WO2013118011A1 - Separating xylene isomers - Google Patents
Separating xylene isomers Download PDFInfo
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- WO2013118011A1 WO2013118011A1 PCT/IB2013/050593 IB2013050593W WO2013118011A1 WO 2013118011 A1 WO2013118011 A1 WO 2013118011A1 IB 2013050593 W IB2013050593 W IB 2013050593W WO 2013118011 A1 WO2013118011 A1 WO 2013118011A1
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- Prior art keywords
- xylene
- isomers
- isomer
- metal complex
- octahedral metal
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- 150000003738 xylenes Chemical class 0.000 title claims abstract description 30
- CTQNGGLPUBDAKN-UHFFFAOYSA-N O-Xylene Chemical compound CC1=CC=CC=C1C CTQNGGLPUBDAKN-UHFFFAOYSA-N 0.000 claims abstract description 40
- 238000000034 method Methods 0.000 claims abstract description 36
- 239000000203 mixture Substances 0.000 claims abstract description 29
- 150000004696 coordination complex Chemical class 0.000 claims abstract description 24
- 239000008096 xylene Substances 0.000 claims abstract description 19
- 238000000926 separation method Methods 0.000 claims abstract description 14
- 238000001179 sorption measurement Methods 0.000 claims abstract description 12
- 239000002184 metal Substances 0.000 claims abstract description 10
- 229910052751 metal Inorganic materials 0.000 claims abstract description 10
- 239000007787 solid Substances 0.000 claims abstract description 8
- 150000001450 anions Chemical class 0.000 claims abstract description 6
- 238000003795 desorption Methods 0.000 claims abstract description 3
- 239000003446 ligand Substances 0.000 claims abstract description 3
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 claims description 12
- 125000001424 substituent group Chemical group 0.000 claims description 5
- XEEYBQQBJWHFJM-UHFFFAOYSA-N Iron Chemical compound [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 claims description 4
- 229910052759 nickel Inorganic materials 0.000 claims description 3
- 125000001997 phenyl group Chemical group [H]C1=C([H])C([H])=C(*)C([H])=C1[H] 0.000 claims description 3
- RYGMFSIKBFXOCR-UHFFFAOYSA-N Copper Chemical compound [Cu] RYGMFSIKBFXOCR-UHFFFAOYSA-N 0.000 claims description 2
- FYYHWMGAXLPEAU-UHFFFAOYSA-N Magnesium Chemical compound [Mg] FYYHWMGAXLPEAU-UHFFFAOYSA-N 0.000 claims description 2
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 claims description 2
- 229910052793 cadmium Inorganic materials 0.000 claims description 2
- BDOSMKKIYDKNTQ-UHFFFAOYSA-N cadmium atom Chemical compound [Cd] BDOSMKKIYDKNTQ-UHFFFAOYSA-N 0.000 claims description 2
- 229910017052 cobalt Inorganic materials 0.000 claims description 2
- 239000010941 cobalt Substances 0.000 claims description 2
- GUTLYIVDDKVIGB-UHFFFAOYSA-N cobalt atom Chemical compound [Co] GUTLYIVDDKVIGB-UHFFFAOYSA-N 0.000 claims description 2
- 229910052802 copper Inorganic materials 0.000 claims description 2
- 239000010949 copper Substances 0.000 claims description 2
- 229910052742 iron Inorganic materials 0.000 claims description 2
- 229910052749 magnesium Inorganic materials 0.000 claims description 2
- 239000011777 magnesium Substances 0.000 claims description 2
- 125000002496 methyl group Chemical group [H]C([H])([H])* 0.000 claims description 2
- 239000006104 solid solution Substances 0.000 claims description 2
- 229910052725 zinc Inorganic materials 0.000 claims description 2
- 239000011701 zinc Substances 0.000 claims description 2
- IVSZLXZYQVIEFR-UHFFFAOYSA-N m-xylene Chemical compound CC1=CC=CC(C)=C1 IVSZLXZYQVIEFR-UHFFFAOYSA-N 0.000 description 32
- URLKBWYHVLBVBO-UHFFFAOYSA-N Para-Xylene Chemical group CC1=CC=C(C)C=C1 URLKBWYHVLBVBO-UHFFFAOYSA-N 0.000 description 30
- 238000004817 gas chromatography Methods 0.000 description 11
- 238000000634 powder X-ray diffraction Methods 0.000 description 9
- 150000001875 compounds Chemical class 0.000 description 8
- 239000007788 liquid Substances 0.000 description 8
- 239000012071 phase Substances 0.000 description 7
- OKKJLVBELUTLKV-UHFFFAOYSA-N Methanol Chemical compound OC OKKJLVBELUTLKV-UHFFFAOYSA-N 0.000 description 6
- 238000006243 chemical reaction Methods 0.000 description 6
- 238000002474 experimental method Methods 0.000 description 6
- 239000000243 solution Substances 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 5
- 239000013078 crystal Substances 0.000 description 5
- IAZDPXIOMUYVGZ-UHFFFAOYSA-N Dimethylsulphoxide Chemical compound CS(C)=O IAZDPXIOMUYVGZ-UHFFFAOYSA-N 0.000 description 4
- 238000002425 crystallisation Methods 0.000 description 4
- 230000008025 crystallization Effects 0.000 description 4
- UHOVQNZJYSORNB-UHFFFAOYSA-N Benzene Chemical compound C1=CC=CC=C1 UHOVQNZJYSORNB-UHFFFAOYSA-N 0.000 description 3
- 238000001704 evaporation Methods 0.000 description 3
- 230000008020 evaporation Effects 0.000 description 3
- VLKZOEOYAKHREP-UHFFFAOYSA-N n-Hexane Chemical compound CCCCCC VLKZOEOYAKHREP-UHFFFAOYSA-N 0.000 description 3
- 239000002904 solvent Substances 0.000 description 3
- 239000000126 substance Substances 0.000 description 3
- 238000012360 testing method Methods 0.000 description 3
- YNQLUTRBYVCPMQ-UHFFFAOYSA-N Ethylbenzene Chemical compound CCC1=CC=CC=C1 YNQLUTRBYVCPMQ-UHFFFAOYSA-N 0.000 description 2
- JUJWROOIHBZHMG-UHFFFAOYSA-N Pyridine Chemical class C1=CC=NC=C1 JUJWROOIHBZHMG-UHFFFAOYSA-N 0.000 description 2
- PPBRXRYQALVLMV-UHFFFAOYSA-N Styrene Chemical compound C=CC1=CC=CC=C1 PPBRXRYQALVLMV-UHFFFAOYSA-N 0.000 description 2
- KKEYFWRCBNTPAC-UHFFFAOYSA-N Terephthalic acid Chemical compound OC(=O)C1=CC=C(C(O)=O)C=C1 KKEYFWRCBNTPAC-UHFFFAOYSA-N 0.000 description 2
- VQGHOUODWALEFC-UHFFFAOYSA-N alpha-Phenylpyridine Natural products C1=CC=CC=C1C1=CC=CC=N1 VQGHOUODWALEFC-UHFFFAOYSA-N 0.000 description 2
- 238000004458 analytical method Methods 0.000 description 2
- 238000009835 boiling Methods 0.000 description 2
- 238000010668 complexation reaction Methods 0.000 description 2
- 238000004090 dissolution Methods 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- JVZRCNQLWOELDU-UHFFFAOYSA-N gamma-Phenylpyridine Natural products C1=CC=CC=C1C1=CC=NC=C1 JVZRCNQLWOELDU-UHFFFAOYSA-N 0.000 description 2
- 239000000047 product Substances 0.000 description 2
- 229910004039 HBF4 Inorganic materials 0.000 description 1
- ZMZDMBWJUHKJPS-UHFFFAOYSA-M Thiocyanate anion Chemical compound [S-]C#N ZMZDMBWJUHKJPS-UHFFFAOYSA-M 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 239000002253 acid Substances 0.000 description 1
- 239000003463 adsorbent Substances 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 150000001491 aromatic compounds Chemical class 0.000 description 1
- 238000000429 assembly Methods 0.000 description 1
- 230000000712 assembly Effects 0.000 description 1
- 239000006227 byproduct Substances 0.000 description 1
- 239000003153 chemical reaction reagent Substances 0.000 description 1
- 239000003795 chemical substances by application Substances 0.000 description 1
- 238000013375 chromatographic separation Methods 0.000 description 1
- 239000006184 cosolvent Substances 0.000 description 1
- 239000010779 crude oil Substances 0.000 description 1
- 238000004821 distillation Methods 0.000 description 1
- 230000000694 effects Effects 0.000 description 1
- 239000000446 fuel Substances 0.000 description 1
- 239000000417 fungicide Substances 0.000 description 1
- 239000001257 hydrogen Substances 0.000 description 1
- 229910052739 hydrogen Inorganic materials 0.000 description 1
- 125000004435 hydrogen atom Chemical group [H]* 0.000 description 1
- ZMZDMBWJUHKJPS-UHFFFAOYSA-N hydrogen thiocyanate Natural products SC#N ZMZDMBWJUHKJPS-UHFFFAOYSA-N 0.000 description 1
- 239000000543 intermediate Substances 0.000 description 1
- 238000011835 investigation Methods 0.000 description 1
- 238000006317 isomerization reaction Methods 0.000 description 1
- 239000007791 liquid phase Substances 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000001404 mediated effect Effects 0.000 description 1
- 239000012621 metal-organic framework Substances 0.000 description 1
- LGRFSURHDFAFJT-UHFFFAOYSA-N phthalic anhydride Chemical class C1=CC=C2C(=O)OC(=O)C2=C1 LGRFSURHDFAFJT-UHFFFAOYSA-N 0.000 description 1
- 229920006391 phthalonitrile polymer Polymers 0.000 description 1
- 230000000704 physical effect Effects 0.000 description 1
- 239000000049 pigment Substances 0.000 description 1
- 229920003023 plastic Polymers 0.000 description 1
- 239000004033 plastic Substances 0.000 description 1
- 229920000642 polymer Polymers 0.000 description 1
- 238000000746 purification Methods 0.000 description 1
- UMJSCPRVCHMLSP-UHFFFAOYSA-N pyridine Natural products COC1=CC=CN=C1 UMJSCPRVCHMLSP-UHFFFAOYSA-N 0.000 description 1
- 238000011084 recovery Methods 0.000 description 1
- 229920005989 resin Polymers 0.000 description 1
- 239000011347 resin Substances 0.000 description 1
- 238000007873 sieving Methods 0.000 description 1
- 238000000638 solvent extraction Methods 0.000 description 1
- 238000002336 sorption--desorption measurement Methods 0.000 description 1
- 239000007858 starting material Substances 0.000 description 1
- 239000000758 substrate Substances 0.000 description 1
- 238000003786 synthesis reaction Methods 0.000 description 1
- 238000002411 thermogravimetry Methods 0.000 description 1
- 238000001757 thermogravimetry curve Methods 0.000 description 1
- 229910052723 transition metal Inorganic materials 0.000 description 1
- 150000003624 transition metals Chemical class 0.000 description 1
- 239000010457 zeolite Substances 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07F—ACYCLIC, CARBOCYCLIC OR HETEROCYCLIC COMPOUNDS CONTAINING ELEMENTS OTHER THAN CARBON, HYDROGEN, HALOGEN, OXYGEN, NITROGEN, SULFUR, SELENIUM OR TELLURIUM
- C07F15/00—Compounds containing elements of Groups 8, 9, 10 or 18 of the Periodic Table
- C07F15/0006—Compounds containing elements of Groups 8, 9, 10 or 18 of the Periodic Table compounds of the platinum group
-
- C—CHEMISTRY; METALLURGY
- C07—ORGANIC CHEMISTRY
- C07C—ACYCLIC OR CARBOCYCLIC COMPOUNDS
- C07C7/00—Purification; Separation; Use of additives
- C07C7/148—Purification; Separation; Use of additives by treatment giving rise to a chemical modification of at least one compound
- C07C7/152—Purification; Separation; Use of additives by treatment giving rise to a chemical modification of at least one compound by forming adducts or complexes
Definitions
- This invention relates to a method of separating xylene isomers from each other in order to produce separate isomers that have different commercial and industrial applications.
- Xylenes are important chemicals, but many commercial and industrial applications require separation and purification of the three isomeric forms, namely ortho-, meta- and para-xylene. They are represented by the formulae:- ortho-xylene
- the xylenes constitute the so-called C8 aromatic compounds derived from crude oil that serve as starting materials for the synthesis of many important chemical intermediates. These include terephthalic acid, phthalic anhydrides, phthalonitriles and styrene, which are used in the production of polymers, plastics, resins, pigments and fungicides. Moreover, xylenes are often added to motor fuel as an anti-knocking agent.
- the octahedral metal complex In its pure form the octahedral metal complex typically exists as a nonporous phase (a), and the formation of a host-guest adduct generally involves entrapment of the guest during crystallization of the host from solution. Therefore, the majority of studies report solvent-mediated enclathration processes
- the guest-included forms can exhibit a variety of structural arrangements; cage-, layer- and channel-inclusion compounds (referred to as ⁇ , ⁇ and ⁇ phases, respectively) are all possible, depending on the geometric relationship between the host and the guest. In some cases the host arrangement is preserved after guest removal and the sorption-desorption processes can be cycled by means of solid-vapor reactions.
- a process for the separation of at least one xylene isomer from another xylene isomer in which the xylene isomers are present in a mixture of xylene isomers the process involving the sorption of xylene as a guest into a host complex and desorption therefrom whereby at least a primary separation of the isomers is achieved, the process being characterised in that the host complex includes at least one solid octahedral metal complex of the general formula 1
- the divalent metal to be selected from nickel, magnesium, iron, cobalt, copper, zinc, and cadmium; for the four substituents R to be selected from methyl and phenyl; and for X to be selected from CI " , Br “ and (NCS) " .
- Still further features of the invention provide for an initial xylene isomer mixture to contain all three isomers in which instance the process is initially carried out to separate out predominantly one isomer followed by a second sorption process in which a second isomer is separated from the third isomer utilising an otherwise generally similar process; for the separation process to include a solid solution of two or more of the octahedral metal complexes; and for the contacting of solid and vapour to be carried out in a chamber that is at least partially evacuated prior to introduction of the vapour containing the xylene isomers.
- Figure 1 illustrates graphically the behaviour of the three different isomers insofar as extent of reaction as a function of time is concerned;
- Figure 2 is illustrates graphically the behaviour of the three different isomers insofar as the thermal stability of the inclusion compounds is concerned.
- the pure octahedral metal complex of Formula 2 was precipitated by slow evaporation of a methanolic solution. The quality and dimensions of the crystals appeared to depend on the initial concentration of the solution as well as the rate of evaporation.
- Microcrystalline octahedral metal complex of the Formula 2 could be re-dissolved in an appropriate solvent and layered with a number of organic liquids to yield various inclusion compounds. For example, dissolution of the octahedral metal complex in hot dimethyl sulfoxide (dmso), followed by layering with benzene, resulted in the formation of the Werner clathrate [Ni(NCS) 2 (para-phenylpyridine) 4 ]-4benzene.
- polycrystalline octahedral metal complex of Formula 2 was exposed to the pure isomers in separate experiments, and the extent of guest uptake was recorded gravimetrically as a function of time (See Figure 1 ). Sorption of the guest occurred slowly in air, but the rate of uptake was significantly enhanced when the reaction chamber was evacuated prior to admitting the vapor. After each experiment, the product was analyzed by means of PXRD, which confirmed the formation of the corresponding phase resulting from solution growth.
- the three inclusion compounds prepared by solid-vapor reactions were analyzed by means of thermogravimetric analysis in order to investigate guest release as a function of temperature (See Figure 2).
- the inclusion compound octahedral metal complex of Formula 2 and para-xylene appears to be the least robust since the guest molecules are released with an onset temperature of approximately 46.1 °C.
- the release of the meta- isomer commences at 72.5 °C and that for the ortho- isomer at 74.8 °C, which shows that ortho-xylene inclusion compound is only slightly more thermally stable than the meta-xylene inclusion compound.
- thermogram for the ortho-xylene inclusion compound showed a small loss (0.3%) between 20 and 30 °C and this observation is in agreement with the sorption experiment, which showed that the reaction proceeded to slightly over 100% completion for the expected host-guest stoichiometry. Excess solvent was most likely adsorbed onto the crystallite surfaces. This phenomenon was not observed for the other two isomers.
- the composition of the liquid xylene mixture was determined by means of gas chromatography (GC). After each experiment, the xylene guest molecules were extracted from the polycrystalline products using hexane and their composition was determined using GC analysis. The results are consistent with the PXRD analyses, confirming that the Werner complex shows remarkable discrimination in favor of ortho-xylene in the presence of meta-xylene and/or para-xylene.
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- General Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Analytical Chemistry (AREA)
- Oil, Petroleum & Natural Gas (AREA)
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- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
Abstract
A process is provided for the separation of at least one xylene isomer from a mixture with at least one other xylene isomer. The process involves the sorption of xylene as a guest into a host complex and desorption therefrom whereby at least a primary separation of the isomers is achieved. The host complex includes at least one solid octahedral metal complex of the general formula (Formula I) (I) in which M is a divalent metal, the four substituents R are selected independently of each other to provide the octahedral metal complex with required ligand properties, and X is one or more coordinating anions that may be the same or different. Contacting is carried out with the octahedral metal complex in a solid state and the xylene isomer mixture in the vapour phase.
Description
SEPARATING XYLENE ISOMERS
FIELD OF THE INVENTION
This invention relates to a method of separating xylene isomers from each other in order to produce separate isomers that have different commercial and industrial applications.
BACKGROUND TO THE INVENTION
Xylenes are important chemicals, but many commercial and industrial applications require separation and purification of the three isomeric forms, namely ortho-, meta- and para-xylene. They are represented by the formulae:- ortho-xylene
meta-xylene
para-xylene
Owing to the similar physical properties of the three isomers of xylene (e.g. boiling points of 144.5, 139.1 and 138.2 °C for ortho-, meta- and para-xylene respectively), separation of the three isomers is an expensive and inefficient undertaking. Distillation with a view to producing commercial grade reagents requires more than 150 theoretical plates to isolate the higher boiling ortho- xylene and up to 360 plates to further separate meta- and para-xylene. Viable alternative methods include crystallization, absorption, sieving, complexation, isomerisation and a number of hybrid strategies that involve solid-liquid separation. Some of these processes involve the use of highly reactive chemicals such as complexation with tetrafluoroboric acid (HBF4) in order to separate meta-xylene. More recently, methods that exploit zeolites to separate para-xylene from a vapor mixture have been explored but, owing to low efficiency, these methods have not yet been implemented on an industrial scale. Indeed, in this type of application the amount of xylene adsorbed (and separated) in each cycle could be relatively small in comparison to the adsorbent (1 % by weight) and the use of a desorbent is often necessary.
Related methods that employ metal-organic frameworks have been reviewed by Zhou et al and attempts to engineer hydrogen bonded assemblies for the selective inclusion of xylenes have also been reported.
More than fifty years ago a method for the separation of xylenes was suggested that involves enclathration by octahedral metal complexes with the
general formula [ML4X2] (where M is a transition metal, X is an anion and L is a pyridine derivative). These so-called Werner clathrates were implemented for chromatographic separation of xylenes but were not deemed sufficiently viable for industrial application. Indeed, crystallization from solution involves a costly recovery process and also leads to the formation of byproducts that reduce selectivity.
In its pure form the octahedral metal complex typically exists as a nonporous phase (a), and the formation of a host-guest adduct generally involves entrapment of the guest during crystallization of the host from solution. Therefore, the majority of studies report solvent-mediated enclathration processes
The guest-included forms can exhibit a variety of structural arrangements; cage-, layer- and channel-inclusion compounds (referred to as β, γ and δ phases, respectively) are all possible, depending on the geometric relationship between the host and the guest. In some cases the host arrangement is preserved after guest removal and the sorption-desorption processes can be cycled by means of solid-vapor reactions.
SUMMARY OF THE INVENTION
In accordance with this invention there is provided a process for the separation of at least one xylene isomer from another xylene isomer in which the xylene isomers are present in a mixture of xylene isomers, the process involving the sorption of xylene as a guest into a host complex and desorption therefrom whereby at least a primary separation of the isomers is achieved, the process being characterised in that the host complex includes at least one solid octahedral metal complex of the general formula 1
and in that contacting is carried out with the at least one octahedral metal complex in the solid state and the xylene isomer mixture in the vapour phase. Further features of the invention provide for the divalent metal to be selected from nickel, magnesium, iron, cobalt, copper, zinc, and cadmium; for the four substituents R to be selected from methyl and phenyl; and for X to be selected from CI", Br" and (NCS)". Still further features of the invention provide for an initial xylene isomer mixture to contain all three isomers in which instance the process is initially carried out to separate out predominantly one isomer followed by a second sorption process in which a second isomer is separated from the third isomer utilising an otherwise generally similar process; for the separation process to include a solid solution of two or more of the octahedral metal complexes; and for the contacting of solid and vapour to be carried out in a chamber that is at least partially evacuated prior to introduction of the vapour containing the xylene isomers.
In order to demonstrate the operation of the invention and the various differences in properties of the different isomers as guests in a host octahedral metal complex, various tests were carried out utilising the octahedral metal complex in which the substituent R was a phenyl moiety in the para- position; the metal was nickel; and the coordinating anions were thiocyanate (NCS)" anions. The octahedral metal complex had the following formula 2:-
Further details of the tests conducted and the conclusions reached follow in the detailed description below. In this description reference will be made to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings:-
Figure 1 illustrates graphically the behaviour of the three different isomers insofar as extent of reaction as a function of time is concerned; and,
Figure 2 is illustrates graphically the behaviour of the three different isomers insofar as the thermal stability of the inclusion compounds is concerned.
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS In the tests carried out using the octahedral metal complex of Formula 2 that may be described as [Ni(NCS)2(para-phenylpyridine)4] and as a Werner clathrate, was shown to demonstrate significant selectivity for ortho-xylene over meta-xylene and para-xylene from a ternary mixture, and then similar selectivity for meta-xylene over para-xylene from a binary mixture. This implies that a single approach that is based on pressure-swing adsorption can be employed to isolate all three isomers.
The pure octahedral metal complex of Formula 2 was precipitated by slow evaporation of a methanolic solution. The quality and dimensions of the crystals appeared to depend on the initial concentration of the solution as well as the rate of evaporation. Microcrystalline octahedral metal complex of the Formula 2 could be re-dissolved in an appropriate solvent and layered with a number of organic liquids to yield various inclusion compounds. For example, dissolution of the octahedral metal complex in hot dimethyl sulfoxide (dmso), followed by layering with benzene, resulted in the formation of the Werner clathrate [Ni(NCS)2(para-phenylpyridine)4]-4benzene.
Similarly, when the dmso solution was layered with a commercial C8 mixture, [Ni(NCS)2(para-phenylpyridine)4]-2ortho-xylene and [Ni(NCS)2(para- phenylpyridine)4]-2meta-xylene are formed together with a [Ni(NCS)2(para- phenylpyridine)4]-2dmso-para-xylene mixed clathrate.
The pure phases of octahedral metal complex of Formula 2 with ortho-xylene and octahedral metal complex of Formula 2 with meta-xylene were formed directly by the evaporation of a mixture of methanol and ortho-xylene or meta-xylene, respectively, and were confirmed by powder X-ray diffraction (PXRD) analysis.
An analogous procedure afforded the previously unknown crystalline phase [Ni(NCS)2(para-phenylpyridine)4]-3para-xylene when para-xylene is used as a co-solvent with methanol. PXRD also indicated that an as-yet unidentified phase is produced concomitantly.
During investigation of solid-vapour reactions as a means of forming the respective xylene clathrates, polycrystalline octahedral metal complex of Formula 2 was exposed to the pure isomers in separate experiments, and the extent of guest uptake was recorded gravimetrically as a function of time (See Figure 1 ). Sorption of the guest occurred slowly in air, but the rate of uptake was significantly enhanced when the reaction chamber was evacuated prior to admitting the vapor. After each experiment, the product was analyzed by means of PXRD, which confirmed the formation of the corresponding phase resulting from solution growth.
The three inclusion compounds prepared by solid-vapor reactions were analyzed by means of thermogravimetric analysis in order to investigate guest release as a function of temperature (See Figure 2). The inclusion compound octahedral metal complex of Formula 2 and para-xylene appears to be the least robust since the guest molecules are released with an onset temperature of approximately 46.1 °C. The release of the meta- isomer
commences at 72.5 °C and that for the ortho- isomer at 74.8 °C, which shows that ortho-xylene inclusion compound is only slightly more thermally stable than the meta-xylene inclusion compound. It was noted that the thermogram for the ortho-xylene inclusion compound showed a small loss (0.3%) between 20 and 30 °C and this observation is in agreement with the sorption experiment, which showed that the reaction proceeded to slightly over 100% completion for the expected host-guest stoichiometry. Excess solvent was most likely adsorbed onto the crystallite surfaces. This phenomenon was not observed for the other two isomers.
Competition experiments were carried out by exposing polycrystalline octahedral metal complex of Formula 2 to the vapors derived from liquid mixtures of the xylene isomers as shown in Table 1 .
TABLE 1
ortho-, meta-, para-xylene mixture in a 1 :1 :1 ratio
PXRD ortho-xylene
% composition in liquid (GC analysis) 33.1 : 36.9 : 30.0
% composition in crystals (GC analysis) 94.9 : 3.5 : 1 .5 ortho-, meta-xylene mixture in a 1 :1 ratio
PXRD ortho-xylene
o,
Vo composition in liquid (GC analysis) 50.3 : 49.6
Vo composition in crystals (GC analysis) 97.29 : 2.8 ortho-, para-xylene mixture in a 1 :1 ratio
PXRD ortho-xylene
% composition in liquid (GC analysis) 50.1 : 49.9
% composition in crystals (GC analysis) 97.69 : 2.4
meta-, para-xylene mixture in a 1 :1 ratio
PXRD meta-xylene
% composition in liquid (GC analysis) 55.7 : 44.3
% composition in crystals (GC analysis) 94.1 : 5.9
Before each sorption experiment the composition of the liquid xylene mixture was determined by means of gas chromatography (GC). After each experiment, the xylene guest molecules were extracted from the polycrystalline products using hexane and their composition was determined using GC analysis. The results are consistent with the PXRD analyses, confirming that the Werner complex shows remarkable discrimination in favor of ortho-xylene in the presence of meta-xylene and/or para-xylene.
In the absence of ortho-xylene, the host reprioritizes its preference, becoming highly selective towards meta-xylene. The selectivity trend ortho-xylene» meta-xylene» para-xylene does not comport well with the trend in thermal stability, and not at all with the kinetics of sorption (rate constants of 1 .35x10"2, 4.72x10"2and 2.73 x10"2 min"1 were determined for ortho-xylene, meta-xylene and para-xylene). It appears that the isomeric selectivity exhibited by octahedral metal complex of Formula 2 is dominated by thermodynamic rather than kinetic factors.
It has been shown that the host of Formula 2 readily absorbs any of the three xylene isomers in a solid-vapor process, but that significant differences in selectivity are observed when the isomers are mixed. Conceptually, this implies that in cases where there is a large difference in selectivity x for three guests A, B, and C such that xA » xB » xC, then it is possible to utilize the same protocol (even the same device) to first separate A from B and C, and then to repeat the process to separate B and C. This is important for implementing separation technologies in commercial applications where such selectivity is required, as is indeed the case for xylenes.
The proof of concept demonstrated here should be amenable to applications involving pressure-swing adsorption technology whereby a single solventless process can be implemented, in principle, in order to effect complete separation of three or more isomers. Previously reported methods of separating xylenes using Werner clathrates were not viable because they required dissolution, crystallization and solvent extraction. This study revisits the use of Werner clathrates, showing that the energy-intensive, time-consuming and costly liquid-phase steps can be eliminated in a pressure-swing adsorption process that employs a substrate that exhibits prioritized selectivity.
It is envisaged that a number of other octahedral metal complexes will behave in a similar manner.
Claims
1 . A process for the separation of at least one xylene isomer from another xylene isomer in which the xylene isomers are present in a mixture of xylene isomers, the process involving the sorption of xylene as a guest into a host complex and desorption therefrom whereby at least a primary separation of the isomers is achieved, the process being characterised in that the host complex includes at least one solid octahedral metal complex of the general formula 1
and in that contacting is carried out with the at least one octahedral metal complex in the solid state and the xylene isomer mixture in the vapour phase.
A process as claimed in claim 1 in which the divalent metal is selected from nickel, magnesium, iron, cobalt, copper, zinc, and cadmium.
3. A process as claimed in either one of claims 1 or 2 in which the four substituents R are selected from methyl and phenyl.
4. A process as claimed in any one of the preceding claims in which X is selected from CI", Br" and (NCS)".
5. A process as claimed in any one of the preceding claims in which an initial xylene isomer mixture contains all three isomers in which instance the process is initially carried out to separate out predominantly one isomer followed by a second sorption process in which a second isomer is separated from the third isomer utilising an otherwise generally similar process.
6. A process as claimed in any one of the preceding claims in which the separation process include a solid solution of two or more of the octahedral metal complexes.
7. A process as claimed in any one of the preceding claims in which the contacting of solid and vapour is carried out in a chamber that is at least partially evacuated prior to introduction of vapour containing the xylene isomers.
8. A process as claimed in any one of the preceding claims in which the octahedral metal complex has the following formula 2:-
5
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
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| US10239044B2 (en) | 2014-06-06 | 2019-03-26 | Northwestern University | Carbohydrate-mediated purification of petrochemicals |
| CN114805006A (en) * | 2022-05-13 | 2022-07-29 | 中国石油大学(北京) | Method and device for separating p-xylene from xylene mixture |
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| US3013091A (en) * | 1958-11-18 | 1961-12-12 | Union Oil Co | Vapor phase separation of organic compounds utilizing werner complexes |
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| US3013091A (en) * | 1958-11-18 | 1961-12-12 | Union Oil Co | Vapor phase separation of organic compounds utilizing werner complexes |
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Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
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| US10239044B2 (en) | 2014-06-06 | 2019-03-26 | Northwestern University | Carbohydrate-mediated purification of petrochemicals |
| CN114805006A (en) * | 2022-05-13 | 2022-07-29 | 中国石油大学(北京) | Method and device for separating p-xylene from xylene mixture |
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