WO2013160292A1

WO2013160292A1 - Process for separating mixtures comprising straight-chain and branched alkanes by adsorption on zif-77

Info

Publication number: WO2013160292A1
Application number: PCT/EP2013/058376
Authority: WO
Inventors: David DUBBELDAM; Rajamani Krishna
Original assignee: Universiteit Van Amsterdam
Priority date: 2012-04-27
Filing date: 2013-04-23
Publication date: 2013-10-31

Abstract

The invention pertains to a process for separating a mixture of alkanes which comprises straight-chain alkanes and branched alkanes, wherein the mixture is contacted with a molecular sieve under adsorption conditions resulting in adsorption of at least part of the alkane mixture onto the molecular sieve to form an adsorbed fraction, and subsequently subjecting the molecular sieve to desorption conditions thereby desorbing at least part of the adsorbed fraction from the molecular sieve, wherein the molecular sieve is a molecular sieve of ZIF-77 topology and pore size. It has been found that the use of a molecular sieve with ZIF-77 topology and pore size results in a process with a very high selectivity. More in particular, it has been found that a material with this structure shows a high adsorption selectivity, defined as the adsorbed amount of the total of straight-chain alkane and mono-methyl branched alkane divided by the absorbed amount of the total of higher-branched alkane. High adsorption selectivity leads to smaller equipment volumes, and lower capital and energy requirements.

Description

PROCESS FOR SEPARATING MIXTURES COMPRISING STRAIGHT - CHAIN AND BRANCHED ALKANES BY ADSORPTION ON ZIF-77

The present invention pertains to a process for separating a mixture of alkanes, which comprises straight-chain alkanes and branched alkanes into at least two fractions. Mixtures of straight-chain and branched alkanes are formed in many hydrocarbon conversion processes, for example in oil refining, but also in other processes. A case where the separation of alkanes with different degrees of branching is of particular importance is in the manufacture of gasoline. More specifically, separation of the isomers of hexane (n- hexane (C6) , 2-methylpentane (2MP) , 3-methylpentane (3MP) , 2, 2-dimethylbutane (22DMB) , and 2 , 3-dimethylbutane (23DMB) is a process of significant importance in the petroleum industry in the context of octane enhancement. The octane number increases with the degree of branching. Branched, especially di-branched, isomers are the preferred components of gasoline. In the petroleum industry catalytic isomerization is used to convert linear alkanes into their branched isomers. The effluent of a paraffin isomerization reactor contains a mixture of normal alkanes, mono-methyl alkanes and di-methyl alkanes. Traditionally, only the normal alkanes are separated from the isomeric mixture by molecular sieving using LTA-type zeolite, and these linear alkanes are recycled to the

isomerization reactor.

Research has been carried out into other molecular sieve materials suitable for this separation. For example, US4367364 describes a process for separating normal paraffins from cyclic and branched-chain hydrocarbons by contacting the mixture at adsorbent conditions with an adsorbent comprising silicalite. The normal paraffin is adsorbed onto the silicalite .

In this case, the mono-methyl alkanes and di-methyl alkanes are collected for inclusion into the gasoline pool. However, the di-methyl alkanes have the highest octane numbers and are the most desirable components. Therefore, a more efficient approach would be to separate only the di-methyl alkanes as product and recycle the normal and the mono-methyl alkanes to the isomerization reactor.

This is the approach followed in US4717784. This reference describes a process for the production of an isomerate

gasoline blending component by contacting a feed containing normal paraffins, mono-methyl-branched paraffins, and higher- branched paraffins with a separatory sieve which preferably is ferrierite. The normal paraffins and mono-methyl-branched paraffins are adsorbed onto the separatory sieve, while the higher-branched paraffins are not. The normal paraffins and mono-methyl-branched paraffins are desorbed from the molecular sieve, and recycled to the isomerisation unit.

US4804802 describes a process for the production of an

isomerate gasoline blending component which uses a combination of two separatory sieves. The first sieve, which preferably is calcium 5A, selectively adsorbs the n-alkanes. The second sieve selectively adsorbs the mono-methyl branched paraffins. The higher-branched isomers are not adsorbed.

A problem associated with the processes described above is that the separation process shows a relatively low selectivity for on the one hand, the straight chain and monomethyl alkanes with their relatively low octane numbers, and on the other hand the higher-branched alkanes with the more desirable octane numbers . There is therefore need for a process with a higher adsorption selectivity, allowing better separation between these groups of compounds. This allows on the one hand for better

separation under the same process conditions and amount of molecular sieve and on the other hand for milder process conditions and less molecular sieve while obtaining the same degree of separation. Higher adsorption selectivity leads to smaller equipment volumes and lower capital and energy

requirements per volume of isomerate processed. The present invention provides such a process.

The present invention therefore pertains to a process for separating a mixture of alkanes which comprises straight-chain alkanes and branched alkanes, wherein the mixture is contacted with a molecular sieve under adsorption conditions resulting in adsorption of at least part of the alkane mixture onto the molecular sieve to form an adsorbed fraction, and subsequently subjecting the molecular sieve to desorption conditions thereby desorbing at least part of the adsorbed fraction from the molecular sieve, wherein the molecular sieve is a

molecular sieve of ZIF-77 topology and pore size.

It has been found that the use of a molecular sieve with ZIF- 77 topology results in a process with a very high selectivity. More in particular, it has been found that a material with this structure shows a high adsorption selectivity, defined as the adsorbed amount of the total of straight-chain alkane and mono-methyl branched alkane divided by the absorbed amount of the total of higher-branched alkane. High adsorption

selectivity leads to smaller equipment volumes, and lower capital and energy requirements. The invention will be discussed in more detail below, with reference to the following figures, without being limited thereto or thereby.

Figure 1 shows the simulated adsorption selectivity as a function of the total pressure at 433 K (160°C) for various nanoporous materials.

Figure 2 shows the simulated pulse breakthrough curves of hexane isomers in ZIF-77 at 433 K (160°C) and 100 kPa total pressure .

Figure 3 shows the simulated pulse breakthrough curves of hexane isomers in MFI at 433 K (160°C) and 100 kPa total pressure .

Figure 4 shows the simulated pulse breakthrough curves of hexane isomers in CoBDP at 433 K (160°C) and 100 kPa total pressure.

Figure 5 shows the simulated pulse breakthrough curves of hexane isomers in MgMOF at 433 K (160°C) and 100 kPa total pressure .

Figure 6 shows the simulated pulse breakthrough curves of C5- C7 alkane isomers in ZIF-77 at 433 K (160°C) and 260 kPa total pressure .

Figure 7 shows a process overview of a C5/C6/C7 isomerisation / separation process using ZIF-77 in the separation step.

Figure 8 (A and B) shows equimolar mixture adsorption isotherms at industrial conditions as a function of pressure for

erionite (ERI) (Fig. 8A) and ZIF-77 (Fig. 8B) . Erionite is capable of separating only the linear alkane (n-hexane (C6) nC6) from the remainder (mono- and dibranched alkanes, 2- methylpentane (2MP) , 3-methylpentane (3MP) , 2 , 2-dimethylbutane (22DMB) , and 2 , 3-dimethylbutane (23DMB) . Figure 8B shows that ZIF-77 is able to separate both linear and mono-branched alkanes from the di-branched molecules. The inventors believe that this is due to the fact that ERI- and LTA-type zeolites have 8-ring window and pore windows in the 4-5 Angstrom range. The nanoporous material is therefore only accessible to linear alkanes due to size-exclusion. Zeolites having a ZIF-77 topology allow also larger molecules in the structure, but shows a crucial order-of-magnitude difference in adsorption between linear, mono-branched, and dibranched molecules. In contrast to previous technology based on size-exclusion, molecular sieves of ZIF-77 topology and pore size the ZIF-77 are able to separate both linear and mono-branched alkanes from the di-branched molecules. More generally, molecular sieves of ZIF-77 topology and pore size ZIF-77 are capable of separating linear from mono-branched from di-branched alkanes over a wide range of chain-lengths.

The temperature of 433 K is just for illustration; the

extraordinary separation properties of ZIF-77 discussed hereunder holds for a much broader temperature ranges, including those typically used in practical processing

schemes, i.e. 370 K to 550 K (97-277°C).

The data in Figure 1 were obtained using state-of-the-art molecular simulation technique of Configurational-Bias Monte Carlo simulations whose accuracy for the prediction of adsorption of alkanes in a wide variety of nanoporous

materials has been established by extensive comparisons with experimental data. The simulation methodology is described e.g. in Dubbeldam, D. et al . , United Atom Forcefield for

Alkanes in Nanoporous Materials, J. Phys . Chem. B 108, 12301- 12313 (2004) and Vlugt, T. J. H., Krishna, R. & Smit, B., Molecular simulations of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite, J. Phys. Chem. B 103, 1102-1118 (1999) .

The adsorption selectivity of ZIF-77 has been found to be of the order of two factors, i.e. 100 times, higher than the adsorption selectivity of other molecular sieves. Reference made to Figure 1, which illustrates this. This figure shows the simulated adsorption selectivity as a function of the total pressure at 433 K for various nanoporous materials. ZIF- 77 has a significantly higher selectivity, as compared to other nanoporous materials (Note the log-scale on the y-axis) . The only material which comes in the same range is MFI, of which family ZSM-5 and silicalite are members. However, MFI only shows a high selectivity at 100 bar. This means that for an MFI-containing system the high selectivity can only be obtained at pressures of 100 bar, while a molecular sieve of

ZIF-77 topology can be applied at much lower pressures, making for a process with increased economic performance.

A further important advantage of ZIF-77 over MFI zeolite is that it allows fractionation of a mixture of C5, C6 and C7 isomers into individual components. For illustration,

reference is made to Figure 2, which shows the simulated pulse breakthrough curves of hexane isomers in ZIF-77 at 433K

(160°C) and 100 KPa total pressure. As can be seen from this figure, the dimethylisomers show a peak at a time range which is an order of magnitude smaller than the monomethyl isomers, which in turn show a peak at an order of magnitude smaller than the normal (straight chain) hexane. The breakthrough of the dibranched molecules is first, followed by the

breakthrough of the mono-branched compounds, while the

breakthrough of the straight-chain compounds is last. This means that the use of a molecular sieve of ZIF-77 topology makes it possible to separate the di-methyl substituted compounds in great selectivity from the monosubstituted and straight-chain compounds. Further, the use of a molecular sieve of ZIF-77 topology also allows separation of the

monomethyl compounds from the straight-chain compounds.

In contrast, MFI does not allow such separation. As can be seen from Figure 3, for MFI the pulse breakthrough curve for 2 , 3-dimethyl butane overlaps substantially with that of 3- methyl butane. This means that under these conditions MFI is not suitable for separation of 2, 3-dimethyl butane and 3- methyl butane. Further, as the pulse breakthrough curve for n- hexane overlaps with that of 2-methyl propane, separation of these compounds is also not possible under these conditions. To illustrate that the adsorption profile for ZIF-77 is indeed something special, reference is made to Figures 4 and 5, which present the breakthrough curves for hexane isomers at 433K and 100 kPa total pressure for CoBDP and MgMOF. For CoBDP it can be seen that while 2,2-dimethyl butane shows a separate peak, all other compounds, including the 2, 3-dimethyl butane, show overlapping curves, which means that they cannot be separated at the stipulated conditions. For MgMOF the curves for the dimethyl compounds are more distinct than for MFI and CoBDF, but still substantially less than for ZIF-77. Also for MgMOF separation of the dimethyl compounds from the monomethyl and straight chain compounds will not be possible. It is noted that it was surprising to find that molecular sieves of ZIF-77 topology and pore size are at all capable for adsorbing normal alkanes and mono-methyl alkanes. The diameter of the pores of the molecular sieve of this structure was found to be 3.6 A (R. Banerjee, A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O'Keeffe, O.M. Yaghi, High-throughput

synthesis of zeolitic imidazolate frameworks and application to C02 capture, Science, 319, 939-943, 2008) . Given the theoretical diameter of an alkane molecule of 4.3 A for n- hexane, 5.0 A for 3-methyl propane, 5.6 A for 2, 3-dimethyl butane, and 6.2 A for 2,2-dimethyl butane, it would be

expected that the pores would be too narrow to allow

adsorption of any of these compounds. However, it was found that these compounds are in fact adsorbed and are able to diffuse through the structure with the exception of 2,2- dimethyl butane. The exclusion of 2,2-dimethyl butane and low diffusivity of 2,3-dimethyl butane further enhance the

separation capacity of ZIF-77. Apparently the structure of the molecular sieve interacts with the structure of the alkane in such a manner that these compounds are in fact adsorbed.

The invention and its various embodiments and advantages are elucidated in more detail below. The molecular sieve with ZIF-77 topology preferably is ZIF-77 or a topological isomorph of ZIF-77. In one embodiment, ZIF-77 is used. ZIF-77 is known in the art. The structure (unit cell and atomic positions) of ZIF-77 is described in R. Banerjee,

A. Phan, B. Wang, C. Knobler, H. Furukawa, M. O'Keeffe, O.M. Yaghi, High-throughput synthesis of zeolitic imidazolate frameworks and application to C02 capture, Science, 319, 939- 943, (2008) .

The manufacture of ZIF-77 is also known in the art. It is described, e.g., in WO2008/140788, in particular in [00207] and [00208] thereof. The synthesis of ZIF-77 is described in detail in the supplemental material of R. Banerjee, A. Phan,

B. Wang, C. Knobler, H. Furukawa, M. O'Keeffe, O.M. Yaghi, High-throughput synthesis of zeolitic imidazolate frameworks and application to C02 capture, Science, 319, 939-943, (2008). As topological isomorphs of ZIF-77 all ZIF-77 like materials can be imagined with substitution of different metal instead of zinc, and minor modification to the nitroimidazole linker. Preferred substitutions of zinc comprise Fe(II), Co (II) and Cu (II) .

Preferred topological isomorphs of ZIF-77 include structures having zeolite framework topologies in which all tetrahedral atoms are transition metals (denoted with 'M'), and all bridging ones are imidazolate (IM) units.

ZIF-77 is based on M=Zn(II) and R=hydrogen.

nitroimidazole linker

Preferred topological isomorphs of ZIF-77, comprise a

nitroimidazole linker, wherein M is a transition metal, preferably selected from the group consisting of Fe(II), Co (II) and Cu(II) . Preferably, R is selected from the group consisting of Br, F, CI, NH2 and CH3. The process according to the invention may be used for the separation of n-alkanes from branched alkanes. The invention is particularly suitable for the separation of n-alkanes and monomethyl alkanes on the one hand and higher-branched, in particular dimethyl alkanes and higher-branched materials on the other hand. The alkane may have 4-20 carbon atoms. In one embodiment, alkane has 4-12 carbon atoms, more in particular 4-8 carbon atoms, still more in particular 5-7 carbon atoms.

The process according to the invention is also suitable for separating mixtures containing n-alkanes of different chain lengths and their respective isomers. In one embodiment, the process according to the invention is used for separating a mixture comprising C4-C8 n-alkanes and their isomers, in particular monomethyl and dimethyl isomers. The process is particularly suitable for separating a mixture comprising C5- C7 n-alkanes and their isomers, in particular monomethyl and dimethyl isomers. Separation of mixtures containing n-alkanes of different chain lengths and their respective isomers, which is of particular interest in the production of gasoline, will be discussed in more detail further on in this specification.

In the process according to the invention a mixture of

alkanes, which comprises straight-chain alkanes and branched alkanes, is contacted with a molecular sieve under adsorption conditions resulting in adsorption of at least part of the alkane mixture onto the molecular sieve to form an adsorbed fraction. In one embodiment, part of the mixture is adsorbed onto the molecular sieve to form an adsorbed fraction and part of the mixture is not adsorbed onto the molecular sieve. The non-adsorbed fraction is separated from the molecular sieve.

The adsorption step takes place under adsorption conditions. Adsorption conditions are those conditions of temperature and pressure at which the fraction to be adsorbed is adsorbed onto the molecular sieve. Suitable conditions include an increased pressure, e.g., a pressure in the range of 1 bar to 1000 bar. It has been found that the high selectivity of ZIF-77 allows the use of lower pressures, which is highly attractive from a technical and economical point of view. Therefore, in one embodiment, the process is carried out at a pressure of 1-100 bar, more in particular at a pressure of 1-60 bar, still more in particular at a pressure if 1-40 bar.

The adsorption reaction can, e.g., take place at a temperature in the range of 370 to 550 K (97-277°C) . Adsorption takes place in the gas phase. The same goes for the desorption step, which will be discussed in more detail below.

After the adsorption step, the molecular sieve is subjected to desorption conditions to remove at least part of the adsorbed fraction from the molecular sieve. The desorption step can be carried out by means known in the art, e.g., by contacting the molecular sieve with a desorbent material at increased

temperature and/or reduced pressure. Suitable desorbent materials may be gases such as gaseous hydrocarbons such as methane or ethane, or other gases, such as nitrogen on

hydrogen. Suitable desorbent materials may also be liquids, as long as they can be separated from the product fraction. The crux of the desorption step is to purge at least part of the adsorbed fraction from the molecular sieve. It is within the scope of the skilled person to select a suitable desorbent material and accompanying desorbent conditions. In one

embodiment, the desorption step is carried out by contacting the molecular sieve with a gas stream, in particular a stream comprising one or more of hydrogen, nitrogen, methane, or ethane. The temperature may, e.g., be in the range of 370 to 550K (97-277°C) . The pressure may be in the range of, e.g., 1- 20 bar. As indicated above in the discussion of Figure 2, it has been found that the adsorption characteristics of molecular sieve of ZIF-77 typology are such that the energy of adsorption of different compounds is quite specific. This means that the use of a molecular sieve of ZIF-77 topology and pore size makes it possible to separate the di-methyl substituted compounds in great selectivity from the monosubstituted and straight-chain compounds .

Due to the difference in adsorption efficiency for the

monomethyl-substituted compounds and the straight-chain compound, it may even be possible by proper selection of the desorption conditions to sequentially desorb different

fractions from the molecular sieve. This allows separation of the adsorbed fraction in different fractions. It is noted that as the individual dibranched molecules also show individual peaks, it is possible not only to separate the dibranched molecules from the monobranched and straight chain isomers, but also to separate the individual dibranched isomers from each other. The present invention therefore also pertains to a process wherein the step of subjecting the molecular sieve to desorption conditions is carried out in at least two steps, wherein in a first step the desorption conditions are such that a first part of the adsorbed fraction is desorbed from the molecular sieve, and in a second step the desorption conditions are such that a second part of the adsorbed

fraction is desorbed from the molecular sieve, with the chemical composition of the desorbed fraction being different. It is within the scope of the skilled person to determine on a case-by-case basis by routine trial and error which desorption conditions should be applied to desorb which fractions. One parameter that is believed to be particularly suitable in this respect is the pressure. By applying a stepwise reduction in pressure the sequential desorption of different fractions can be achieved. It is also possible to separate an isomer mixture into different fractions by applying a sequence of adsorption- desorption steps.

The process according to the invention is applied by

contacting the feedstock with a molecular sieve of ZIF-77 topology. This can for example be carried out by feeding the feedstock to a reactor containing the molecular sieve in particulate form. The molecular sieve may be employed, for example, in the form of a fixed bed, fluidized bed, or moving bed. As explained above, the molecular sieve may be subjected alternatingly to adsorbent and desorbent conditions. In the simplest embodiment of the invention the process setup

comprises a single reactor, which is alternatingly operated in adsorption and desorption mode. In another embodiment the process is operated in a setup comprising at least two reactors comprising the molecular sieve, with the feed mixture being provided to a reactor that is operated under adsorption conditions, and a desorbent medium being provided to a reactor operated under desorbent conditions. The preferred

configuration is a fixed bed of adsorbent. The preferred mode of operation is continuous, wherein the bed is alternated between adsorbent and desorbent conditions. In some

embodiments this may be indicated as a "pressure-swing

adsorption setup".

The present invention finds application in the separation of numerous different alkane mixtures. The invention provides the use of a molecular sieve as defined herein for separating a higher branched alkane and a lower branched alkane or for separating different dibranched alkanes from each other, in particular for separating 2 , 2-dibranched alkanes from other di-branched alkanes. Preferably, separating said a higher branched alkane and a lower branched alkane comprises

a) separating a linear alkane from a mono-branched and/or di- branched alkane

b) separating a mono-branched alkane from a linear and/or a di-branched alkane

c) separating a di-branched alkane from a linear and/or a mono-branched alkane

d) separating a linear alkane from a mono-branched alkane e) separating a linear alkane from a di-branched alkane f) separating a mono-branched alkane from a di-branched alkane and/or

g) separating a di-branched alkane from a mono-branched alkane.

In a preferred embodiment, the invention is applied in

separating gasoline-fraction alkanes. In this embodiment the feedstock is a composition comprising at least 50 wt . % of alkanes with 4 to 8 carbon atoms, the composition comprising at least one of n-alkane and mono-methyl alkane with 4 to 8 carbon atoms, and dimethyl alkane with 4-8 carbon atoms. The feedstock is contained with a molecular sieve of ZIF-77 topology under such conditions that the n-alkanes and the monomethylalkanes are adsorbed onto the molecular sieve, and the dimethylalkanes are not adsorbed. The dimethyl alkanes that are not adsorbed are separated from the molecular sieve. The molecular sieve is subsequently subjected to desorption conditions to desorb at least part of the adsorbed fraction from the molecular sieve. In a preferred embodiment the feedstock is a composition comprising at least 50 wt . % of alkanes with 5 to 7 carbon atoms, the composition comprising at least one of n-alkane and mono-methyl alkane with 5 to 7 carbon atoms, and dimethyl alkane with 5-7 carbon atoms

Within the context of the present specification the phrase "a compound is adsorbed" means that at least 80% of the compound as present in the starting mixture is adsorbed onto the molecular sieve, in particular at least 90 ~6 , more in

particular at least 95%. Within the context of the present specification the phrase "a compound is not adsorbed" means that at most 20% of the compound as present in the starting mixture is adsorbed onto the molecular sieve, in particular at most 10%, more in particular at most 5%.

The effectiveness of this process is illustrated in Figure 6. This figure shows the simulated pulse breakthrough curves of C5-C7 alkane isomers in ZIF-77 at 433 K (160°C) and 260 kPa total pressure. As can be seen from this figure, the

separating properties of a molecular sieve with ZIF-77

topology are such that the C5-C7 dimethyl-alkanes show an adsorption profile which is clearly distinct from the

adsoption profile of the C5-C7 monomethyl alkanes and C5-C7 n- alkanes. As the C5-C7 dibranched products have the highest octane value, this surprising property allows the separation of high-octane value material from the lower-octane value material. This can, if so desired, be provided to an

isomerisation unit where it is converted to higher-branched material .

In one embodiment the feedstock comprises at least 50 wt . % of alkanes with the stipulated number of carbon atoms, 4-8, in particular 5-7, preferably at least 70 wt.%, more preferably at least 80 wt.%. Other components may be present, as long as they do not interfere with the separation process. It is preferred for the feedstock to comprise at least 90 wt.% of alkanes with 4 to 8, in particular 5 to 7 carbon atoms, more preferably at least 95 wt.%. In one embodiment, the feedstock is derived from an isomerisation reactor. In this embodiment it is preferred if the effluent desorbed from the molecular sieve, which consists for at least 80 wt.% of n-alkanes and monomethyl alkanes is recycled back to the isomerisation unit. Isomerisation processes are known in the art. Generally they comprise providing a feedstock to be isomerised to a reactor comprising an isomerisation catalyst under isomerisation conditions .

The invention is illustrated by the following examples, without being limited thereto or thereby.

Example 1

ZIF-77 is contacted with a feedstock containing a mixture of n-hexane, 2-methyl-pentane, 3-methyl pentane, 2,2-dimethyl butane, and 2,3-dimethyl butane. The reaction conditions are a temperature of 433K (160°C) and an above-atmospheric pressure. The feedstock is in the gas phase. After a certain time, the non-adsorbed fraction is removed from the feedstock. It will consist mainly of 2,2-dimethyl butane, and 2,3-dimethyl butane. The molecular sieve is subjected to a desorption step by containing it with a desorbent material, e.g. a gas stream comprising one or more of hydrogen, nitrogen, methane, or ethane at a temperature in the range of 370 to 550K (97-277°C) and a pressure in the range of, e.g., 1-20 bar.

Example 2

The adsorption of a mixture of CIO n-alkane (decane) and various n-methyl isomers thereof onto ZIF-77 was subjected to computer modelling using state-of-the-art molecular simulation technique of Configurational-Bias Monte Carlo simulations whose accuracy for the prediction of adsorption of alkanes in a wide variety of nanoporous materials has been established by extensive comparisons with experimental data. The method has been described in Dubbeldam, D. et al . , United Atom Forcefield for Alkanes in Nanoporous Materials, J. Phys . Chem. B 108, 12301-12313 (2004) and Vlugt, T. J. H., Krishna, R. & Smit, B., Molecular simulations of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite, J.

Phys. Chem. B 103, 1102-1118 (1999).

The following table summarises the results, modelled at different pressures: Compound Loading (mol/kg) at Loading (mol/kg) at

1 Pa pressure, 433K 100 Pa pressure, (160°C) 433K (160°C) n-decane 0.0318339050 0.4234596693

2 -methyl-nonane 0.0038122609 0.2280755365

3-methyl-nonane 0.0019725067 0.1734125415

4 -methyl-nonane 0.0008021988 0.0923721302

2, 7-dimethyl-octane 0.0008480325 0.0846708362

2, 4-dimethyl-octane 0.0000669159 0.0035902011

2, 2-dimethyl-octane 0.0000002159 0.0000573462

As can be seen from the table, the n-decane loading is

substantially higher than the loading of any other compounds. The adsorption decreases with an increase of sterical

hindering applied onto the hydrocarbon backbone. The molecules where the methyl-substituent is nearer the end of the chain show a higher loading than the molecules where the methyl substituent is nearer middle of the chain. The presence of two methyl-substituents leads to a further decrease in the

adsorption, especially where they are closer together,

resulting in increased sterical hinderance. The 2 , 2-dimethyl- octane is hardly adsorbed at all. From this table it can be seen that ZIF-77 can be used to adsorb the n-decane and the monomethyl-nonanes from the mixture, while the dimethyl octanes are adsorbed to a lesser extent. Proper selection of the adsorption pressure allows influencing of the product to be adsorbed. Example 3

A mixture of C5-C7 alkanes is provided to an isomerisation reactor that comprises an isomerisation catalyst, e.g., a Group VIII noble metal on a mordenite. The effluent from the isomerisation reactor, which comprises n-alkanes, monomethyl- isomers, and dimethyl isomers, is provided to a reactor

comprising ZIF-77. In that reactor the linear and mono- branched isomers are adsorbed onto the ZIF-77. The dibranched material is not adsorbed, and removed from the reactor. The linear and monobrached materials are desorbed from the ZIF-77, and recycled to the isomerisation reaction. This process is illustrated in Figure 7. Example 4

comprising erionite. In that reactor the linear and mono- branched isomers are adsorbed onto the erionite. Erionite is capable of separating only the linear alkane (n-hexane (C6) nC6) from the remainder (mono- and dibranched alkanes, 2- methylpentane (2MP) , 3-methylpentane (3MP) , 2 , 2-dimethylbutane (22DMB) , and 2 , 3-dimethylbutane (23DMB) . The results are shown in Figure 8A.

Claims

1. Process for separating a mixture of alkanes which

comprises straight-chain alkanes and branched alkanes, wherein the mixture is contacted with a molecular sieve under

adsorption conditions resulting in adsorption of at least part of the alkane mixture onto the molecular sieve to form an adsorbed fraction, and subsequently subjecting the molecular sieve to desorption conditions thereby desorbing at least part of the adsorbed fraction from the molecular sieve, wherein the molecular sieve is a molecular sieve of ZIF-77 topology and pore size.

2. Process according to claim 1, wherein the molecular sieve is ZIF-77 or a topological isomorph of ZIF-77.

3. Process according to any one of the preceding claims, wherein the alkane mixture is contacted with a molecular sieve under adsorption conditions whereby part of the mixture is adsorbed onto the molecular sieve to form an adsorbed fraction and part of the mixture is not adsorbed onto the molecular sieve, separating the non-absorbed fraction from the molecular sieve, and subsequently subjecting the molecular sieve to desorption conditions thereby desorbing at least part of the adsorbed fraction from the molecular sieve.

4. Process according to any one of the preceding claims, wherein the step of subjecting the molecular sieve to

desorption conditions is carried out in at least two steps, wherein in a first step a first part of the adsorbed fraction is desorbed from the molecular sieve, and in a second step a second part of the adsorbed fraction is desorbed from the molecular sieve.

5. Process according to any one of the preceding claims, wherein the mixture comprises n-alkanes and branched alkanes.

6. Process according to claim 5, wherein the mixture

comprises n-alkanes, monomethyl alkanes, and higher-branched alkanes .

7. Process according to claim 6, wherein the mixture

comprises n-alkane, mono-methyl-isomers of said n-alkane, and dimethyl isomers of said n-alkane.

8. Process according to any one of the preceding claims wherein the alkanes have 4-20 carbon atoms, in particular alkane has 4-12 carbon atoms, more in particular 4-8 carbon atoms, still more in particular 5-7 carbon atoms.

9. Process according to any one of the preceding claims, wherein the feedstock is a mixture comprising C4-C8 n-alkanes and their isomers, in particular monomethyl and dimethyl isomers .

10. Process according to claim 9, wherein the feedstock is a mixture comprising C5-C7 n-alkanes and their isomers, in particular monomethyl and dimethyl isomers.

11. Process according to claim 9 or 10 wherein the feedstock is derived from an isomerisation reactor and the effluent desorbed from the molecular sieve, which consists for at least 80 wt . % of n-alkanes and monomethyl alkanes is recycled back to the isomerisation reactor. 12 Process according any one of claims 1-11 for separating a higher branched alkane and a lower branched alkane or for separating different dibranched alkanes from each other. 13. Process according to claim 12, wherein separating said higher branched alkane and lower branched alkane comprises a) separating a linear alkane from a mono-branched and/or di^¬ branched alkane

b) separating a mono-branched alkane from a linear and/or a di-branched alkane

c) separating a di-branched alkane from a linear and/or a mono-branched alkane

g) separating a di-branched alkane from a mono-branched alkane . 14. Use of a molecular sieve as defined in claim 1 or 2 for separating a higher branched alkane and a lower branched alkane or for separating different dibranched alkanes from each other. 15. Use according to claim 14, wherein separating said higher branched alkane and lower branched alkane comprises

a) separating a linear alkane from a mono-branched and/or di^¬ branched alkane

b) separating a mono-branched alkane from a linear and/or a di-branched alkane

c) separating a di-branched alkane from a linear and/or a mono-branched alkane

g) separating a di-branched alkane from a mono-branched alkane .