JP7660876B2 - Adsorbent for alkane and method for separating isooctane - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act 1. Announcement on a website Posting address: https://chemistry-europe. onlinelibrary. wiley. com/doi/abs/10.1002/chem. 202000043 Download from the above posting address Chemistry-A European Journal, vol. 26, Issue 38, pages 8393-8399 Posting date: March 31, 2020

The present invention relates to an alkane adsorbent and a method for separating isooctane.

Alkanes are saturated hydrocarbons that are linear, branched, or cyclic alkanes. Even today, with the development of advanced separation and refining methods and molecular recognition, it is difficult to effectively separate these alkanes, which have little electronic bias. For this reason, there is a demand for the development of materials that can effectively separate alkanes. In particular, the separation of alkanes from gasoline is considered an essential technology for improving the octane number. Gasoline is a mixture of alkanes and alkenes with carbon numbers of 4 to 11, and aromatic compounds. Gasoline with a research octane number (RON) of 96 or more is classified as high-octane, and gasoline with a RON of 86 or more is classified as regular. This RON is an index evaluated by the volume ratio of isooctane in gasoline to isooctane in a n-heptane mixture.

Because the boiling points of isooctane and n-heptane (99°C and 98°C) are close, it is difficult to separate n-heptane by distillation to obtain high-RON gasoline. There are two methods for increasing the RON content of gasoline: (1) thermal reforming using high temperature and pressure, and (2) catalytic reforming using a platinum-immobilized catalyst, with (2) being the predominant method in Japan. Linear alkanes are increased in RON content by converting them into branched alkanes in a reaction using an isomerization catalyst.

However, this isomerization reaction has the disadvantage that once the reaction from linear alkanes to branched alkanes has progressed to a certain point, the reaction stops (because the reaction reaches equilibrium). For this reason, various methods are used to remove unreacted linear alkanes. One of these is to separate them by contacting them with an adsorbent filled with zeolite 5A. Generally, alkanes themselves are saturated hydrocarbon compounds composed of carbon-carbon and carbon-hydrogen bonds. In other words, alkanes have low polarization (low polarity), and sufficient interaction with the adsorbent cannot be expected, so it takes time to separate the unreacted linear alkanes from the branched alkanes.

In addition to zeolite 5A, the potential of carbon molecular sieves, new zeolite materials, metal-organic frameworks (MOFs), and other materials as adsorbents has been reported (Non-Patent Document 1), but effective separation is difficult for the reasons mentioned above. Therefore, it is necessary to develop new methods and materials that can separate low-polarity alkanes with high selectivity. The development of materials that can effectively recognize the shape and size of alkanes and selectively adsorb them in a short period of time is considered to be an important research area that can contribute to the refining of high RON gasoline in the future.

Barcia, P. S., Silva, J. A. C., Rodriguez, A. E. Adsorption dynamics of C5‐C6 isomerate fractions in zeolite beta for the octane improvement of gasoline. Energy Fuels 2010, 24, 1931-1940

Based on the above, the objective of the present invention is to provide an adsorbent with a new structure that is capable of adsorbing alkanes in a shorter time than conventional methods.

After extensive research, the inventors discovered that the above problems can be solved by using an adsorbent containing a supramolecular organic crystal of a specific thiacalixarene structure as the active ingredient.

The first aspect of the present invention is an adsorbent for alkanes, characterized in that the active ingredient is a supramolecular organic crystal constructed from thiacalixarene represented by the following general formula (I):

In the first aspect of the present invention, the alkane to be adsorbed is preferably a branched alkane.

In the first aspect of the present invention, the alkane to be adsorbed is preferably isooctane.

The second invention is a method for separating isooctane, which includes an adsorption step in which the adsorbent of the first invention selectively adsorbs isooctane from a mixture of isooctane and n-heptane.

In the second invention, it is preferable to include a step of removing the crystallization solvent contained in the supramolecular organic crystal by heating the adsorbent of the first invention before the adsorption step.

The adsorbent of the present invention is capable of adsorbing alkanes in a shorter time than conventional methods.

FIG. 1 shows the results of a competitive adsorption experiment of the activated crystals towards a binary alkane mixture. FIG. 2 shows the results of the alkane vapor sorption advantage of the activated crystals. FIG. 3(a) shows the vapor adsorption isotherms of the activated crystals for 2,2-dimethylbutane and 2,3-dimethylbutane, FIG. 3(b) shows the vapor adsorption isotherms of the activated crystals for cyclopentane, cyclohexane, and cycloheptane, and FIG. 3(c) shows the vapor adsorption isotherms of the activated crystals for n-pentane, n-hexane, and n-heptane. FIG. 4 is an explanatory diagram showing an experimental method for the adsorption experiment of isooctane onto activated crystals. Figure 5 shows the results of nuclear magnetic resonance spectroscopy (NMR) of the crystals that had adsorbed isooctane vapor taken out of the sample tube. The bottom image is an enlarged view. FIG. 6(A) is a powder X-ray diffraction (PXRD) pattern of a supramolecular organic crystal of thiacalixarene of formula (I) immediately after preparation, FIG. 6(B) is a PXRD pattern of an activated crystal obtained by removing cyclohexane from the supramolecular organic crystal, and FIG. 6(C) is a PXRD pattern of the activated crystal after exposure to isooctane. FIG. 7 is a graph showing the adsorption equilibrium time of the activated crystals for isooctane. Figure 8 shows the adsorption isotherms for isooctane and n-heptane. In the figure, the black circles and circles indicate the adsorption and desorption of isooctane, while the black triangles and triangles indicate the adsorption and desorption of n-heptane. 9 shows the ¹ H NMR spectrum measured for the crystals obtained in a competitive adsorption experiment from a mixture of isooctane and n-heptane. The lower part shows an enlarged view. FIG. 10 shows the results of investigating the selectivity when the volume ratio of isooctane to n-heptane was changed. FIG. 11 shows the results of repeatedly adsorbing isooctane onto the activated crystal.

<Adsorbent>
The adsorbent of the present invention has as its active ingredient a supramolecular organic crystal constructed from thiacalixarene represented by the following general formula (I), and is an adsorbent having an alkane as its adsorption target.

The characteristic of the adsorbent of the present invention is that it can effectively separate "alkanes", preferably "branched alkanes", and more preferably "isooctane", which is used in the RON evaluation criteria, by taking advantage of the properties of thiacalixarene supramolecular organic crystals, which can effectively recognize the structure (shape and size) of alkanes.

The active ingredient of the adsorbent of the present invention, "a three-dimensional supramolecular organic crystal having channel-type flexible adsorption sites constructed by the assembly of thiacalixarene molecules 1," has the following characteristics.
(1) The ability to recognize the shape and size of alkanes and "gate" the adsorption site on/off;
(2) The characteristic of rapid adsorption of alkanes accompanied by a gate opening effect.
(3) Flexible structure;
These characteristics allow for effective capture of only a specific "alkane", preferably a "branched alkane", more preferably only isooctane. In addition, since isooctane can be selectively and effectively separated, it is highly expected to be used as an adsorbent capable of achieving a high RON.

(Thiacalixarenes)
The thiacalixarene constituting the adsorbent of the present invention is a cyclic compound composed of elemental sulfur and aromatic groups, and is a tetramer, i.e., a thiacalix[4]arene, in which the opposing phenol units are inverted to form a 1,3-alternate type.
Each aromatic ring has a Br group at the 4-position and an OPr group at the 1-position.

(Supramolecular organic crystals)
The above-mentioned thiacalixarene forms a supramolecular organic crystal containing cyclohexane. By heating the supramolecular organic crystal composed of the above-mentioned thiacalixarene and cyclohexane, the cyclohexane can be released from the crystal. In this specification, the crystal obtained after removing cyclohexane from the above-mentioned supramolecular organic crystal is called an "activated crystal."
The temperature in the preparation of the activated crystals is preferably 80° C. or higher and 150° C. or lower, more preferably 90° C. or higher and 120° C. or lower. The activated crystals are preferably prepared by heating under reduced pressure, but the degree of reduced pressure is not particularly limited.

The inventors discovered that the activated crystal obtained after removing cyclohexane from the supramolecular organic crystal is non-porous, yet is able to recognize the shape and size of alkanes and adsorb alkane vapor. This phenomenon occurs when the vapor pressure of the alkane reaches a certain level, causing the adsorption gate in the crystal to fully open (gate opening effect), and the alkane vapor is rapidly adsorbed.

In addition, the crystals are not constructed with strong bonds such as covalent bonds or coordinate bonds, but are formed by the aggregation of thiacalixarenes of formula (I) through relatively weak intermolecular interactions. This means that the crystal structure is flexible, and the vapor adsorption experiments conducted by the inventors have revealed that the size of the alkanes that can be adsorbed can be within a wide range.

(Subject to absorption)
The activated crystals recognize the shape and size of alkanes and specifically adsorb the vapor of the alkanes. Examples of the adsorbable alkanes include branched alkanes such as 2,2-dimethylbutane, 2,3-dimethylbutane, and isooctane, and cyclic alkanes such as cyclopentane, cyclohexane, and cycloheptane.
On the other hand, the activated crystals do not adsorb vapors of straight-chain alkanes, such as n-heptane, n-hexane, and n-pentane.

The time required to adsorb the target alkanes, i.e., the time it takes for the adsorbent's adsorption capacity to reach saturation, is approximately 3 to 5 hours.

The adsorbent of the present invention is expected to have functionality as an RON improver by combining the ability to recognize the shape and size of alkanes with flexible structural features. In other words, it is expected to function as an RON improver that has the function of selectively adsorbing branched alkanes from a mixture of branched alkanes and linear alkanes.
The adsorbent of the present invention can be used as an adsorbent that selectively adsorbs isooctane from a mixture of isooctane as a branched alkane and n-heptane as a linear alkane, and can be used as a RON improver.

<Method for separating isooctane>
(Adsorption process)
The method for separating isooctane of the present invention comprises an adsorption step of contacting a mixture of isooctane and n-heptane with the above-described adsorbent of the present invention to selectively adsorb isooctane from the mixture.

As shown in the examples below, the method of contacting the mixture of isooctane and n-heptane with the adsorbent of the present invention is preferable.

The time required for selectively adsorbing isooctane from a mixture of isooctane and n-heptane onto the adsorbent, i.e., the time until the adsorption capacity of the adsorbent becomes saturated, is between 40 and 60 hours.

When the partial pressure of isooctane reaches a certain level, the adsorption gate of the adsorbent opens fully and adsorption begins, so it is preferable to set the partial pressure of isooctane to 4 kPa or more.

(Crystallization Solvent Removal Step)
It is preferable to have a step of removing the crystallization solvent from the supramolecular organic crystals in the adsorbent of the present invention to form activated crystals, prior to the above-mentioned adsorption step.
The crystallization solvent removal step can be carried out by heating the supramolecular organic crystal. Preferably, the supramolecular organic crystal can be heated under reduced pressure. The degree of reduced pressure is not particularly limited. The heating temperature is preferably 80° C. or higher and 150° C. or lower, and preferably 90° C. or higher and 120° C. or lower.
Examples of the crystallization solvent contained in the supramolecular crystal include cyclohexane contained in the supramolecular crystal when it is formed, and isooctane contained in the crystal after the above-mentioned adsorption step.

<Preparation of Activated Crystals>
A supramolecular organic crystal of thiacalixarene represented by the following formula (I) was heated overnight at 100° C. under reduced pressure to prepare an activated crystal from which cyclohexane contained in the supramolecular organic crystal was removed.

<Adsorption experiment of alkane mixture onto activated crystals (evaluation of the adsorption advantage of alkanes)>
The results of a competitive adsorption experiment of the activated crystals with a binary alkane mixture are shown in Figure 1. The amount of each alkane adsorbed was determined by ¹ H NMR.
[a] The number of adsorbed alkanes is a value measured at equilibrium. The number of alkanes was calculated from the integral value of the measured ¹ H NMR chemical shift of the alkane, assuming that the thiacalixarene of formula (I) is one molecule.
The measurements were performed by placing 300 mg of activated crystals in an open 15 mL vial in a 50 mL vial containing 5 mL of a 1:1 (v/v) alkane mixture for 2 days at ambient temperature and pressure.

As shown in Figure 1, various alkane mixtures showed different adsorption advantages. The adsorption advantages of alkane vapors to activated crystals are summarized in Figure 2.

3 shows the adsorption isotherms of the activated crystals for branched alkanes (a), cyclic alkanes (b), and linear alkanes (c). The measurements were performed at 298K.
It was shown that no linear alkanes were adsorbed at all (Figure 3(c)). On the other hand, the branched alkanes 2,2-dimethylbutane and 2,3-dimethylbutane, as well as cyclopentane, cyclohexane, and cycloheptane, were adsorbed. Figure 3(b) shows that the order of adsorption priority for cyclic alkanes is cyclopentane < cyclohexane < cycloheptane, meaning that the larger the size of the cyclic alkane, the more preferentially it is adsorbed. The priority when branched alkanes are also included is as shown in Figure 2.

<Adsorption experiment of isooctane onto activated crystals>
An experiment was carried out in which isooctane was adsorbed onto the activated crystals prepared above. The experimental method is shown in Figure 4. A 15 mL sample tube containing the activated crystals was placed in a 50 mL sample tube containing isooctane, and the tube was covered and left to stand for two days.

The crystals that had adsorbed isooctane vapor were removed from the sample tube, and the results of nuclear magnetic resonance spectrum analysis (NMR) are shown in Figure 5. As shown in the enlarged view at the bottom of Figure 5, the chemical shifts of isooctane that had been incorporated into the activated crystals were detected at 0.89 ppm, 0.90 ppm, 1.17 ppm, and 1.65 ppm. It was confirmed that the crystals had adsorbed isooctane.

Next, the powder X-ray diffraction (PXRD) patterns of the crystals taken out of the sample tube are shown in Figure 6. From the bottom, they are the PXRD pattern (A) immediately after the preparation of the supramolecular organic crystal of thiacalixarene of formula (I), the PXRD pattern (B) of the activated crystal obtained by removing cyclohexane from the supramolecular organic crystal, and the PXRD pattern (C) after exposing the activated crystal to isooctane.

By exposing the activated crystals to isooctane, they show a PXRD pattern similar to that of the thiacalixarene supramolecular organic crystal immediately after preparation (A). This suggests that the activated crystals adsorb areoctane, changing the crystal structure to a shape similar to that of the thiacalixarene supramolecular organic crystal. In other words, this means that isooctane has been incorporated into the position where cyclohexane was previously incorporated.

<Adsorption equilibrium time and adsorption isotherm of isooctane onto activated crystals>
To investigate the adsorption equilibrium time for isooctane on the activated crystals, the amount of isooctane adsorbed at each time point after exposure to isooctane was calculated by ^1H NMR nuclear magnetic resonance spectroscopy. The exposure experiment was carried out using the experimental equipment shown in Figure 4 in <Adsorption experiment of isooctane on activated crystals>. The exposure time was 1-19 hours, and the ratio of isooctane per thiacalixarene molecule as the amount of adsorption is shown in Figure 7. It was revealed that the activated crystals rapidly adsorbed isooctane between 1 and 4 hours, and the amount of isooctane adsorbed after 4 hours reached equilibrium.

Next, the adsorption isotherms for isooctane and n-heptane are shown in Figure 8. In the figure, ● and ◯ indicate the adsorption and desorption of isooctane, while ▲ and △ indicate the adsorption and desorption of n-heptane. Adsorption of isooctane began abruptly at a vapor pressure of approximately 4 kPa, with the maximum amount of adsorption being 15 mL. This abrupt adsorption phenomenon means that the adsorption sites in the activated crystals are turned ON and are taking in isooctane. As for desorption, no desorption was observed even when the pressure in the system was reduced, and it was observed that isooctane was retained stably. On the other hand, this experiment proved that the activated crystals do not exhibit any adsorption ability for n-heptane. From the above, it can be concluded that this activated crystal recognizes and adsorbs isooctane.

<Adsorption experiment from mixed vapor of isooctane and n-heptane>
In order to verify whether the activated crystals adsorb only isooctane, a competitive adsorption experiment was carried out from a mixture of isooctane and n-heptane. The volumetric mixing ratio of isooctane and n-heptane was set to 1:1, and the experiment was carried out using the experimental equipment shown in Figure 4 shown in <Adsorption experiment of isooctane to activated crystals>. Figure 9 shows the ¹ H NMR spectrum measured for the crystals obtained when the exposure time to the mixed vapor was set to 2 days. No chemical shift of n-heptane was observed, and only the chemical shift of isooctane was detected. The chemical shifts of isooctane incorporated into the activated crystals are 0.89 ppm, 0.90 ppm, 1.17 ppm, and 1.65 ppm. In other words, it can be concluded that the crystals selectively adsorb only isooctane.

Next, the selectivity when the volume ratio of isooctane to n-heptane was changed was also investigated. The volume ratios were isooctane:n-heptane = 0.2:1, 0.4:1, 0.6:1, and 0.8:1. This experiment was performed using the experimental equipment shown in Figure 4 in <Adsorption experiment of isooctane to activated crystals>. The adsorption amounts of isooctane and n-heptane were calculated from the integrated intensity of ^1H NMR. The ratio of isooctane per thiacalixarene molecule as the adsorption amount is shown in Figure 10. The exposure time was 5 to 168 hours.
First, this experiment confirmed by ¹ H NMR that the activated crystals did not adsorb n-heptane at any volume ratio. The amount of isooctane adsorbed was shown to be greater with a higher ratio of isooctane. In a competitive adsorption experiment, it was revealed that the adsorption equilibrium of isooctane was 48 hours.

<Repeated adsorption experiment of isooctane (repeated use of activated crystals)>
The results of repeated adsorption of isooctane onto the activated crystals are shown in Figure 11. As shown in Figure 4, a 15 mL sample tube containing the activated crystals was placed into a 50 mL sample tube containing isooctane, the tube was capped, and the tube was left to stand for two days (first adsorption).
The amount of isooctane adsorbed (mL/g) was then calculated by ¹ H NMR of the nuclear magnetic resonance spectrum. The crystals adsorbing isooctane were heated overnight at 100° C. under reduced pressure to remove the isooctane contained therein, and the activated crystals were reprepared (desorption treatment).
The re-prepared activated crystals were subjected to the second adsorption treatment and the second desorption treatment in the same manner as above, and these treatments were repeated up to five times.

As shown in Figure 11, it was found that repeated adsorption and desorption of isooctane gradually enables adsorption at a lower vapor pressure. In other words, it was found that the adsorption capacity of the adsorbent of the present invention can be improved by performing the solvent adsorption and desorption process preferably 1 to 5 times, more preferably 3 to 5 times, and even more preferably 4 to 5 times before use.

The adsorbent of the present invention can effectively separate alkanes, preferably branched alkanes, and more preferably isooctane, by taking advantage of the properties of thiacalixarene supramolecular organic crystals that can effectively recognize the structures (shapes and sizes) of alkanes.
Furthermore, the method for separating isooctane of the present invention is suitable for use as a method for improving the RON of gasoline.

Claims (5)

An adsorbent for alkanes, comprising as an active ingredient a supramolecular organic crystal constructed from thiacalixarene represented by the following general formula (I):

The adsorbent according to claim 1, wherein the alkane to be adsorbed is a branched alkane. The adsorbent according to claim 1, wherein the alkane to be adsorbed is isooctane. A method for separating isooctane, comprising an adsorption step in which isooctane is selectively adsorbed from a mixture of isooctane and n-heptane onto the adsorbent according to claims 1 to 3. The method for separating isooctane according to claim 4, further comprising a step of removing the crystallization solvent contained in the supramolecular organic crystal by heating the adsorbent according to claims 1 to 3 prior to the adsorption step.