ZEOLITE-BASED ADSORBENTS BASED ON ZEOLITE X WITH A LOW BINDER CONTENT AND A LOW OUTER SURFACE AREA, PROCESS FOR PREPARING THEM AND USES THEREOF

Abstract

The invention relates to an adsorbent comprising a zeolite-based phase and a non-zeolite-based phase, said adsorbent having: an outer surface area of less than or equal to 30 m.sup.2.Math.g.sup.−1, preferably less than or equal to 20 m.sup.2.Math.g.sup.−1, a zeolite-based phase comprising at least one zeolite of FAU structure of X type, and a pore diameter distribution, determined by mercury intrusion according to standard ASTM D 4284-83 and expressed by the volume distribution dV/d log DHg, in which DHg is the apparent pore diameter and V is the pore volume, the mode of which is between 100 nm and 250 nm, limits inclusive.

The invention also relates to a process for preparing the said adsorbent and to the uses thereof, especially for separating xylene isomers.

Claims

1. An adsorbent comprising a zeolite-based phase and a non-zeolite-based phase, wherein said adsorbent: has an outer surface area of less than or equal to 30 m.sup.2.Math.g.sup.−1, a pore diameter distribution, determined by mercury intrusion according to standard ASTM D 4284-83 and expressed by the volume distribution dV/d log D.sub.Hg, wherein D.sub.Hg is the apparent pore diameter and V is the pore volume, the mode of which is between 100 nm and 250 nm, limits inclusive, and the zeolite-based phase comprises at least one zeolite of FAU structure of X type.

2. The adsorbent according to claim 1, wherein the pore diameter distribution corresponds to a unimodal distribution.

3. The adsorbent according to claim 1, having a micropore volume, evaluated via the t-plot method from the nitrogen adsorption isotherm at a temperature of 77 K, which is greater than 0.200 cm.sup.3.Math.g.sup.−1.

4. The adsorbent according to claim 1, wherein the adsorbent has a content of non-zeolite-based phase between 2% and 8% by weight relative to the total weight of the adsorbent.

5. The adsorbent according to any one of the preceding claim 1, further comprising barium or barium and potassium.

6. The adsorbent according to claim 1, having macropores and the mesopores, wherein a total volume contained in the macropores and the mesopores as measured by mercury intrusion according to standard ASTM D4284 83 is between 0.15 cm.sup.3.Math.g.sup.−1 and 0.5 cm.sup.3.Math.g.sup.−1, limits inclusive.

7. The adsorbent according to claim 6 having a ratio defined as (macropore volume)/(macropore volume+mesopore volume) of between 0.2 and 1, limits inclusive.

8. The adsorbent according to claim 1, further having an Si/Al atomic ratio of between 1.00 and 1.50, limits inclusive.

9. A process for preparing the adsorbent according to claim 1, comprising the steps of: a) agglomeratinq crystals of at least one FAU zeolite having an outer surface area as measured by nitrogen adsorption, of greater than 20 m.sup.2.Math.g.sup.−1, limits inclusive, with a binder and also with an amount of water which allows the forming of an agglomerated material, followed by drying and calcination of the agglomerated material; b) carrying out zeolitization of all or part of the binder by placing the agglomerated material obtained in step a) in contact with an aqueous basic solution, optionally in the presence of at least one structuring agent; c) optionally, removing the structuring agent optionally present; d) carrying out cationic exchange(s) of the agglomerated material of step b) or c) by placing in the agglomerated material in contact with a solution of barium ions or of barium ions and potassium ions; e) optionally, carrying out additional cationic exchange of the agglomerated material of step d) by placing the agglomerated material in contact with a solution of potassium ions; f) washing and drying of the agglomerated material obtained in step d) or e), at a temperature of between 50° C. and 150° C.; and g) producing the zeolite-based adsorbent by activating the agglomerated material obtained in step f) under a stream of a gas selected from the group consisting of oxidizing gases and inert gases, wherein the gas is at a temperature of between 100° C. and 400° C.

10. The process according to claim 9, wherein the agglomeration binder used in step a) contains at least one zeolitizable clay.

11. The process according to claim 9, wherein the FAU zeolite crystals used during the agglomeration step (step a) have a number-mean diameter of between 1 μm and 20 μm, limits inclusive.

12. The process according to claim 9 wherein the FAU zeolite used in step a) is a hierarchically porous FAU zeolite.

13. A process, comprising using an adsorbent according to claims 1 as an adsorption agent in: separating C8 aromatic isomer fractions or, separating substituted toluene isomers or, separating cresols, or separating polyhydric alcohols.

14. A process for the gas-phase or liquid-phase separation of xylene isomers using at least one adsorbent according to claim 1.

15. A process for separating para-xylene from a feedstock of aromatic isomer fractions containing 8 carbon atoms, using, as para-xylene adsorption agent, an adsorbent according to claim 1.

16. The process according to claim 15, wherein the process is performed in a counter-current simulated moving bed adsorption unit, under the following operating conditions: number of beds: 4 to 24; number of zones: at least 4 operating zones, each being located between a feed point and a withdrawal point; temperature between 100° C. and 250° C.; pressure between the bubble pressure of xylenes (or of toluene when toluene is chosen as desorbent) at the process temperature and 3 MPa; ratio of the flow rates of desorbent to feedstock to be treated: 0.7 to 2.5; recycling rate between 2 and 12, cycle time, corresponding to the time between two injections of desorbent onto a given bed: between 4 and 25 minutes.

17. The process according to claim 16, wherein the desorbent is toluene or para-diethylbenzene.

18. The process according to claim 16 wherein a water content in the inlet streams constituted by the feedstock and/or desorbent streams is adjusted to between 20 ppm and 150 ppm.

19. The process according to claim 10, wherein the one zeolitizable clay is chosen from the group consisting of kaolins, kaolinites, nacrites, dickites, halloysites and metakaolins, and mixtures thereof.

20. The adsorbent according to claim 1 having a micropore volume, evaluated via the t-plot method from the nitrogen adsorption isotherm at a temperature of 77 K, which is between 0.205 cm.sup.3.Math.g.sup.−1 and 0.300 cm.sup.3.Math.g.sup.−1.

21. The adsorbent according to claim 1 having a micropore volume, evaluated via the t-plot method from the nitrogen adsorption isotherm at a temperature of 77 K, which is between 0.205 cm.sup.3.Math.g.sup.−1 and 0.290 cm.sup.3.Math.g.sup.−1.

Description

EXAMPLES

Example A

Synthesis of Hierarchically Porous FAU Zeolite

[0175] FAU zeolite with a high outer surface area is synthesized directly according to the article by Inayat et al. (Angew. Chem. lnt. Ed., (2012), 51, 1962-1965). [0176] Step 1): Preparation of the Growth Gel in the Reactor Stirred with Archimedean Screw at 300 rpm

[0177] A growth gel is prepared in a stainless-steel reactor equipped with a heating jacket, a temperature probe and a stirrer, by mixing an aluminate solution containing 119 g of sodium hydroxide (NaOH), 128 g of alumina trihydrate (Al.sub.2O.sub.3.3H.sub.2O, containing 65.2% by weight of Al.sub.2O.sub.3) and 195.5 g of water at 25° C. over 25 minutes, with a stirring speed of 300 rpm, with a silicate solution containing 565.3 g of sodium silicate, 55.3 g of NaOH and 1997.5 g of water at 25° C.

[0178] The stoichiometry of the growth gel is as follows: 3.48 Na.sub.2O/Al.sub.2O.sub.3/3.07 SiO.sub.2/180 H.sub.2O. Homogenization of the growth gel is performed with stirring at 300 rpm for 25 minutes at 25° C. [0179] Step 2): Introduction of the Structuring Agent into the Reaction Medium

[0180] 27.3 g of a solution of TPOAC at 60% in MeOH are introduced into the reaction medium with a stirring speed of 300 rpm (TPOAC/Al.sub.2O.sub.3 mole ratio=0.04). After 5 minutes of homogenization, the stirring speed is lowered to 50 rpm. [0181] Step 3): Maturation Phase

[0182] The reaction medium is kept stirring at 50 rpm at 25° C. for 22 hours, and crystallization is then started. [0183] Step 4): Crystallization

[0184] The stirring speed is maintained at 50 rpm, and the reactor jacket is set to a nominal value of 80° C. so that the reaction medium rises in temperature to 75° C. over 80 minutes. After 72 hours at a stage of 75° C., the reaction medium is cooled by circulating cold water through the jacket to stop the crystallization. [0185] Step 5): Filtration/Washing

[0186] The solids are recovered on a sinter and then washed with the permuted water to neutral pH. [0187] Step 6): Drying/Calcination

[0188] In order to characterize the product, drying is performed in an oven at 90° C. for 8 hours, and the loss on ignition of the dried product is 22% by weight.

[0189] Calcination of the dried product, which is necessary to release both the microporosity (water) and the mesoporosity by removing the structuring agent, is performed with the following temperature profile: 30 minutes of increase to 200° C., then 1 hour at a stage of 200° C., then 3 hours of increase to 550° C., and finally 1.5 hours at a stage of 550° C.

[0190] The crystals obtained are identified by x-ray diffraction (XRD analysis) as being faujasite crystals. Chemical analysis of the solid gives an Si/Al atomic ratio=1.24. The number-mean diameter of the crystals of the mesoporous zeolite (or hierarchically porous zeolite) thus obtained is 4.5 μm.

[0191] The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.260 cm.sup.3.Math.g.sup.−1 and 90 m.sup.2.Math.g.sup.−1 expressed per gram of dry adsorbent.

Example B

Synthesis of Non-Mesoporous Zeolite X crystals with an Si/Al Atomic Ratio=1.25, a Number-Mean Diameter of 1.0 μm and an Na/Al Atomic Ratio =1

[0192] A gel of molar composition 3.5 Na.sub.2O−2.8 SiO.sub.2−Al.sub.2O.sub.3−130 H.sub.2O is prepared by mixing the following reagents: sodium silicate, sodium aluminate and water. The gel is matured at 35° C. for 20 hours, and crystallization is performed for 4 hours at 100° C.

[0193] The crystals obtained after filtration and washing are identified by x-ray diffraction (XRD analysis) as being faujasite crystals. Chemical analysis of the solid gives an Si/Al atomic ratio=1.25. The number-mean diameter of the zeolite crystals is 1.0 μm. The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.345 cm.sup.3.Math.g.sup.−1 and 2 m.sup.2.Math.g.sup.−1 expressed per gram of dry adsorbent.

Preparation of the Zeolite-Based Adsorbents

[0194] A homogeneous mixture is prepared and 1600 g of crystals of NaX zeolite prepared according to the procedures described in Examples A or B are agglomerated with 350 g of kaolin (expressed as calcined equivalent) and 130 g of colloidal silica sold under the trade name Klebosol® 30 (containing 30% by weight of SiO.sub.2 and 0.5% of Na.sub.2O) with the amount of water allowing extrusion of the mixture. The loss on ignition of the paste before extrusion is 44%. Extrudates 1.6 mm in diameter are formed. The extrudates are dried overnight in a ventilated oven at 80° C. They are then calcined for 2 hours at 550° C. under a stream of nitrogen, and then for 2 hours at 550° C. under a stream of dry, decarbonated air and then crushed so as to recover grains with an equivalent diameter equal to 0.4 mm.

Example 1: (comparative)

Preparation of a Zeolite-Based Adsorbent in Crushed Form with a Type X Zeolite, the Zeolite Crystals 1.0 μm in Size, and a Binder of Non-Zeolitized Kaolin Type

[0195] Granules (200 g) obtained from the powder synthesized in Example B are exchanged using a 0.5 M barium chloride solution at 95° C. in 4 steps. At each step, the volume ratio of solution to mass of solid is 20 ml.Math.g.sup.−1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to free it of the excesses of salt. It is then activated at a temperature of 250° C. for 2 hours under a stream of nitrogen.

[0196] The degree of barium exchange is 97% and the loss on ignition is 5.4%. The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.226 cm.sup.3.Math.g.sup.−1 and 167 m.sup.2.Math.g.sup.−1.

[0197] The total volume of the macropores and mesopores measured by mercury porosimetry is 0.32 cm.sup.3.Math.g.sup.−1. The volume fraction of the macropores to the total volume of the macropores and mesopores is equal to 0.87.

[0198] The pore diameter distribution is determined from the analysis by mercury intrusion performed on the adsorbent and represented by the volume distribution dV/d log D.sub.Hg as a function of the apparent diameter of the pores D.sub.Hg. The distribution shows a distinct peak in the region of the macropores corresponding to a unimodal distribution about a mode equal to about 350 nm.

[0199] The mechanical strength is also measured according to the method presented in the description of the invention. The pressure required to obtain 0.5% of fines is 2.2 MPa.

Example 2: (comparative)

Preparation of a Zeolite-Based Adsorbent in Crushed Form with a Zeolite of X Type, the Zeolite Crystals 1.0 μm in Size, and a Binder of Zeolitized Kaolin Type

[0200] Granules (200 g) obtained from the powder synthesized in Example B are placed in a glass reactor equipped with a jacket regulated at a temperature of 100° C.±1° C., and 1.5 L of an aqueous sodium hydroxide solution of concentration 1 M are then added and the reaction medium is left stirring for a time of 3 hours.

[0201] The agglomerates are then washed in 3 successive operations of washing with water followed by emptying the reactor. The washing efficiency is ensured by measuring the final pH of the washing waters, which is between 10.0 and 10.5.

[0202] These agglomerates are exchanged using a 0.5 M barium chloride solution at 95° C. in 4 steps. At each step, the volume ratio of solution to mass of solid is 20 ml.Math.g−.sup.1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to free it of the excesses of salt. It is then activated at a temperature of 250° C. for 2 hours under a stream of nitrogen.

[0203] The degree of barium exchange is 97% and the loss on ignition is 5.3%. The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.249 cm.sup.3.Math.g.sup.−1 and 5 m.sup.2.Math.g.sup.−1.

[0204] The total volume of the macropores and mesopores measured by mercury porosimetry is 0.29 cm.sup.3.Math.g.sup.−1. The volume fraction of the macropores to the total volume of the macropores and mesopores is equal to 0.97.

[0205] The pore diameter distribution is determined from the analysis by mercury intrusion performed on the adsorbent and represented by the volume distribution dV/d log D.sub.Hg as a function of the apparent pore diameter D.sub.Hg (cf. FIG. 1). The distribution shows a distinct peak in the region of the macropores corresponding to a unimodal distribution about a mode equal to about 360 nm.

[0206] The content of non-zeolite-based phase is equal to 5% by weight, as measured by XRD, using as reference the starting zeolite crystals that have undergone the same barium exchange.

[0207] The mechanical strength is also measured according to the method presented in the description of the invention. The pressure required to obtain 0.5% of fines is 2.5 MPa.

Example 3: (comparative)

Preparation of a Zeolite-Based Adsorbent in Crushed Form with a Zeolite of HPX Type, the Zeolite Crystals 4.5 μm in Size, and a Binder of Non-Zeolitized Kaolin Type

[0208] Granules (200 g) obtained from the powder synthesized in Example A are exchanged using a 0.7 M barium chloride solution at 95° C. in 4 steps. At each step, the volume ratio of solution to mass of solid is 20 ml.Math.g.sup.−1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to free it of the excesses of salt. It is then activated at a temperature of 250° C. for 2 hours under a stream of nitrogen.

[0209] The degree of barium exchange is 97% and the loss on ignition is 5.5%. The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.192 cm.sup.3.Math.g.sup.−1 and 70 m.sup.2.Math.g.sup.−1.

[0210] The total volume of the macropores and mesopores measured by mercury porosimetry is 0.33 cm.sup.3.Math.g.sup.−1. The volume fraction of the macropores to the total volume of the macropores and mesopores is equal to 0.6.

[0211] The mechanical strength is also measured according to the method presented in the description of the invention. The pressure required to obtain 0.5% of fines is 2.1 MPa.

[0212] The pore diameter distribution is determined from the analysis by mercury intrusion performed on the adsorbent and represented by the volume distribution dV/d log D.sub.Hg as a function of the apparent pore diameter D.sub.Hg in FIG. 1. The distribution shows a peak and a shoulder in the region of the macropores, describing a bimodal distribution with a first mode at about 140 nm and a second in the region of the very small macropores, at about 55 nm.

Example 4 (according to the invention)

Preparation of a Zeolite-Based Adsorbent in Crushed Form with Crystals of HPX Type 4.5 μm in Size, and a Binder of Zeolitized Kaolin Type

[0213] Granules (200 g) obtained from the powder synthesized in Example A are placed in a glass reactor equipped with a jacket regulated at a temperature of 100° C.±1° C., and 1.5 L of an aqueous sodium hydroxide solution of concentration 1 M are then added and the reaction medium is left stirring for a time of 3 hours.

[0214] The agglomerates are then washed in 3 successive operations of washing with water followed by emptying the reactor. The washing efficiency is ensured by measuring the final pH of the washing waters, which is between 10.0 and 10.5.

[0215] These agglomerates are exchanged using a 0.5 M barium chloride solution at 95° C. in 4 steps. At each step, the volume ratio of solution to mass of solid is 20 ml.Math.g.sup.−1 and the exchange is continued for 4 hours each time. Between each exchange, the solid is washed several times so as to free it of the excesses of salt. It is then activated at a temperature of 250° C. for 2 hours under a stream of nitrogen.

[0216] The degree of barium exchange is 96% and the loss on ignition is 5.3%. The micropore volume and the outer surface area measured according to the t-plot method from the nitrogen adsorption isotherm at 77 K after degassing under vacuum at 400° C. for 10 hours are, respectively, 0.260 cm.sup.3.Math.g.sup.−1 and 12 m.sup.2.Math.g.sup.−1.

[0217] The total volume of the macropores and mesopores measured by mercury porosimetry is 0.29 cm.sup.3.Math.g.sup.−1. The volume fraction of the macropores to the total volume of the macropores and mesopores is equal to 0.9.

[0218] The pore diameter distribution is determined from the analysis by mercury intrusion performed on the adsorbent and represented by the volume distribution dV/d log D.sub.Hg as a function of the apparent pore diameter D.sub.Hg in FIG. 1. The distribution shows a distinct peak in the region of the macropores corresponding to a unimodal distribution about a mode equal to about 190 nm.

[0219] FIG. 1 represents the pore diameter distribution curves determined from the analysis by mercury intrusion performed on the adsorbents of Examples 2 to 4.

[0220] The content of non-zeolite-based phase is equal to 5% by weight, as measured by XRD, using as reference the starting zeolite crystals that have undergone the same barium exchange.

[0221] The mechanical strength is also measured according to the method presented in the description of the invention. The pressure required to obtain 0.5% of fines is 2.5 MPa.

Example 5

[0222] A breakthrough test (frontal chromatography) is then performed on these adsorbents to evaluate their efficiency. The amount of adsorbent used for this test is about 34 g.

[0223] The procedure for obtaining the breakthrough curves is as follows: [0224] filling of the column with the sieves and insertion in the test bench, [0225] filling with the desorption solvent at room temperature, [0226] gradual increase to the adsorption temperature under a stream of solvent (5 cm.sup.3/min), [0227] injection of solvent at 30 cm.sup.3/min when the adsorption temperature is reached, [0228] solvent/feedstock permutation to inject the feedstock (30 cm.sup.3.Math.min.sup.−1); [0229] the injection of the feedstock is then maintained for a time sufficient to reach thermodynamic equilibrium (i.e. until the concentration of solvent in the effluent is zero), and [0230] collection and analysis of the breakthrough effluent.

[0231] The desorption solvent used is para-diethylbenzene. The composition of the feedstock is as follows: [0232] para-xylene: 45% by weight [0233] meta-xylene: 45% by weight [0234] isooctane: 10% by weight (this is used as a tracer for estimating the non-selective volumes and does not participate in the separation).

[0235] The test is performed with an adsorption temperature of 175° C. The pressure is sufficient for the feedstock to remain in the liquid phase, i.e. 1.5 MPa. The surface velocity (flow rate/column cross section) of circulation of the liquid at the test temperature is about 1.3 cm.Math.s.sup.−1 for all of the tests.

[0236] The selectivity for para-xylene relative to meta-xylene is calculated by material balance. The breakthrough results are given in Table 1 below:

TABLE-US-00001 TABLE 1 Adsorption EHTP Mechanical Adsorbent PX/MX selectivity capacity (%) (%) Cl strength Example 1 3.35 14.2 6.3  7.6 2.2 (comparative) Example 2 3.52 17.4 8.4  7.3 2.5 (comparative) Example 3 2.66 13.4 2.6 13.7 2.1 (comparative) Example 4 3.19 17.6 3.7 15.2 2.5 (invention) Key PX = Para-Xylene; MX = Meta-Xylene Adsorption capacity expressed in % (cm.sup.3 of C.sub.8-aromatics adsorbed per cm.sup.3 of column) EHTP = Equivalent height of theoretical plates measured on para-xylene expressed in % of column length [00002]$\mathrm{Cl}=\frac{\mathrm{selectivity}\times \mathrm{capacity}}{\mathrm{EHTP}}$

[0237] Relative to the results obtained with the adsorbent of Examples 1 and 2, a marked improvement in the mass transfer on the adsorbent of Example 4 is found, reflected by the considerably reduced equivalent height of theoretical plates.

[0238] Relative to the results obtained with the adsorbent of Example 3, a marked improvement in the selectivity for para-xylene with respect to meta-xylene (+17%) and a marked increase in the adsorption capacity are found for the adsorbent of Example 4.

[0239] The CI index combining all of these parameters, capacity, selectivity and EHTP, makes it possible to evaluate the impact of the compromise between selectivity, capacity and mass transfer: the higher the index, the better the compromise. It is noted that the CI indices calculated on the adsorbents based on HPX crystals, namely on the adsorbents of Examples 3 and 4 (Example 4 being according to the invention), are very markedly superior to the indices calculated on the adsorbents of Examples 1 and 2.

[0240] The highest calculated CI index is obtained on the adsorbent of Example 4 according to the invention, and, consequently, this adsorbent will be the most efficient for the separation of para-xylene.

[0241] The zeolite-based adsorbent according to the invention combines good mechanical strength, good adsorption selectivity for para-xylene, a high adsorption capacity and rapid transportation of molecules within the adsorbent.