between ethylene and ethane (Į < 1.5 (Barclay, Flebbe, and Manley (1982))) or propylene and propane (Į = 1.09 (Sloley (2001))) makes it a very energy intensive process. Distillation is the largest energy consumer within the chemical industry. Besides the distillation of crude oil, petroleum and LPG (of which much larger amounts are produced for use as fuel), the US annual energy demand for the cryogenic olefin/paraffin separation is up to 130 PJ (Table 1.2). For The Netherlands this separation of ethylene/ethane and propylene/propane mixtures accounts for 5-15 PJ per annum (Vente, (2006)). Based on the production data provided in Table 1.1, and an annual growth of 3-5%, the annual worldwide energy demand for olefin/paraffin separation will be over 300 PJ for approximately 130 Mton of olefins.
Besides the high energy demands, the safety requirement for cryogenic distillation will also be very demanding, since the flammable compressed and cooled liquefied gas could explode in case of cooling failure, in particular since the olefin and paraffin themselves are integrated with the cooling system of the separation process.
1.3 Light olefin/paraffin separation processes
The continuing rise in crude oil prices and the growing responsibility to reduce the emissions of greenhouse gases resulted in many research projects to find alternative separation processes to fulfil the growing demand of olefins and to reduce the energy demands (Bryan (2004); Eldridge (1993)). Most attention is paid to absorption, adsorption and membrane based processes, often aided by a bonding of the olefin, e.g. via ʌ-complexation.
1.3.1 Absorption
released and the olefin can desorb from the complex. The solution contains the M+-ion, either Cu+ or Ag+. To this purpose metal-salts, such as AgNO3, AgClO4 or AgBF4, are dissolved at a high concentration in water or another solvent. The use of Cu+ salts in water is not preferred, because of the low stability of Cu+ in water. Here options are CuTFA (cuprous trifluoroacetate) in propionitrile or aromatic solvents or CuAlCl4 in toluene. For AgNO3 ionic liquids have been reported to be attractive solvents (Kang et al. (2006); Won et al. (2005)).
C
C
M
+
+
-+
+
+
-+
C
C
M
+
+
-+
+
+
-+
Fig. 1.2: Schematic picture of the Dewar-Chatt model
for ʌ-complexation (Safarik and Eldridge (1998)).
1.3.2 Physical adsorption
Separation via physical adsorption onto a porous solid can be based on a couple of physical differences between the olefin and paraffin. For size exclusion only one component should be able to enter the internal pore structure of the adsorbent. If both components can enter the pore, differences in the diffusivity may allow kinetic separation. Alternatively, separation may be possible at equilibrium, either via an affinity or entropy driven adsorption.
Another possibility is to use the differences in diffusion rate of the molecules. When the pores of the adsorbent are close to the size of the molecules, the transport can be limited by the constraints of the pore size. Via this kinetic separation the fastest diffusing component will initially be the main component in the interior of the adsorbent, while the other component will primarily remain outside. In practical operation the adsorption should then be stopped before equilibrium is reached.
In reality there is a grey area of overlap between these two ways to separate olefin/paraffin mixtures. The influence of the temperature and the framework flexibility sometime allow the adsorption of a component whose critical diameter is larger than the rigid pore diameter mentioned in literature. However the diffusion of this component will be much slower and can be almost zero.
The last method to achieve a selective adsorption of the olefins is based on the affinity of and/or packing efficiency in the adsorbent towards the components in the mixture. Both components can penetrate the adsorbent at a sufficiently high diffusion rate and have an observable adsorption capacity. A separation based on the difference in adsorption affinity of the components relates to their adsorption enthalpies. In case of a separation based on the packing efficiency, the entropy is controlling the selectivity. The entropy effects become important at high loadings. In a practical process a binary equilibrium will be established in the adsorbent. Zeolites with a larger pore, which allow the adsorption of both components, like NaX or 5A (Da Silva and Rodrigues (1999); Huang et al. (1995); Järvelin and Fair (1993)), mesoporous structures (Grande et al. (2004); Newalkar et al. (2003)), and Kureha activated carbon (Zhu et al. (2005)) did show a selective adsorption of the olefins.
1.3.3 Supported ʌ -complex adsorbents
The selectivity of adsorbents can be improved by the introduction of metal-ions that selectively bind to the olefins. An example is the introduction of Cu+ or Ag+-ions on the support. They form a relatively stable ʌ-complex with the olefin, while the paraffin only adsorbs via the weak Van der Waals physical adsorption forces. To obtain a large number of adsorption sites on the supports, a high dispersion of the metal-ions is required. Examples of supports modified with M+-ions are: resins (Hirai, Kurima, and Komiyama (1986); Wu et al. (1997)), Ȗ-Al2O3 (Blas, Vega, and Gubbins (1998)), SiO2 (Padin et al. (2000); Rege, Padin, and Yang (1998)), pillared clays (Choudary et al. (2002)), carbons (Hirai, Komiyama, and Keiichiro (1988); Mei et al. (2002)), mesoporous silica (Grande et al. (2005)) and zeolites (Pearce (1988); Takahashi, Yang, and Yang (2002)).
adsorption/desorption columns have been developed to obtain the highest purity and to use the compression/decompression power most efficiently. Adsorptive separation of light olefin/paraffin mixtures is still limited to the research stage. An adsorptive separation, especially if continuous membrane operation would be used, could result in a decrease of the energy cost to ~ 60% of that used nowadays for cryogenic distillation of light olefin/paraffin mixtures (Eldridge (2005)).
1.3.4 Membranes
Membranes allow the conversion of a discontinuous adsorption based separation of light olefin/paraffin mixtures into a continuous operation mode. In membranes the component with the larger diffusivity and/or strongest adsorption affinity will be selectively transported through the membrane layer, while the other components remain on the retentate side of the membrane. The continuous membrane operation has the advantage that no energy is lost in the compression/decompression cycles for PSA operations or the temperature cycles for TSA operations.
Olefin/paraffin selective membranes can be constructed from polymers (or carbonized polymers) or zeolites (Giannakopoulos and Nikolakis (2005)). Since the flow through the membrane is primarily controlled by the diffusion rate, a key factor for these membranes is to obtain a high selectivity while maintaining a high flux. Thicker membranes could result in a high selectivity, but they also increase the diffusion length. Thinner membranes could show a lower selectivity because of defects in the membrane layer. These defects are pinholes through which transport of all components occurs, which can become larger than the diffusive flow through the pores of the membrane thus reducing the selectivity of the membrane.
Polymeric membranes are also used to separate the gas stream from a solution containing a M+-salt (Duan, Ito, and Ohkawa (2003); Nymeijer et al. (2004)). The olefins will diffuse through the membrane and absorb in the solution. Via another membrane separation unit, or at the other side of a ‘supported liquid membrane’, the olefins will desorb from the solution and are collected at the low pressure side.
1.4 Multi-component adsorption
In the other cases, like for the olefin/paraffin separation, information about the influence of the other components on the adsorption behaviour of each individual component will be required. The other components will also adsorb, and therefore the available volume (or surface area) for each component will be restricted by the others. Because of this competition between all components for the volume (or surface area) available in the adsorbent and the volume available in the gas phase, the adsorbed amounts will deviate from their single component adsorption isotherms.
To describe this multi-component adsorption several models have been developed (Yang (1987)). Starting from the single component Langmuir adsorption isotherms, two models have often been applied for binary adsorption. The single component Langmuir isotherms can be extended to a multi-component isotherm. The adsorption capacity of component i in a mixture of N components is then determined via the following equation:
¦
N j j j i i i i p K p K q q 1 max (1.1)The constants in the equation are then considered to be (a simple function of) the adsorption constants obtained from the measurement of single component isotherms. A similar modification can be applied to some of the other single component isotherm models.
This extension is only valid if the saturation capacities of both components are the same. If the saturation capacities differ, which is generally the case, entropy considerations enter the picture and only a thermodynamic consistent theory will be appropriate to predict the mixture adsorption. An alternative model for mixture adsorption that uses the data of the single component isotherms is the Ideal Adsorbed Solution (IAS) theory (Myers and Prausnitz (1965)). For this model the thermodynamic approach used for liquid-vapour equilibrium is extended to the gas-adsorbent equilibrium. For an ideal gas the chemical potential of each component in the adsorbed phase is given by:
( )
ln ) ( ) , , ( S 1 0 J 0 S Pi T x gi T RgT ixiPi (1.2)
where gi0(T) is the molar Gibbs free energy of component i at 101.325 kPa, Ȗi is the activity
coefficient of component i in the adsorbed phase, xi is the mol fraction of component i in the
adsorbed phase and Pi0(ʌ) is defined as the equilibrium pressure for pure component i at the
spreading pressure ʌ. In the ‘Ideal’ case the activity coefficient of all components is equal to
The spreading pressure of each component is calculated from the single component isotherm. Therefore the following integral has to be solved:
³
w 0 0 0 0 ln ) ( ) ( i P p i i q p p a RT PS
(1.3)
In this equation the single component isotherms are used to calculate loading qi0(p) at each pressure. At the binary equilibrium pressure, the spreading pressure of all components will be equal. For the mixed gas phase the chemical potential is given by:
y P RT T g y T i i i( , , ) ( ) ln 0 1 S P (1.4)
where yi is the mol fraction in the gas phase and P the total pressure.
In case of thermodynamic equilibrium the chemical potential of the gas phase and the adsorbed phase will be equal. For the Ideal Adsorbed Solution (IAS), the combination of Eq. 1.2 and 1.4 results in the following expression which should be satisfied for each component in the mixture: ) ( 0 S i i iP xP y (1.5)
Further the following equations are required to calculate the Ideal Adsorbed Solution composition: 1
¦
¦
yi xi (1.6)¦
0 1 i i t q x q (1.7) t i i xq q (1.8)where qi0 is the amount of component i adsorbed at spreading pressure ʌ in the absence of the other components.
1.5 Outline of the thesis
In this introduction an overview of current practice and alternative techniques to separate the light olefin/paraffin mixtures was given. As alternative, adsorptive separation is considered. Therefore the theoretical background of the Ideal Adsorbed Solution (IAS) theory for mixture adsorption was presented in this introduction chapter. The central theme of this thesis is the development, optimization and application of potential zeolite-based adsorbents for olefin/paraffin separation.
Specifically Faujasite zeolites and CuCl modified Faujasite zeolites are investigated for the separation of ethane/ethylene and propane/propylene mixtures. In Chapter 2 the synthesis optimization of uniform large zeolite NaX crystals is described. In these large crystals CuCl is dispersed and the adsorbent is characterized by different techniques such as X-Ray Diffraction (XRD), Thermo Gravimetric Analysis (TGA), DRIFT and FTIR transmission spectroscopy.
The observed colour changes and changes in the morphology of CuCl/NaX samples after prolonged storage in the atmosphere, resulted in a study to the stability of the CuCl/Faujasite zeolites. The effect of the exposure to humid air or water vapour on these adsorbents is presented in Chapter 3.
The single component adsorption results of ethane, ethylene, propane and propylene on NaX and the optimized CuCl/NaX adsorbent and their modelling are presented in Chapter 4.
For the actual application in an adsorption process, binary adsorption data will be required. Therefore a new adsorption breakthrough setup was constructed. In Chapter 5 a description of the setup is presented. Binary adsorption equilibrium data were determined for ethylene/ethane and propylene/propane (50:50) mixtures. The data obtained with these measurements are compared with the predictions of the Ideal Adsorbed Solution (IAS) theory using the single component isotherms presented in Chapter 4.
In Chapter 6 a summary of the previous chapters is given, the performance of the CuCl/NaX and NaX adsorbents for the separation of light olefin/paraffin mixtures is evaluated, possible process options are discussed, and the final conclusions are drawn.
1.6 List of symbols
a Specific area of the adsorbent [m2 kg-1]
gi0 Molar Gibbs free energy of component i [J mol-1]
Ki Adsorption constant for component i [Pa-1]
N Number of components in gas mixture [-]
P Total pressure [Pa-1]
Pi0 Equilibrium pressure for pure component i at ʌ [Pa-1]
qi Adsorbed amount of component i [mol kg-1]
xi Mol fraction of component i in the adsorbed phase [-] yi Mol fraction of component i in the gas phase [-]
Į Relative volatility [-]
Ȗi Activity coefficient of component i [-]
ȝi Chemical potential of component i [J mol -1]
ʌ Spreading pressure of the adsorbed phase [Pa m]
1.7 References
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Barrer, R. M., Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London (1978).
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Separations by Adsorption via Pi-Complexation, Fluid Phase Equilibr. 150 (1998) 117-124.
Bryan, P. F., Removal of Propylene from Fuel-Grade Propane, Separ. Purif. Rev. 33 (2004) 157-182.
Buyanov, R. A. and Pakhomov, N. A., Catalysts and Processes for Paraffin and Olefin Dehydrogenation, Kinet. Catal. 42 (2001) 64-75.
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Choudary, N. V., Kumar, P., Bhat, T. S. G., Cho, S. H. and Han, S. S., Adsorption of Light Hydrocarbon Gases on Alkene-Selective Adsorbent, Ind. Eng. Chem. Res. 41 (2002) 2728-2734.
Da Silva, F. A. and Rodrigues, A. E., Adsorption Equilibria and Kinetics for Propylene and Propane over 13X and 4A Zeolite Pellets, Ind. Eng. Chem. Res. 38 (1999) 2051-2057. Do, D. D., Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London
(1998).
Duan, S., Ito, A. and Ohkawa, A., Separation of Propylene/Propane Mixture by a Supported Liquid Membrane Containing Triethylene Glycol and a Silver Salt, J. Membrane Sci. 215 (2003) 53-60.
Eldridge, R. B., Olefin/Paraffin Separation Technology: A Review, Ind. Eng. Chem. Res. 32 (1993) 2208-2212.
Eldridge, R. B., Brainstorming Session Background Information as Part of the Hybrid Technology Workshop Separations Research Program in Austin, TX, USA, (2005).
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Grande, C. A., Araujo, J. D. P., Cavenati, S., Firpo, N., Basaldella, E. and Rodrigues, A. E., New Pi-Complexation Adsorbents for Propane-Propylene Separation, Langmuir 20 (2004) 5291-5297.
Grande, C. A., Firpo, N., Basaldella, E. and Rodrigues, A. E., Propane/Propylene Separation by SBA-15 and Pi-Complexated AG-SBA-15, Adsorption 11 (2005) 775-780.
Grande, C. A. and Rodrigues, A. E., Propane/Propylene Separation by Pressure Swing Adsorption using Zeolite 4A, Ind. Eng. Chem. Res. 44 (2005) 8815-8829.
Herberhold, M., Metal Pi-Complexes: Part II: Specific Aspects, Elsevier, New York (1974). Hirai, H., Komiyama, M. and Keiichiro, W., Solid Adsorbent for Unsaturated Hydrocarbon
and Process for Separation of Unsaturated Hydrocarbon from Gas Mixture, US Patent 4 747 855 (1988).
Hirai, H., Kurima, K. and Komiyama, M., Selected Solid Ethylene Adsorption Composed of Copper (I) Chloride and Polystyrene Resin Having Amino Groups, Polym. Mater. Sci. Eng. 55 (1986) 464-468.
Honig, J. M. and Reyerson, L. H., Adsorption of Nitrogen, Oxygen, and Argon on Rutile at Low Temperatures; Applicability of the Concept of Surface Heterogeneity, J. Phys. Chem. 56 (1952) 140-146.
Huang, H. Y., Padin, J. and Yang, R. T., Comparison of Pi-Complexations of Ethylene and Carbon Monoxide With Cu+ and Ag+, Ind. Eng. Chem. Res. 38 (1999) 2720-2725.
Huang, Y.-H., Liapis, A. I., Xu, Y., Crosser, O. K. and Johnson, J. W., Binary Adsorption and Desorption Rates of Propylene-Propane Mixtures on 13X Molecular Sieves, Separ. Technol. 5 (1995) 1-11.
Humphrey, J. L. and Keller, G. E., Separation Process Technology, McGraw-Hill, New York (1997).
Järvelin, H. and Fair, J. R., Adsorptive Separation of Propylene-Propane Mixtures, Ind. Eng. Chem. Res. 32 (1993) 2201-2207.
Kang, S. W., Char, K., Kim, J. H., Kim, C. K. and Kang, Y. S., Control of Ionic Interaction in Silver Salt-Polymer Complexes with Ionic Liquids: Implications for Facilitated Olefin Transport, Chem. Mater. 18 (2006) 1789-1794.
Kotelnikov, G. R., Komarov, S. M., Bespalov, V. P., Sanfilippo, D. and Miracca, I., Application of FBD Processes for C-3-C-4 Olefins Production from Light Paraffins, Stud. Surf. Sci. Catal. 147 (2004) 67-72.
Kotelnikov, G. R., Komarov, S. M., Titov, V. I. and Bespalov, V. P., New Propylene Production Process, Petro. Chem. 41 (2001) 422-427.
Kroschwitz, J. I., Kirk-Othmer Encyclopedia of Chemical Technology, Wiley, New York (1995) Vol. 9 pp. 877 and Vol. 20 pp. 257.
Mei, H., Hu, C. G., Liu, X. Q. and Yao, H. Q., Study of Activated Carbon Supported CuCl for Ethylene/Ethane Separation by Adsorption: Effects of Oxidative Treatment, New Carbon Mat. 17 (2002) 33-37.
Moulijn, J. A., Makkee, M. and van Diepen, A., Chemical Process Technology, John Wiley & Sons, Chichester, England (2001).
Myers, A. L. and Prausnitz, J. M., Thermodynamics of Mixed-Gas Adsorption, AIChE J. 11 (1965) 121-127.
Newalkar, B. L., Choudary, N. V., Turaga, U. T., Vijayalakshmi, R. P., Kumar, P., Komarneni, S. and Bhat, T. S. G., Potential Adsorbent for Light Hydrocarbon Separation: Role of SBA-15 Framework Porosity, Chem. Mater. 15 (2003) 1474-1479.
Nymeijer, D. C., Visser, T., Assen, R. and Wessling, M., Composite Hollow Fiber Gas-Liquid Membrane Contractors for Olefin/Paraffin Separation, Sep. Purif. Technol. 37 (2004) 209-220.
Olson, D. H., Camblor, M. A., Vilaescusa, L. A. and Kuehl, G. H., Light Hydrocarbon Sorption Properties of Pure Silica Si-CHA and ITQ-3 and High Silica ZSM-58, Micropor. Mesopor. Mat. 67 (2004) 27-33.
Orica Limited, Ethylene Chemical Fact Sheet: Ethylene Production: Process Flow Diagram, (1999).
Padin, J., Rege, S. U., Yang, R. T. and Cheng, L. S., Molecular Sieve Sorbents for Kinetic Separation of Propane/Propylene, Chem. Eng. Sci. 55 (2000) 4525-4535.
Pearce, G. K., Selective Adsorption and Recovery of Organic Gases using Ion-Exchanged Faujasite, US Patent 4 717 398 (1988).
Rege, S. U., Padin, J. and Yang, R. T., Olefin/Paraffin Separation by Adsorption: Pi-Complexation vs. Kinetic Separation, AIChE J. 44 (1998) 799-809.
Reine, T.A., Olefin/Paraffin Separation by Reactive Adsorption, PhD Thesis, The University of Texas, Austin, USA (2004).
Ruthven, D. M., Farooq, S. and Knaebel, K. S., Pressure Swing Adsorption, VCH Publishers, New York (1994).
Safarik, D. J. and Eldridge, R. B., Olefin/Paraffin Separations by Reactive Absorption: A Review, Ind. Eng. Chem. Res. 37 (1998) 2571-2581.
Sloley, A. W., Distillation Operations Manual, www.distillationgroup.com (2001).
Takahashi, A., Yang, F. H. and Yang, R. T., New Sorbents for Desulfurization by Pi-Complexation: Thiophene/Benzene Adsorption, Ind. Eng. Chem. Res. 41 (2002) 2487-2496.
Ter Horst, J. H., Bromley, S. T., Van Rosmalen, G. M. and Jansen, J. C., Molecular Modelling of the Transport Behaviour of C-3 and C-4 Gases through the Zeolite DD3R, Micropor. Mesopor. Mat. 53 (2002) 45-57.
The Association of Petrochemical Producers in Europe (Appe), Western European Market Review, www.petrochemistry.net (2006).
Butterworth-Toth, J., State Equations of Solid-Gas Interface Layers, Acta. Chim. Hung. 69 (1971) 311-328.
Vente, J., Hydrocarbon Separation, Energy research Centre of the Netherlands (ECN), (2006). Vlugt, T. J. H., Zhu, W., Kapteijn, F., Moulijn, J. A., Smit, B. and Krishna, R., Adsorption of
Linear and Branched Alkanes in the Silicalite-1, J. Am. Chem. Soc. 120 (1998) 5599-5600. Wang, S. and Zhu, Z. H., Catalytic Conversion of Alkanes to Olefins by Carbon Dioxide
Oxidative Dehydrogenation-A Review, Energ. Fuel. 18 (2004) 1126-1139.
Weyten, H., Luyten, J., Keizer, K., Willems, L. and Leysen, R., Membrane Performance: the Key Issues for Dehydrogenation Reactions in a Catalytic Membrane Reactor, Catal. Today 56 (2000) 3-11.
Won, J., Kim, D. B., Kang, Y. S., Choi, D. K., Kim, H. S., Kim, C. K. and Kim, C. K., An Ab Initio Study of Ionic Liquid Silver Complexes as Carriers in Facilitated Olefin Transport Membranes, J. Membrane Sci. 260 (2005) 37-44.
Wu, Z. B., Han, S. S., Cho, S. H., Kim, J. N., Chue, K. T. and Yang, R. T., Modification of Resin-Type Adsorbents for Ethane/Ethylene Separation, Ind. Eng. Chem. Res. 36 (1997) 2749-2756.
Yang, R. T., Gas Separation by Adsorption Processes, Imperial College Press, London (1987).
Zhu, W., Groen, J. C., Van Miltenburg, A., Kapteijn, F. and Moulijn, J. A., Comparison of Adsorption Behaviour of Light Alkanes and Alkenes on Kureha Activated Carbon, Carbon 43 (2005) 1416-1423.
2
Synthesis and characterization of
CuCl/Faujasite for selective olefin
adsorption
The synthesis of large NaX crystals and the dispersion of CuCl in Faujasite zeolites have been optimized. TGA and XRD show a maximum dispersion capacity of 36 wt% for NaX and 43 wt% for NaY, corresponding to approximately 10 CuCl molecules per super cavity. SEM and TEM analysis showed that the CuCl was well dispersed inside the zeolite crystal. A DRIFT study, with CO as a probe molecule confirmed that at 623 K CuCl was dispersed in the zeolite. It also showed that the adsorption of CO on CuCl is stronger than on the Na+-ion of the Faujasite zeolite, due to its strong ʌ-complexation with Cu+.2.1 Introduction
The separation of olefin/paraffin mixtures can be achieved by adsorption using the selective interaction of the olefin with metal-ions, like Cu+ or Ag+, forming a ʌ-complex (Herberhold (1974); Chapter 1 of this thesis). The Cu+ is the cheaper element of these two and shows a higher affinity with the olefins (Yang (2003)) and is therefore chosen as the selective metal-ion in this study. In order to create a large number of adsorption sites for the olefin, these ions have to be dispersed over a large surface area. Supports with a large surface areas on which these metal-ions were dispersed earlier are resins (Hirai, Kurima, and Komiyama (1986); Wu et al. (1997)), Ȗ-Al2O3 (Blas, Vega, and Gubbins (1998); Yang and Kikkinides (1995)), carbons (Hirai, Komiyama, and Keiichiro (1988); Mei et al. (2002)), SiO2 (Padin et al. (2000); Rege, Padin, and Yang (1998)), pillared clays (Choudary et al. (2002)), mesoporous silica (Grande et al. (2005)) and zeolites (Pearce (1988); Takahashi, Yang, and Yang (2002)). Compared to the other supports, zeolites have a very uniform accessible nanoporous crystal structure, large surface areas and a good thermal and chemical stability.
2.1.1 Zeolites
There are several types of zeolites discovered so far, both natural and artificial (Baerlocher and McCusker (2006)). Zeolites are widely used in the petrochemical industry as catalysts or adsorbents for several processes (Moulijn, Makkee, and van Diepen (2001)). For the use of the selective adsorption of olefins, the zeolite should have pores which are large enough to allow the adsorbing gases to enter the cavities via its window openings and to allow a fast diffusion of these gases in the interior of the zeolite crystal. The selectivity of the zeolites towards the olefins can be further improved by the dispersion of metal-ions in the zeolite cavities. A good interaction of these dispersed metal-ions with the support would be preferred in order to prevent the continuous loss of metal-ion adsorption sites, e.g. via sintering. Because of the positive charge of the metal-ion, a negatively charged structure would result in a good interaction. In zeolite structures negative charges can be formed in the structure by replacing part of the Si-atoms with Al-atoms. This is quantified by the so-called Si/Al-ratio of the zeolite. During the synthesis this negative charge will be compensated by positive ions (e.g. Na+) in the synthesis solution. These ions will remain in the cavities of the zeolite at the end of the synthesis, but can later on be exchanged by other ions.
6-structure. For zeolite X this ratio should be between 1.0-1.5, while for zeolite Y a ratio above 1.5 is used (Breck (1974)). The cations, which counterbalance the charge of the zeolite framework, are located in the sites indicated in Fig. 2.1. For zeolite Y the sites I, I’ and II are (partially) occupied (Fitch, Jobic, and Renouprez (1986)), while for zeolite X sites I, I’, II and III’ are (partially) occupied (Olson (1995)).
A fast adsorption equilibrium throughout the crystal is best achieved when small crystals are used. However small crystals would result in a severe pressure drop when they are used in packed bed adsorption columns, which is not preferred in research applications. To overcome this pressure drop, larger pellets can be pressed from the small crystals. In many cases a binder would be required to maintain the pellet structure. These pellets could result in an additional transport barrier for the adsorption and desorption of the gases and would complicate modelling. Another option is to synthesize large zeolite crystals. For the research application of the large zeolite crystals in adsorption measurements important factors are the phase purity and particle size distribution of the synthesized crystals.
ƍ
ƍ
ƍ
S6R
D6R
sodalite
cage
super
cavity
ƍ
ƍ
ƍ
S6R
D6R
sodalite
cage
super
cavity
Fig. 2.1: Schematic picture of the Faujasite crystal structure and the cation sites.
2.1.2 Cu+-ion dispersion in zeolites
To improve the selectivity of the zeolite for the selective adsorption of olefins, Cu+-ions will be dispersed in the zeolite structure. Several techniques have been developed, including ion-exchange and monolayer dispersion (Yang (2003)).
of the ions in the solution, the nature of the cations, the temperature, the anions of the salt, the solvent, and the structure of the zeolite. The exchange properties of zeolites are widely used in processes, both in industry (catalyst manufacture) and by consumers (softening of water).
A typical procedure for the ion-exchange of a zeolite would start with dissolving a salt containing the preferred metal-ion in water or an other solvent. The solution is contacted with the zeolite, either batch-wise by suspending the zeolite particles in the solution, or continuously by circulating the solution through a bed of zeolite crystals. After the exchange the zeolites are filtered and washed with water. In order to enhance the degree of ion-exchange in a batch (or semi-batch) process, the procedure is often carried out multiple times. Unfortunately this procedure does not work for Cu+ salts, since they are insoluble in water and will oxidize to Cu2+ in the solution. A possible alternative is to perform the ion-exchange with a Cu2+-salt. The Cu2+-ions are exchanged as [Cu2+OH]+ and can be converted to Cu+ by thermal auto-reduction at 573-723 K in helium, whereby the following reactions occur (Takahashi et al. (2001)):
2 [CuOH]+ĺ [Cu2O]2+ + H2O [Cu2O]2+ĺ 2 Cu+ + ½ O2
After completion of this auto-reduction reaction the Cu+ exchanged form of the zeolite will be obtained.
2.1.2.2 Monolayer dispersion
Dispersion of salt onto solid substances can be achieved using two techniques, either via thermal dispersion (Chen and Sachtler (1998); Xie et al. (1996)) or via incipient wetness impregnation. For thermal dispersion of a salt onto a solid surface, the two solids are thoroughly mixed. The mixed solids are heated to a temperature between the Tamman temperature of the salt and its melting point (Tm). Above the Tamman temperature, approximately ½ Tm, the crystal structure of the salt becomes more flexible and mobile. The salt molecules can diffuse over the surface of the support and a dispersed layer is formed. This process is also known as a ‘solid state ion-exchange’.
In this study large crystal CuCl/NaX adsorbents will be synthesized. Therefore first the synthesis of large crystals of NaX will be optimized and these will be characterized using XRD, SEM and N2-physisorption. Thereafter CuCl will be dispersed on the surface of the pores of Faujasite zeolites using the thermal dispersion method. Compared to the other techniques this is the most simple and effective method and high loadings can be obtained. In most cases an ion-exchange or an incipient wetness impregnation would require an additional reduction step, which would complicate the synthesis procedure and would affect the reproducibility. Since the thermal dispersion process of CuCl on Faujasite is poorly characterized in the literature, it is further optimized using various characterization techniques, like Thermogravimetric analysis (TGA), XRD, SEM/EDX, TEM and FTIR spectroscopy. For the IR study, CO will be used as a probe molecule in the DRIFT-cell.
In order to characterize the adsorbent for its application in an adsorptive olefin/paraffin separation process, the adsorbent should show a higher affinity for one of the components in the mixture. A quick screening can be performed by studying the surface complexes on the adsorbent by means of FTIR spectroscopy. Unfortunately the gas phase of the olefins and paraffins also absorb part of the infrared light. A preliminary study in the DRIFT cell indicated that the absorbance of the adsorbed components could hardly be distinguished from the gas phase contribution at atmospheric pressures. Therefore the adsorption of ethane, ethylene, propane and propylene was tested in the low-pressure IR transmission cell.
2.2 Experimental
2.2.1 Synthesis of zeolite NaX
Large zeolite NaX crystals were synthesized using two recipes reported in the literature (Charnell (1971); Qiu et al. (1998)) and a modified version of the recipe of Qiu (Van Miltenburg et al. (2006)). Following the recipe of Qiu, an Al-solution was made by dissolving NaAlO2 (Riedel-de Haën) in a sodium hydroxide solution in water. Triethanolamine (J.T. Baker) was added as a stabilizing and buffering or complexing agent. The solution was filtered twice through a 0.2 ȝm filter to remove remaining particulates and other impurities. A Si-solution was made by dissolving Aerosil 90 (kindly provided by Degussa) in water. Finally triethanolamine was added to the Si-solution. Both solutions were aged for about 1 hour after which they were mixed together. The molar ratio of the ingredients was: SiO2 : NaAlO2 : NaOH : TEA : H2O = 1 : 1.4 : 4.3 : 2 : 222. The gel was kept at 353 K so the crystallization of the zeolites could occur. After 2-3 weeks the crystals were filtered, washed thoroughly and dried at 353 K for 1 day. To remove small silica particles attached to the crystals, they were ultrasonically cleaned in ethanol and dried afterwards at 373 K in air. The obtained crystals were divided into several sieve fractions.
The synthesized NaX crystals were analyzed with XRD (Bruker-AXS D5005, CuKĮ) and SEM (Philips XL20, 15kV). The Si/Al-ratio of the zeolite was determined by ICP-OES (PerkinElmer Optima 3000DV). The Na content in the zeolite was determined by AAS. The porous properties were determined by volumetric N2-physisorption at 77 K (Quantachrome Autosorb-6B).
The particle size distributions were determined with a Mastersizer S, with a 300 mm RF lens in a flow-through cell. The particles were dispersed in demineralised water. To break up agglomerated particles, the dispersion was exposed for 4 minutes to ultrasonic vibrations.
2.2.2 Synthesis of CuCl/Faujasite
Physical mixtures were prepared by mixing different amounts of CuCl (Fluka) with the synthesized NaX crystals or commercial NaY (Zeolyst CV100, Si/Al = 2.55). These physical mixtures were slowly heated (1 K min-1) in the quartz reactor to 623 K in flowing argon with a rate of 100 ml (STP) min-1 and at this temperature the samples were heated for typically 4 h. Thereafter heating was stopped and the temperature slowly returned to room temperature.
To limit the influence of the exposure to the ambient atmosphere to the minimum, some mixtures were also prepared in a ¼” stainless steel tube, which could be closed from the atmosphere with two valves. The physical mixtures were fixed in a 3 cm long tube between two 1 mm thick stainless steel frits, with pore openings of 0.5 ȝm. After the dispersion procedure, the two valves were closed and the samples were removed and stored in a glove box under nitrogen to prevent the exposure of the CuCl to ambient air.
The TGA experiments of Faujasite and of the physical mixtures of CuCl and Faujasite were performed in a Mettler Toledo TGA/SDTA851e. Depending on the composition for the experiments, a sample amount ranging from 15 to 40 mg was inserted in an alumina TGA cup of 70 ȝl. For all TGA experiments the volume of the sample in the cup was approximately equal; therefore the amount of Faujasite was similar for all experiments.
Once the samples containing NaY were inserted in the TGA, they were purged for 30 min at 298 K with a helium flowrate of 100 ml (STP) min-1. The temperature was then slowly raised (1 K min-1) to 373 K and this temperature was kept for 1 h. Then the temperature was further increased to 623 K at 2 K min-1. After that a temperature of 623 K was kept for typically 4 h. To investigate the effect of a longer dispersion time, samples with an excess of CuCl were heated at 623 K for 8h.
The XRD patterns were recorded for CuCl/Faujasite and for physical mixtures of CuCl and Faujasite with a Philips PW1830/40 diffractometer using CuKĮ radiation. The dispersed samples were synthesized beforehand in the quartz reactor, after which the XRD patterns were recorded immediately to limit exposure to the ambient air.
The DRIFT experiments of CO adsorption were performed in a Nicolet Nexus FTIR at 323 K. The DRIFT-cell was equipped with KBr windows and absorption spectra were recorded with a nitrogen cooled MCT detector. CO adsorption was performed on NaY and 43 wt% CuCl/NaY (weight percentage based on the dry NaY mass). To minimize the exposure to the air, the dispersion of CuCl on NaY was achieved inside the DRIFT cell by increasing the temperature to typically 623 K at 1 K min-1 in flowing helium with a rate of 100 ml (STP) min-1 and at this temperature the samples were heated for 4 h. Before measurement the samples were rapidly cooled (>150 K min-1) to 323 K, at which temperature the adsorption of a mixture of 5 vol% CO in helium on the adsorbent was monitored. The desorption was achieved by returning to the pure helium flow. Evolved gas analysis by a mass spectrometer confirmed that the gas phase composition change was established within seconds. In order to calculate the absorbance, the background spectra of the samples after the dispersion of CuCl on NaY, but before CO adsorption were recorded at 323 K. To investigate the effect of the CuCl-dispersion a physical mixture of 43 wt% CuCl and NaY was only heated to 423 K for 4 h to evaporate most of the adsorbed water, after which the temperature was rapidly decreased to 323 K for CO adsorption.
The synthesized CuCl/NaX was further characterized using SEM (Philips XL20, 15kV) and TEM (Philips CM30T, 300 kV, with LINK EDX system). For the TEM analysis a suspension of ground small crystals in hexane was dropped on a microgrid carbon polymer supported on an aluminium grid, followed by drying at ambient conditions, all in an argon glovebox. Samples were transferred to the microscope in a special vacuum-transfer sample holder under exclusion of air.
2.2.3 Adsorption of light olefins and paraffins studied with FTIR
The adsorption behaviour of light olefins and paraffins was investigated using a low-pressure IR transmission cell equipped with KBr windows (Miura and Gonzalez (1982)). An electric heating coil around the transmission cell allowed the control of the temperature inside the vacuum chamber up to 398 K. The transmission cell was installed inside an FTIR spectrometer (Nicolet Magna-IR 860) and absorption spectra were recorded using the DTGS detector. The transmission cell was connected to a gas introduction/evacuation system, of which the flowsheet is shown in Fig. 2.2. From a fixed volume (B) a small amount of a gas mixture could be introduced to the transmission cell.
lines of the setup and the transmission cell via the 1st rotary pump. When all valves were opened completely and the pressure slowly started to decrease, the temperature of the transmission cell was slowly increased to 398 K in steps of 10 K. During heating the pressure in the cell was continuously monitored so the heating could be stopped wherever the pressure started to increase too rapidly. At the higher temperatures the adsorbed water started to desorb and evaporated from the adsorbent wafer. Once the pressure in the mixture chamber (PB in Fig. 2.2) was below 10-2 mbar, valve V1 was slowly opened. This way the turbo pump could further reduce the pressure. Because of the slow pressure decrease and slow temperature increase applied in this experimental procedure, the pressure throughout the setup and across the wafer would be approximately equal and would therefore reduce the chance of cracking the thin wafer.
Once the pressure in the fixed volume (PB) remained stable at the minimum detection limit (1.0 10-7 mbar), the temperature was lowered to 323 K. In order to calculate the absorbance, a background IR spectrum was recorded at this temperature. Ethane, ethylene, propane and propylene adsorption were investigated on both samples. Before the introduction of a small amount of gas inside the fixed volume, the valves to the 1st rotary pump and the turbo pump were closed (V1, V2 and V5). By slowly opening valve V3, a small gas volume was introduced in the fixed volume (B). The pressure (PB) of the volume was increased to approximately 50 mbar. This gas sample was then introduced to the transmission cell by
valve V2. At several intermediate pressures, this valve was closed and an IR spectrum was recorded after the pressure had stabilized. When the pressure in the chambers was below 10-2 mbar, valve V1 was also opened completely and all gases were fully desorbed. The introduction of 50 mbar of gas and the slow reduction in pressure was performed and analyzed twice for each gas on each adsorbent to investigate the reproducibility of the experiment and the reversibility of the adsorption/desorption process. After the two duplicate experiments for each gas, the sample was reheated to 398 K to completely desorb all adsorbed components, after which another gas was investigated.
2.2.4 Gases
The gases used in the experiments were all supplied by HoekLoos and had the following purities: helium 4.6 (> 99.996%), 5 vol% CO 2.0 (> 99.0%) in helium 4.6 (> 99.996%), argon 4.6 (> 99.996%), ethane 3.0 (>99.9%), ethylene 2.8 (99.8%), propane 3.5 (>99.95%) and propylene 3.5 (99.95%).
2.3 Results
2.3.1 Synthesis of zeolite NaX
Table 2.1: Elemental analysis and porous properties of the zeolites. Geographical location Si/Al-ratio
[-] Na/Al-ratio [-] SBET [m2 g-1] Vmicro [cm3 g-1] NaX (Charnell) 1.2 1.0 749 0.29 NaX (Qiu) 1.3 1.0 852 0.33 NaX (Modified) 1.3 1.0 857 0.33 NaY (Zeolyst, CV100) 2.55 1.0 875 0.33
a
0 10 20 30 40 50 60 0 20 40 60 80 100 R e la tiv e in te n s it y [-] 0 20 40 60 80 100 R e la tiv e in te n s it y [-] 2ș [-]a
0 10 20 30 40 50 2ș [-] 60b
0 10 20 30 40 50 6 2ș [-] 0 0 20 40 60 80 100 R e la tiv e in te n si ty [-] P A P P P Pb
0 10 20 30 40 50 6 2ș [-] 0 0 20 40 60 80 100 R e la tiv e in te n si ty [-]b
0 10 20 30 40 50 6 2ș [-] (Charnell) 0 0 20 40 60 80 100 R e la tiv e in te n si ty [-] P A P P P P P A P P P P (Qiu, dp< 50ȝm)c
0 10 20 30 40 50 60 2ș [-] 0 20 40 60 80 100 R e la tiv e in te n s it y [-] P A P P P Pc
0 10 20 30 40 50 6 2ș [-] 0 0 20 40 60 80 100 R e la tiv e in te n s it y [-]c
0 10 20 30 40 50 6 2ș [-] 0 0 20 40 60 80 100 R e la tiv e in te n s it y [-]d
0 10 20 30 40 50 6 2ș [-] 0 0 20 40 60 80 100 R e la tiv e in te n s it y [-] P A P P P Pd
0 10 20 30 40 50 6 2ș [-] 0 0 20 40 60 80 100 R e la tiv e in te n s it y [-]d
0 10 20 30 40 50 6 2ș [-] P A P P P P P A P P P P (Qiu, dp> 50ȝm) 0 0 20 40 60 80 100 R e la tiv e in te n s it y [-] P A P P P P P A P P P P (Modified, dp< 50ȝm) 0 10 20 30 40 50 60 2ș [-] 0 20 40 60 80 100 R e la tiv e in te n s it y [-]e
0 10 20 30 40 50 2ș [-] 0 20 40 60 80 100 R e la tiv e in te n s it y [-]e
60 (Modified, dp< 50ȝm) 0 0 10 20 30 40 50 6 2ș [-] 0 20 40 60 80 100 R e la tiv e in te n s it y [-]f
0 10 20 30 40 50 6 2ș [-] 0 20 40 60 80 100 R e la tiv e in te n s it y [-] 0f
(Literature NaX)0 10 20 30 40 50 60 2ș [-] 0 20 40 60 80 100 R e la tiv e in te n s ity [-]
g
0 10 20 30 40 50 6 2ș [-] 0 20 40 60 80 100 R e la tiv e in te n s ity [-] 0g
(Literature NaP) 0 0 10 20 30 40 50 6 2ș [-] 0 20 40 60 80 100 R e la ti v e in te n s it y [-]h
0 10 20 30 40 50 6 2ș [-] 0 20 40 60 80 100 R e la ti v e in te n s it y [-] 0h
(Literature NaA)Fig. 2.3: XRD pattern of (g) NaP (literature) and (h) NaA (literature) (Baerlocher and McCusker (2006)).
a
b
b
c
d
e
Fig. 2.4: SEM images of: (a) NaX (Charnell recipe), (b) NaX dp<50 ȝm (Qiu recipe), (c) NaX dp>50 ȝm (Qiu recipe),
The particle size distribution of the crystals obtained from the recipe of Qiu and our modified crystals are plotted in Fig. 2.5. Also the distribution after 4 minutes exposure to ultrasonic vibrations is included. Due to this ultrasonic treatment the average particle size decreases and additional fines were formed. The modified recipe resulted in a smaller fraction of fines before and after ultrasonic vibrations and gave a more uniform particle size distribution. 0 1 dp[μm] Pr o b a b ili ty [-] 10 100 1000 5 10 15 a b d c 0 1 dp[μm] Pr o b a b ili ty [-] 10 100 1000 5 10 15 a b d c
Fig. 2.5: Particle size distribution of: (a) Qiu recipe (short dashed line, ), (b) Qiu recipe 4 min ultrasonic treated (dotted line, ), (c) modified recipe (solid line, ) and (d) modified recipe 4 min ultrasonic treated (long dashed line, ).
2.3.2 Synthesis of CuCl/Faujasite
The TGA patterns of NaY and of the physical mixtures of CuCl and NaY are shown in Fig. 2.6a. The hydrated NaY powders lost 21 wt% of the initial mass of the zeolite sample upon heating from 298 to 373 K, corresponding to regions I-III in Fig. 2.6a. A further temperature increase from 373 K to 623 K resulted in an extra mass loss of 3 wt%. For all the physical mixtures of CuCl and NaY, the mass loss below 623 K is about 24 wt% on the basis of the initial mass of NaY in the mixtures. For an amount of CuCl in the mixture below 43 wt%, the mass of the mixture sample remains constant at 623 K for 4 h, while for a higher amount of CuCl in the mixture, a decrease in the mass is still observed.
In Fig. 2.7 the loading of a dispersion of CuCl in NaY is calculated for a dispersion time of 4 and 8 h. Below a loading of 43 wt%, the longer dispersion time has no effect on the loading of CuCl remaining on the surface of the dispersed CuCl/NaY zeolite. Above 43 wt%, a longer dispersion time results in a lower amount of CuCl still present in the mixture, due to the sublimation of the excess amount of CuCl.
T e m p e ra tur e [ K ] 0 100 200 300 400 500 0.7 0.8 0.9 1.0 Time [min] Re la ti v e we ig h t [-] 300 400 500 600 I II III IV V 47 wt% 43 wt% 0 wt% 57 wt% 25 wt%
a
NaY Tem p e ra tur e [ K ] 0 100 200 300 400 500 0.7 0.8 0.9 1.0 Time [min] Re la ti v e we ig h t [-] 300 400 500 600 I II III IV V 47 wt% 43 wt% 0 wt% 57 wt% 25 wt%a
0 100 200 300 400 500 0.7 0.8 0.9 1.0 Time [min] Re la ti v e we ig h t [-] 300 400 500 600 I II III IV V 47 wt% 43 wt% 0 wt% 57 wt% 25 wt%a
NaY Re la ti v e we ig h t [-] 0 0.7 Time [min] 300 Te m p e ra tu re [K] 200 400 600 800 1000 0.8 0.9 1.0 400 500 600 I II III IV V VI VII 36 wt% 24 wt% 0 wt% 46 wt% 12 wt%b
NaX Re la ti v e we ig h t [-] 0 0.7 Time [min] 300 Te m p e ra tu re [K] 200 400 600 800 1000 0.8 0.9 1.0 400 500 600 I II III IV V VI VII 36 wt% 24 wt% 0 wt% 46 wt% 12 wt%b
NaXFig. 2.6: Thermogravimetric analysis profiles of heat treatment in helium of: (a) NaY (Zeolyst) and physical mixtures of CuCl and NaY, and (b) NaX (Charnell recipe) and of physical mixtures of CuCl and NaX.
0 0
Initital loading of CuCl [wt%]
R e m a in ing load ing o f C u C l [w t%] 20 40 60 20 40 60 43wt% 8h 4h 0 0
Initital loading of CuCl [wt%]
R e m a in ing load ing o f C u C l [w t%] 20 40 60 20 40 60 43wt% 8h 4h
Fig. 2.7: Remaining CuCl loading after heat treatment procedure in helium of physical mixtures of CuCl and NaY.
0 2ș [-] Int ens it y [A .U .] 10 20 30 40 50 28.5 47.4 6.2 10.2 0 wt% 25 wt% 13 wt% 36 wt% 47 wt% 57 wt%
a
23.7 0 2ș [-] Int ens it y [A .U .] 10 20 30 40 50 28.5 47.4 6.2 10.2 0 wt% 25 wt% 13 wt% 36 wt% 47 wt% 57 wt%a
23.7 28.5 0 2ș [-] In tens it y [A .U .] 10 20 30 40 50 47.4 6.2 10.2 0 wt% 25 wt% 13 wt% 36 wt% 47 wt% 57 wt%b
43 wt% 41 wt% 23.7 28.5 0 2ș [-] In tens it y [A .U .] 10 20 30 40 50 47.4 6.2 10.2 0 wt% 25 wt% 13 wt% 36 wt% 47 wt% 57 wt%b
43 wt% 41 wt% 23.7Fig. 2.8: XRD patterns of NaY (Zeolyst) with different amounts of CuCl: (a) physical mixtures, (b) CuCl dispersed in NaY.
Initial weight fraction of CuCl [-]
In tens it y at 2 ș = 6. 2 [· 1 0 3A. U .] Physical mixture Dispersed sample 0 0.2 0.4 0.6 0 2.5 7.5 5.0 7.5
a
Initial weight fraction of CuCl [-]
In tens it y at 2 ș = 6. 2 [· 1 0 3A. U .] Physical mixture Dispersed sample 0 0.2 0.4 0.6 0 2.5 7.5 5.0 7.5
a
Initial weight fraction of CuCl [-]
In tens it y at 2 ș = 23. 7 [· 1 0 3A. U .] 0 0.2 0.4 0.6 0 1
b
2 4 5 3Initial weight fraction of CuCl [-]
In tens it y at 2 ș = 23. 7 [· 1 0 3A. U .] 0 0.2 0.4 0.6 0 1
b
2 4 5 3 0Initial weight fraction of CuCl [-]
In tens it y 2 ș = 2 8 .5 [· 1 0 3A. U. ] Int ens it y 2 ș = 4 7 .4 [· 10 3A.U. ] 0.2 0.4 0.6 0 5 10 15 0 2..5 5.0 7.5 Physical mixture Dispersed sample
c
0Initial weight fraction of CuCl [-]
In tens it y 2 ș = 2 8 .5 [· 1 0 3A. U. ] Int ens it y 2 ș = 4 7 .4 [· 10 3A.U. ] 0.2 0.4 0.6 0 5 10 15 0 2..5 5.0 7.5 Physical mixture Dispersed sample
c
Fig. 2.9: Intensity of the NaY reflections at (a) 2ș = 6.2 and (b) 2ș = 23.7 for the physical mixture () and
the dispersed sample (¡) at different starting loadings of CuCl (weight fraction). (c) Intensity of the CuCl reflections at 2ș = 28.5 (V, T) and 2ș = 47.4 (U, S) for the physical mixture (V, U) and the dispersed
The intensities of most of the reflections of NaY are the same for the physical mixtures and the heat treated samples. However the reflections at 2ș = 6.2 and 10.2 are further reduced in the heat treated samples. In Fig. 2.9a-b the intensities of the reflections of NaY at 2ș = 6.2 and 2ș = 23.7 are shown for the physical mixture before and after the CuCl dispersion at 623 K. The reflections at 2ș = 23.7 decrease similarly for both samples, so the NaY zeolite is still present in a similar ratio with CuCl as in the physical mixture. The low angle reflection at 2ș = 6.2 shows a further reduction in the intensity after the CuCl dispersion at 623 K and becomes almost zero at high CuCl loadings.
The intensities of the reflections of CuCl (2ș = 28.5 and 47.4) are plotted in Fig. 2.9c versus the loading of CuCl for the physical mixtures before and after the heat treatment at 623 K for 4 h. For the physical mixtures a linear increase in the intensity of the reflections of CuCl is observed. For all the heated samples there is a decrease in the reflections of CuCl. Below a loading of 43 wt% CuCl the reflections of CuCl disappear almost completely. When the amount of CuCl in the mixture exceeds 43 wt%, the reflections of crystalline CuCl do not disappear completely and a relatively high intensity remains.
For the NaX (Charnell recipe) zeolite the XRD pattern of the dispersed mixtures with CuCl are shown in Fig. 2.10. Like NaY the reflections of CuCl disappear below a saturation loading of 36 wt% for NaX. Above this loading the reflection peaks of CuCl remain visible. The dispersion of CuCl into NaX does not affect the intensities of the reflections of NaX, since the decrease of the NaX reflections follows the same trend as observed for the physical mixture of NaY. The low angle reflections at 2ș = 6.2 and 10.2 decrease more quickly than the other reflections of NaX for higher CuCl loadings.
0 2ș [-] In te n s it y [A.U .] 10 20 30 40 50 46 wt% 36 wt% 24 wt% 12 wt% 0 wt% 28.5 47.4 6.2 10.2 23.7 0 2ș [-] In te n s it y [A.U .] 10 20 30 40 50 46 wt% 36 wt% 24 wt% 12 wt% 0 wt% 28.5 47.4 6.2 10.2 23.7
a
b
c
Fig. 2.11: SEM images of CuCl loaded samples: (a) 20 wt% CuCl/NaX,
(b) 36 wt% CuCl/NaX and (c) 50 wt% CuCl/NaX.
SEM images of typical CuCl/NaX crystals are shown in Fig. 2.11a-c. For higher loadings a layer of external CuCl is observed. The sample of 20 wt% CuCl on NaX shows a relatively smooth surface. At a loading of 36 wt% a very thin layer of CuCl could be observed. At higher loading (e.g. 50 wt% in Fig. 2.11c), this layer becomes thicker. The TEM image of a small CuCl/NaX crystal is shown in Fig. 2.12. EDX analysis confirmed the presence of CuCl in the zeolite crystal. The picture shows darker spots of finely dispersed CuCl throughout the zeolite crystal. Also some external CuCl around this slightly overloaded sample is present.
For the NaY and 43 wt% CuCl/NaY samples, the absorption spectra of the CO stretch vibrations between 2300 – 1900 cm-1 are shown in Fig. 2.13. The adsorption of CO on NaY results in the appearance of two absorption bands at 2169 and 2120 cm-1. The dispersion of CuCl onto NaY results in the appearance of an intense absorption band at 2145-2136 cm-1 as seen in the spectrum of the 43 wt% CuCl/NaY sample. The bands at 2169 and 2120 cm-1 remain present in this spectrum as two shoulders.
2300 0 Wavenumber [cm-1] Ab s o rb a n c e [A. U .] 1900 2000 2100 2200 0.50 0.75 1.00 2145-2136 0.25 43 wt% CuCl/NaY NaY 2169 2120 2300 0 Wavenumber [cm-1] Ab s o rb a n c e [A. U .] 1900 2000 2100 2200 0.50 0.75 1.00 2145-2136 0.25 43 wt% CuCl/NaY NaY 2169 2120
Fig. 2.13: IR absorption spectra of CO adsorption on NaY and 43wt% CuCl/NaY at 323 K and pCO = 5 kPa.
A CO desorption experiment for the 43 wt% CuCl/NaY sample is shown in Fig 2.14. The shoulder bands at 2169 and 2120 cm-1 decrease relatively quickly and are no longer present after 30 min. The absorption band at 2145-2136 initially drops gradually and a small apparent ‘blue shift’ is observed. After 30 min the absorbance of this peak continues to decreases very slowly, but could only be removed completely at an elevated temperature (> 373 K).
2300 0 Wavenumber [cm-1] Ab s o rb a n c e [A.U .] 1900 2000 2100 2200 0.50 0.75 1.00 2145-2136 0.25 43 wt% CuCl/NaY 2300 0 Wavenumber [cm-1] Ab s o rb a n c e [A.U .] 1900 2000 2100 2200 0.50 0.75 1.00 2145-2136 0.25 43 wt% CuCl/NaY
Fig 2.15 shows the absorption spectra of the dried (at 423 K) and the dispersed (at 623 K) 43 wt% CuCl/NaY samples after CO adsorption. At 423 K the absorption band at 2145-2136 cm-1 is very small. Dispersion of CuCl only occurs after the sample is treated at 623 K.
2300 0 Wavenumber [cm-1] A b s o rb anc e [A. U .] 1900 2000 2100 2200 0.50 0.75 1.00 0.25 623 K 423 K 2169 2120 214 5-2 136 2300 0 Wavenumber [cm-1] A b s o rb anc e [A. U .] 1900 2000 2100 2200 0.50 0.75 1.00 0.25 623 K 423 K 2169 2120 214 5-2 136
Fig. 2.15: IR absorption spectra for CO adsorption at 323 K and 5 kPa on mixtures of CuCl and NaY preheated at 423 or 623 K. The spectra were recorded 13 minutes after exposure to CO.
2.3.3 Adsorption of light olefins and paraffin studied with FTIR
IR adsorption spectra of NaY and 43 wt% CuCl/NaY after the introduction of 3.5-5.5 kPa of ethane or ethylene in the transmission cell are shown in Fig. 2.16. Ethylene adsorption resulted in intense absorption bands between 3124-2975 cm-1, 1926-1875 cm-1 and 1530-1380 cm-1. For ethane intense absorption bands between 3000-2890 cm-1 and 1605-1350 cm-1 were observed. As shown in the enlargement of the spectra between 1650-1250 cm-1 in Fig. 2.16d, the adsorption of ethylene on 43wt% CuCl/NaY resulted in the formation of additional bands at 1544, 1421 and 1280 cm-1, which were not observed on NaY (Fig. 2.16b).
150 0 200 0 250 0 300 0 350 0
a
W a v e num be r [c m -1] 0 Abs orb an ce [-] 0. 2 0. 4 C2 H6 Na Y 30 0 0 -2 8 9 0 16 0 5 -1 3 5 0 150 0 200 0 250 0 300 0 350 0a
W a v e num be r [c m -1] 0 Abs orb an ce [-] 0. 2 0. 4 150 0 200 0 250 0 300 0 350 0a
W a v e num be r [c m -1] 0 Abs orb an ce [-] 0. 2 0. 4 C2 H6 Na Y 30 0 0 -2 8 9 0 16 0 5 -1 3 5 0 150 0 200 0 250 0 30 0 0 350 0b
W a ve n u mb er [c m -1 ] 0 Ab sorb an ce[ -] 0. 2 0. 4 1 300 1 400 1 500 16 0 0 0 0. 2 0. 6 0. 8 0. 4 C2 H4 Na Y 3 124 -297 5 1 9 2 6 -187 5 1 5 30 -138 0 150 0 200 0 250 0 30 0 0 350 0b
W a ve n u mb er [c m -1 ] 0 Ab sorb an ce[ -] 0. 2 0. 4 150 0 200 0 250 0 30 0 0 350 0b
W a ve n u mb er [c m -1 ] 0 Ab sorb an ce[ -] 0. 2 0. 4 1 300 1 400 1 500 16 0 0 0 0. 2 0. 6 0. 8 0. 4 1 300 1 400 1 500 16 0 0 0 0. 2 0. 6 0. 8 0. 4 C2 H4 Na Y 3 124 -297 5 1 9 2 6 -187 5 1 5 30 -138 0 150 0 20 0 0 250 0 300 0 350 0c
W a ve n u mb er [c m -1] 0 Abs orb an ce [-] 0. 2 0. 4 0. 6 0. 8 1. 0 C2 H6 Cu Cl /N a Y 3 000 -2 8 9 0 1 605 -1 35 0 150 0 20 0 0 250 0 300 0 350 0c
W a ve n u mb er [c m -1] 0 Abs orb an ce [-] 0. 2 0. 4 0. 6 0. 8 1. 0 150 0 20 0 0 250 0 300 0 350 0c
W a ve n u mb er [c m -1] 0 Abs orb an ce [-] 0. 2 0. 4 0. 6 0. 8 1. 0 C2 H6 Cu Cl /N a Y 3 000 -2 8 9 0 1 605 -1 35 0d
ban ce [-] 150 0 20 0 0 250 0 300 0 35 0 0 Wa v e n u m b e r [c m -1] 0 Abs or 0.4 0.2 0.6 0.8 1.0 1 300 14 0 0 15 0 0 1 600 0 C 2 H4 Cu Cl /N a Y 1 926 -187 5 150 0 20 0 0 250 0 300 0 35 0 0 Wa v e n u m b e r [c m -1] 0 Abs or 0. 2 0. 6 0. 8 0. 4 154 4 128 0 142 1 3 124 -297 5 153 0-1 38 0d
ban ce [-] 0.6 0.4 0.2 0.8 1.0 150 0 20 0 0 250 0 300 0 35 0 0 Wa v e n u m b e r [c m -1] 0 Abs ord
ban ce [-] 0.6 0.4 0.2 0.8 1.0 1 300 14 0 0 15 0 0 1 600 0 1 300 14 0 0 15 0 0 1 600 0. 2 0. 6 0. 8 0. 4 0 C 2 H4 Cu Cl /N a Y 1 926 -187 5 0. 2 0. 6 0. 8 0. 4 154 4 128 0 142 1 3 124 -297 5 153 0-1 38 0 Fig. 2.16: IR absorption spect ra attwo partial pressures for:
