Method for modifying acidic properties of microporous crystalline zeolites

Abstract

The invention relates to a method of treating a microporous crystalline zeolite, such as a Ultrastable Zeolite Y, in particular for modifying acidic properties thereof. This method comprises successive steps of: impregnating said zeolite with a solution containing a tetraalkyl ammonium hydroxide in a liquid vehicle, drying the impregnated zeolite, and contacting the dried zeolite with water vapor at a temperature of between 100 and 200 °C. The zeolite thus obtained is particularly useful for use as a catalyst in fluid catalytic cracking processes.

Description

[0001]METHOD FOR MODIFYING ACIDIC PROPERTIES OF MICROPOROUS CRYSTALLINE ZEOLITES The invention lies in the field of treatment of zeolites in order to improve their characteristics and properties. More particularly, the invention relates to a method of treating a microporous crystalline zeolite, such as a zeolite which has been previously subjected to a dealumination treatment, in order to modify the textural and / or acidic properties thereof. Zeolites are crystalline materials consisting primarily of a three-dimensional framework of SiO4 (silicate) and AlO4- (aluminate) tetrahedra. These tetrahedra are connected by shared oxygen atoms, forming a regular lattice structure with interconnected pores and channels. Zeolites are widely used in many fields taking advantage of their regular microporous structure, strong acidity and ion-exchange capability. One of the most important fields of application of zeolites is catalysis, and more particularly fluid cracking catalysis, for example for petrochemical processing. Fluid catalytic cracking (FCC) is one of the major conversion technologies in the oil refinery industry, widely used for transforming the high boiling point high molecular weight hydrocarbons present in crude oils into lighter and more valuable fractions such as diesel, gasoline and olefinic gases. FCC processes currently produce the majority of the world’s gasoline, this process being in operation at over 300 out of a total of 646 refineries worldwide. Apart from producing gasoline, FCC units are also major producers of propylene and, to a lesser extent, raw materials for petrochemical processes. All of the industrially commercialized FCC catalysts are composed of 10-40% of USY zeolite (Ultra-Stable Y zeolite). The use of USY zeolites as a major catalytic component in FCC processes, in order to convert heavy hydrocarbons into lighter products such as gasoline and diesel, has indeed allowed to revolutionize the refining industry in the 1960s. USY zeolite is a type of Y zeolite that has undergone stabilization treatment through a steaming process which achieves its partial dealumination and the increase of its Si / Al ratio. This treatment improves the thermal and hydrothermal stability of the zeolites, and therefore their ability to withstand harsh operational environments, while preserving at least to some extent their catalytic properties. This treatment reduces the concentration of Brønsted acid sites of the zeolites, which are responsible for its catalytic activity, and generates secondary mesopores. Major research effort has been focused on the modification of the textural and chemical properties of USY zeolites, for increasing their catalytic activity and custom-tailoring their properties according to the feedstock characteristics and desired catalytic outputs manifested by the refineries. The literature is hence rich on USY zeolites modification strategies aiming at increasing the catalysis capacity thereof. An example of such a modification strategy is described in the publication from Gackowski et al., Microporous and mesoporous materials, 2023, 359: 112626, This document discloses that treating USY zeolites with tetrabutyl ammonium hydroxide in aqueous solution results in a slight increase in the concentration of Brønsted and Lewis acid sites thereof. The document EP 0528494 discloses a method of treating a Y-type zeolite, such as a USY zeolite, with an alkaline compound such as tetrapropyl ammonium hydroxide in aqueous phase, in order to increase the hydrophilicity and decrease the unit cell size thereof. The US patent application US 2012 / 258852 discloses a method of preparing mesoporous and / or mesostructured zeolites from a crystalline, substantially microporous zeolite such as a USY zeolite. This method comprises a framework modification step followed by a mesopore introduction step. More particularly, it comprises contacting the zeolite with an acid, then with a base, and finally subjecting it to a thermal treatment at a temperature of at least 450 °C. Document US 10350585 discloses a method for synthesizing zirconium- substituted hierarchical zeolites, such as ZSM-5 zeolites, for catalytic cracking. This method consists if the successive steps of metallation, treatment by framework modification process, ion-exchange process, then demetallation. The framework modification process comprises contacting the zeolite with tetrapropyl ammonium hydroxide for a few hours then subjecting the mixture directly to a hydrothermal treatment. The publication from Han et al., 2024, Microporous and Mesoporous Materials, 365: 112906 discloses a method of generating mesoporosity in H-Y zeolites by a basic treatment. This method comprises contacting the zeolite with a tetraalkyl ammonium hydroxide, washing and drying it, then grinding and calcining it. However, there is currently no satisfactory method for custom modifying characteristics / properties of USY zeolites or, more generally, microporous crystalline zeolites, and in particular those which have been previously subjected to an at least partial dealumination treatment, for example a stabilization treatment through a steaming process. The invention aims at providing such a method. More particularly, the invention aims at providing a method for custom modifying the acidic properties and / or the textural characteristics of microporous crystalline zeolites, in particular those resulting from a dealumination treatment, for example from a stabilization treatment through a water steaming process and / or an acid treatment, such as USY zeolites. The inventors have now developed a method that makes it possible to achieve these objectives. The present invention therefore relates to a method of treating a microporous crystalline zeolite, in particular a microporous crystalline zeolite which is an Ultrastable Zeolite Y, a mordenite, a Beta zeolite, a ZSM-5 zeolite or a ferrierite zeolite, preferably such a microporous crystalline zeolite which has been previously subjected to a dealumination treatment, in particular for modifying acidic and / or textural properties thereof, as well as improving the catalytic efficiency thereof. This method comprises successive steps of: - a / impregnating this zeolite with a solution containing a tetraalkyl ammonium hydroxide in a liquid vehicle, - b / drying the impregnated zeolite obtained in step a / , this method comprising a subsequent step of: - c / contacting the dried zeolite obtained in step b / with water vapor at a temperature of between 100 and 200 °C. A crystalline zeolite is herein defined as a zeolite comprising at least 10 % of crystalline phase. The zeolite to which the method of the invention is applied preferably comprises at least 50 % of crystalline phase, and even more preferably at least 70 % of crystalline phase. By “which has been previously subjected to a dealumination treatment”, it is meant that the microporous crystalline zeolite which is the starting material of the method of the invention has been obtained by subjecting a zeolite to a dealumination treatment. The dealumination treatment to which the zeolite, in particular the USY zeolite, has been submitted prior to the implementation of the method of the invention can be any type of treatment known by itself for achieving the partial dealumination of zeolites. This treatment can for example be a treatment with water steam, as conventionally implemented for preparing USY zeolites from faujasites (Y zeolites). It can otherwise for example be an acid treatment, also known by itself in the art. Preferably, the zeolite, in particular the USY zeolite, to which the method of the invention is applied has been previously subjected to a treatment in the presence of steam and and / or to an acid treatment before being subjected to the method of the invention. It preferably has a molar framework Si / Al ratio higher than 2.5. It is specified here that all the Si / Al ratios mentioned in this description are molar ratios. The method of the invention itself preferably does not comprise any step of contacting the zeolite with an acid, before or after step a / of impregnating the zeolite with a solution containing a tetraalkyl ammonium hydroxide in a liquid vehicle. The zeolite to which the method of the invention is applied is microporous, meaning that it contains micropores, i.e. pores the size of which is less than 2 nm. Depending on the zeolite, and in particular embodiments on the method which has been used for its dealumination, it may or may not contain mesopores, i.e. pores the size of which is between 2 and 50 nm. By way of example, USY zeolites typically comprise small mesopores, the size of which is around 2 nm, and larger mesopores, the size of which is of about 20 to 30 nm. Zeolites which have been subjected to acid leaching are typically devoid of mesopores. The method of the invention advantageously induces a loss of the larger micropores / smaller mesopores (i.e., the size of which is around 2 nm) of the zeolite, thereby leading to the obtention of a zeolite having a decreased overall micropore volume and a higher catalysis efficiency. Moreover, the acidic properties of the zeolite obtained at the end of the method of the invention, in particular the concentration of Brønsted acid sites and Lewis acid sites of this zeolite, can advantageously be tuned as required, by varying the type and amount of tetraalkyl ammonium hydroxide used in the impregnating step a / , according to the specific starting zeolite. In particular, by an appropriate choice of the type and amount of tetraalkyl ammonium hydroxide used in the impregnating step a / , it is possible to achieve a very high increase in the number of Brønsted acid sites and Lewis acid sites of the zeolite. This increase can be as high as +30 %. For some zeolites the amount of Brønsted acid sites can even be increased by up to +60% when using tetrapropyl ammonium hydroxide in a concentration of 0.8 mmol per gram of zeolite in the impregnating step a / . Moreover, in such conditions, it has been discovered by the inventors that the zeolite obtained at the end of the method of the invention presents, on laboratory scale, a superior catalytic activity in a cumene cracking process than the initial untreated zeolite. An increase of 2 % in the cumene conversion rate can be obtained. Such an increase proves to be of crucial importance on an industrial scale. The zeolite obtained at the end of the method of the invention is also more efficient, in particular in that sense that it enables a higher rate of conversion towards the desired products to be obtained, than the initial untreated zeolite, when it is used as a catalyst for fluid catalytic cracking. This is in particular demonstrated by a microactivity test (MAT) for the conversion of a standard vacuum gas oil. In particular embodiments of the invention, the impregnating step a / uses 0.01 to 8 mmol of tetraalkyl ammonium hydroxide(s) per gram of zeolite. It preferably uses 0.04 to 8 mmol, and more preferably 0.1 to 8 mmol, of tetraalkyl ammonium hydroxide(s) per gram of zeolite. If the aim is to increase Brønsted acid site concentration, the impregnating step a / uses preferably 0.2 to 8 mmol, of tetraalkyl ammonium hydroxide(s) per gram of zeolite, and in particular 1 to 8 mmol, for example 3 to 8 mmol, or else 0.2 to 4 mmol, of tetraalkyl ammonium hydroxide(s) per gram of zeolite. Generally speaking, the higher the framework Si / Al ratio of the zeolite, in particular USY zeolite, the less amount of tetraalkyl ammonium hydroxide(s) per gram of zeolite will be necessary in order to increase the Brønsted acid site concentration. If the aim is to rather reduce Brønsted acid site concentration, amounts of tetraalkyl ammonium hydroxide(s) per gram of zeolite of less than 0.2 mmol, for example of between 0.1 and 0.19 mmol, per gram of zeolite will be preferred. In particular embodiments of the invention, the impregnating step a / uses 0.01 to 1 mmol, such as 0.1 to 1 mmol, for example 0.2 to 1 mmol, of tetraalkyl ammonium hydroxide(s) per gram of zeolite. Preferably, it uses 0.19 to 1 mol, for example 0.4 to 1 mmol, of tetraalkyl ammonium hydroxide(s) per gram of zeolite. These ranges of values are advantageously associated with a high change (increase or decrease, depending on the framework Si / Al ratio of the zeolite) in the number of acid sites (both Brønsted acid sites and Lewis acid sites) of the zeolite. An amount of approximately 0.8 mmol of tetraalkyl ammonium hydroxide(s) per gram of zeolite, for example 0.7 to 0.9 mmol / g, is particularly preferred in the context of invention, in particular when the tetraalkyl ammonium hydroxide is tetrapropyl ammonium hydroxide, and the zeolite is a USY zeolite with a framework Si / Al ratio higher than 12, when seeking for a maximal increase in the acidic properties of the zeolite. Furthermore, the best catalytic activity for FCC is advantageously obtained when using 0.1 to 1 mmol of tetraalkyl ammonium hydroxide(s) per gram of zeolite, for example when the zeolite is a USY zeolite with a framework Si / Al ratio of about 10, as demonstrated in the examples described herein below. In alternative embodiments of the invention, the impregnating step a / uses 1.2 to 8 mmol of tetraalkyl ammonium hydroxide(s) per gram of zeolite. Such a range of values advantageously makes it possible to at least partially transform the zeolite into a zeolite having a structure of the MFI type, such as a ZSM-5 zeolite. ZSM-5 is an MFI-structured zeolite containing both silicon and aluminum in its framework, and having a significant catalytic activity. This property is particularly advantageous in the FCC process, where it enhances the yield of olefins from Liquefied Petroleum Gas (LPG). ZSM-5 is a component found in many FCC catalyst formulations, typically constituting a few percent of the catalyst blend, and the production thereof by the method of the invention is therefore highly advantageous. The zeolite to which the method of the invention is applied can be of any type of crystalline microporous zeolite. This zeolite might have previously undergone a dealumination process. The zeolite can in particular be a zeolite of the FAU type, more particularly an Ultrastable Y zeolite (USY zeolite). It can otherwise be a microporous crystalline mordenite (MOR), a Beta zeolite, a ZSM-5 zeolite or a ferrierite (FER) zeolite, which preferably has been previously subjected to a dealumination treatment. The counter-cation of the zeolite is preferably H+. It can otherwise be Na+, K+, etc. In particularly preferred embodiments of the invention, the zeolite is not substituted. By substituted, it is meant here, in a conventional way, that the framework Si and / or Al atoms thereof are partially or totally substituted by one or several other atoms. For example, the zeolite to which the method of the invention is applied is not substituted by any zirconium atom, or any other atom. In particular embodiments of the invention the zeolite has a molar framework Si / Al ratio of between 2.4 and 200, such as between 2.6 and 200, preferably between 2.4 and 50, for example equal to 15. The molar framework Si / Al ratio of the zeolite to which the method of the invention is applied can be determined by any method known by the person skilled in the art. It can for example be determined from the X-ray powder diffraction (XRD) pattern of the zeolite. This XRD pattern can for example be collected using CuKα radiation (1.54059 Å), for example from 5 to 50 °2θ, by any suitable device, such as a PANalytical Empyrean X-ray diffractometer. From the XRD pattern thus obtained, the unit cell parameter a0 can be calculated, by a method well known to the person skilled in the art skilled in the art, such as that described in Burton, A.W. (2009), Powder Diffraction in Zeolite Science, In: Chester, A.W., Derouane, E.G. (eds) Zeolite Characterization and Catalysis, Springer. The framework Si / Al ratio can then be determined from this value a0, using the Fichtner-Schmittler equation: Al / (Al+Si) = 107.1(a0 – 24.238) / 192 where a0is the unit cell parameter in Å calculated from the position of XRD peaks. The framework Si / Al ratio can be easily deduced by the value x thus obtained for Al / (Al+Si), by the equation: Framework Si / Al = (1-x) / x The starting zeolite on which the method of the invention is applied is preferably: - a USY zeolite, i.e., a zeolite presenting a FAU structure and which has been synthesized as Zeolite Y (for example having a framework Si / Al of between 1.5 and 2.3) and has been subsequently subjected to a dealumination treatment in order to obtain a framework Si / Al ratio higher than 2.3; USY zeolites with a framework Si / Al ratio of between 2.5 and 50, preferably of between 6 and 16, are particularly preferred in the context of the invention; - a mordenite, i.e., a zeolite presenting a MOR structure and which has been synthesized as a MOR presenting a framework Si / Al comprised between 5 and 15, and has been subsequently subjected to a dealumination treatment to obtain a framework Si / Al ratio higher than 15; - a ferrierite, i.e., a zeolite presenting a FER structure and having been subjected to a dealumination treatment to obtain a framework Si / Al higher than 8; - a ZSM-5, i.e., a zeolite presenting an MFI structure and having been subjected to a dealumination treatment to obtain a framework Si / Al ratio higher than 10; - or a Beta zeolite, i.e., a zeolite presenting intergrowth of different polymorphs of the zeolite beta family (i.e., BEA, BEB, BEC) and having been subjected to a dealumination treatment to obtain a framework Si / Al ratio higher than 8. In particular embodiments of the invention, the zeolite is a USY zeolite with a framework Si / Al ratio of between 2.5 and 50, preferably of between 6 and 16, and the amount of tetraalkyl ammonium hydroxide(s), for example tetrapropyl ammonium hydroxide, per gram of zeolite, is between 0.1 and 8 mmol, preferably from 0.1 to 1 mmol. The method according to the invention may further respond to one or more of the features described below, implemented individually or in each of their technically operating combinations. The step a / of impregnating the zeolite with a solution containing a tetraalkyl ammonium hydroxide in a liquid vehicle, herein referred to as the “impregnating step”, can be carried out in any way. In preferred embodiments of the invention, it is carried out by immersing the zeolite powder in a bath of the impregnating solution, preferably under stirring. The immersion time is then preferably of at least 5 minutes, for example between 5 and 60 minutes. It may in particular be equal to 30 minutes. A volume of impregnating solution of between 0.5 and 100 mL per gram of zeolite can for example be used. The impregnating step can otherwise be carried out by the so-called Incipient Wetness Impregnation technique, which is well-known by the person skilled in the art. The impregnating step can be carried out at any temperature. It is preferably carried out at a temperature of between 10 and 60 °C, and more preferably at room temperature, i.e. at a temperature of between 20 and 25 °C. It is preferably performed under stirring. The solution containing a tetraalkyl ammonium hydroxide in a liquid vehicle used in the impregnating step, herein referred to as the “impregnating solution”, preferably contains a total concentration of tetraalkyl ammonium hydroxide(s) of between 0.01 and 1 mol / l, more preferably 0.1 mol / l. The pH of the impregnating solution is preferably between 7 and 12. In particularly preferred embodiments of the invention, the impregnating step a / is carried out in the absence of any alkali hydroxide. Therefore, the impregnating solution is preferably devoid of any alkali hydroxide. It is also preferably devoid of acid. The impregnating solution can comprise a single tetraalkyl ammonium hydroxide or a mixture of different tetraalkyl ammonium hydroxides. In preferred embodiments of the invention, the alkyl group of at least one tetraalkyl ammonium hydroxide contained in the impregnating solution, preferably of each of the tetraalkyl ammonium hydroxides contained in the impregnating solution, comprises from 1 to 4 carbon atoms. The impregnating solution preferably comprises, as a tetraalkyl ammonium hydroxide, at least tetrapropyl ammonium hydroxide, i.e., a tetraalkyl ammonium hydroxide wherein the alkyl group comprises 3 carbon atoms. Tetrapropyl ammonium hydroxide is preferably the only tetraalkyl ammonium hydroxide contained in the impregnating solution. The impregnating solution can also, or otherwise, comprise tetramethyl ammonium hydroxide (the alkyl group of which comprises 1 carbon atom), tetraethyl ammonium hydroxide (the alkyl group of which comprises 2 carbon atoms) and / or tetrabutyl ammonium hydroxide (the alkyl group of which comprises 4 carbon atoms). By way of example, the impregnating solution can comprise 0.1 mol / l of tetrapropyl ammonium hydroxide as the sole tetraalkyl ammonium hydroxide of the impregnating solution. The liquid vehicle of the impregnating solution preferably comprises water. It can consist of water. It can also or otherwise comprise, or consist of, an alcohol, preferably a monoalcohol, i.e. a monoalcohol comprising a single hydroxide group, and more preferably ethanol. In particular embodiments of the invention, the vehicle of the impregnating solution consists of a mixture of water and one or several alcohol(s), preferably one or several monoalcohol(s). For example, it can consist of a mixture of water and ethanol. Step b / of drying the impregnated zeolite obtained in step a / , herein referred to as the “drying step” is essential, as it increases the effect of the method of the invention on the properties of the zeolite, compared to the methods of the prior art. The phenomena responsible for this advantageous result will not be prejudged here. However, it can be assumed that this drying step, resulting in the removal of the liquid vehicle, allows for a better impregnation of the zeolite with the tetraalkyl ammonium hydroxide contained in the impregnating solution. This better impregnation, in combination with step c / of the method of the invention, leads to a much better efficiency of the latter. The drying step of the method of the invention can be carried out by any technique known in the art. The drying step is preferably carried out by heating the impregnated zeolite, for example in an oven. In particular embodiments of the invention, the drying step is carried out at a heating temperature of between 30 and 100 °C, preferably at a temperature of between 50 and 100 °C, and more preferably at about 80 °C. The drying step is preferably carried out in appropriate conditions for ensuring a complete, or almost complete, removal of the liquid vehicle of the impregnating solution from the zeolite. Preferably, the drying step is preferably performed until obtaining a content of less than 30% w / w, preferably 10 % w / w, of the liquid vehicle, for example water, impregnating the zeolite. It falls within the skills of the person skilled in the art to determine the duration of the drying step that is adequate to achieve this objective, in particular according to the specific composition of the liquid vehicle, the specific features of the zeolite and the heating temperature applied in the drying step. As an example, the duration of the drying step may be of between 1 and 24 hours. In particularly preferred embodiments of the invention, the method does not comprise contacting the dried zeolite obtained in the drying step b / with liquid water before or during step c / of contacting said dried zeolite with water vapor at a temperature of between 100 and 200 °C. In other words, after the drying step b / , the zeolite is never brought into contact with liquid water until the end of the contacting step c / , and, more preferably, until the method of the invention has been completed. Step c / of contacting the dried zeolite obtained at the end of the drying step b / with water vapor at a temperature of between 100 and 200 °C is herein referred to as the “contacting step” or the “steam assisted conversion (SAC) treatment”. The SAC treatment technique is well-known in itself to the person skilled in the art, who knows perfectly how to implement it. In the context of the invention, it is essential in order to obtain the desired modification(s) of the textural and / or acidic properties of the zeolite. By way of example, the contacting step c / can be carried out by introducing the dried zeolite in an autoclave containing liquid water, in such a way that the zeolite never comes into contact with said liquid water, and heating at the desired temperature. According to the invention, this temperature is comprised between 100 and 200 °C. Such a temperature ensures that all the water contained in the autoclave is in vapor form. The temperature of the water vapor in the contacting step c / is preferably comprised between 150 and 200 °C, for example of about 180 °C. The contacting step c / is preferably carried out for a period of between 2 and 48 hours, for example for about 24 hours. The water vapor pressure in the autoclave is preferably comprised between 1 and 10 bar. In preferred embodiments of the invention, the zeolite is not contacted with any acid substance during the contacting step c / . More generally, it is not subjected to any acid treatment throughout the implementation of the method of the invention. In particular embodiments of the invention, the method comprises, after the contacting step c / , a final step d / of calcinating the zeolite obtained at the end of the contacting step c / . This final step d / of calcinating the zeolite obtained at the end of the contacting step c / , herein referred to as the “calcinating step”, can be carried out by any method known to the person skilled in the art. It can for example be carried out by contacting the zeolite with an air flow at an adequate temperature. The calcinating step of the method of the invention is preferably carried out at a temperature of between 500 and 650 °C, more preferably at about 550 °C. Such a calcination step advantageously achieves a complete elimination of the tetraalkyl ammonium hydroxide(s) which have been used to impregnate the zeolite and which may obstruct the micropores thereof. It favors an increase in the number of Brønsted acid sites and Lewis acid sites of the zeolite. The final step d / of calcinating the zeolite obtained at the end of the contacting step c / may be carried out for a period of between 0.5 and 8 hours, preferably for about 4 hours. It is preferably carried out under an air atmosphere. As explained above, the method of the invention advantageously makes it possible to custom-tailor the characteristics of the initial zeolite, in particular its textural and / or acidic characteristics. In particular, according to its operating conditions, it makes it possible to decrease the number of Brønsted acid sites and Lewis acid sites of the initial zeolite, or to increase this number. This increase can advantageously be very high. It also makes it possible to modify the porosity features of the zeolite, for example suppress the small mesopores thereof. The method of the invention makes it possible to obtain zeolites with increased catalytic activity, in particular when used in a cumene conversion process. By an appropriate choice of its operating conditions, it also makes it possible to control the type of products obtained at the end of such a catalytic reaction, and for example to promote benzene production. The zeolites obtained at the end of the method of the invention can prove advantageously useful for use as catalysts in fluid catalytic cracking and hydrocracking processes, in particular in the oil refinery industry, for transforming high boiling point high molecular weight hydrocarbons present in fossil crude oils into lighter and more valuable fractions such as diesel, gasoline and olefinic gases, or for transforming bio-sourced feedstocks. They may also prove useful for many other applications, depending on their specific characteristics. The features and advantages of the invention will emerge more clearly in the light of the following examples of implementation, provided for illustrative purposes only and in no way limitative of the invention, with the support of figures 1 to 6, in which: - figure 1 shows graphs representing the measured concentrations of Brønsted acid sites (in A / ) and Lewis acid sites (in B / ) in a USY zeolite (of initial framework Si / Al = 15.9) after its treatment by a method according to the invention, as a function of the amount of TPAOH (in mmol) implemented in this method for 0.5 g of USY zeolite – for each graph, the acid site concentration for the untreated USY zeolite is indicated by a dotted line ; - figure 2 shows XRD patterns of untreated USY zeolite (“USY”) (of initial framework Si / Al = 15.9) and this USY zeolite after its treatment by a method according to the invention, implementing different amounts of TPAOH (“mmol TPAOH”) for 0.5 g of the USY zeolite; - figure 3 shows: in A / , a graph representing the micropore volume up to 2 nm of USY zeolite (of initial framework Si / Al = 15.9) after its treatment by a method according to the invention, as a function of the amount of TPAOH (in mmol) implemented in this method for 0.5 g of USY zeolite – micropore volume up to 2 nm measured for the untreated USY zeolite is indicated by a dotted line ; in B / , a graph representing the pore size distribution of this zeolite as obtained after treatment with 0.2 mmol TPAOH (“0.2 mmol TPAOH”) and of the untreated zeolite (“USY”) (derivative of pore volume versus pore width dV / dW as a function of the pore width); - figure 4 shows a graph representing the pores specific surface area for a USY zeolite (of initial framework Si / Al = 15.9) after its treatment by a method according to the invention, as a function of the amount of TPAOH (in mmol) implemented in this method for 0.5 g of USY zeolite – pores specific surface area measured for the untreated USY zeolite is indicated by a dotted line; - figure 5 shows graphs representing the measured concentrations of Brønsted acid sites (in A / ) and Lewis acid sites (in B / ) in a USY zeolite (of initial framework Si / Al = 15.9) after its treatment by a method according to the invention, as a function of the number of carbon atoms of the alkyl chain of the tetraalkyl ammonium hydroxide implemented in this method, each tetraalkyl ammonium hydroxide being implemented in an amount of 0.4 mmol for 0.5 g of USY zeolite; - and figure 6 shows graphs representing, as a function of time, the % of conversion of cumene (A / ), the benzene selectivity (B / ) and the C3-C6 selectivity (C / ) in cumene cracking experiments using, as a catalyst, an untreated USY zeolite (“Zeolite”) (of initial framework Si / Al = 15.9) or the same USY zeolite after treatment by a method according to the invention using 0.4 mmol of TPAOH for 0.5 g of zeolite in the impregnating step (“0.4 mmol TPAOH”). Example 1 – Method of treating USY zeolite Samples of commercial USY zeolite (CBV 720, Zeolyst International, a microporous crystalline zeolite presenting mesopores, having a framework Si. / Al ratio of 15.9) (0.5 g) are impregnated by immersion under stirring in containers containing different volumes of a 0.1 M aqueous solution of tetrapropyl ammonium hydroxide (TPAOH), these volumes comprising the following quantities of TPAOH: 0 mmol, 0.02 mmol, 0.1 mmol, 0.2 mmol, 0.3 mmol, 0.4 mmol, 0.6 mmol, 0.8 mmol, 1.2 mmol, 2 mmol, 4 mmol. After drying the impregnated samples in an oven at 80 °C for 12 h, these powdery samples are introduced into a ceramic crucible, placed into an autoclave containing 5 mL of distilled water and treated with steam at 180 °C for 24 h (Steam assisted conversion (SAC) treatment). The recovered powders are calcined under air flow at 550 °C during 4 h. The framework Si / Al ratio of the initial commercial zeolite was calculated from its X-ray powder diffraction (XRD) pattern, collected using a PANalytical Empyrean X-ray diffractometer using CuKα radiation (1.54059 Å) from 5 to 50 °2θ. The unit cell parameter a0 was calculated, and from this value the framework Si / Al ratio was determined using the Fichtner-Schmittler equation: Al / (Al+Si) = 107.1(a0– 24.238) / 192 where a0 is the cell parameter in Å calculated from the position of XRD peaks. The framework Si / Al ratio can be easily deduced by the value x thus obtained for Al / (Al+Si), by the equation: Framework Si / Al = (1-x) / x Example 2 – Analysis of the zeolites The modified USY zeolites obtained at Example 1 are analyzed as follows. 2.1 / Measurement of acid sites concentrations The amount of Brønsted and Lewis acid sites in the zeolites are determined by pyridine chemisorption followed by Fourier-transform infrared spectroscopy (FTIR). Experiments are carried out in a quartz IR cell equipped with CaF2 windows, connected to vacuum and atmospheric systems, as well as temperature-controlled oven. Prior to analysis, the samples are pressed into self-supported wafers of 2 cm2(0.5 ton) of approximately 15 mg. The acid site concentrations are determined by pyridine adsorption at 150 °C. After establishing a pressure of 1 Torr at equilibrium, the cell is evacuated at 150 °C. The infrared (IR) spectra are recorded at room temperature after the activation period and after pyridine thermodesorption in vacuum (10-3Pa) at 150 °C. The amount of pyridine adsorbed on Brønsted [PyH+] and Lewis [PyL] sites is determined from the integration of bands at 1545 and 1454 cm-1respectively, using extinction coefficients previously determined. All spectra are recorded in the range of 1000 to 4000 cm-1on a Nicolet 5700 FT- NIR apparatus. The results obtained are shown in figure 1, in A / for the Brønsted acid sites and in B / for the Lewis acid sites. It is observed that both the Brønsted acid sites (BAS) concentration and the Lewis acid sites (LAS) concentration can be tuned by varying the amount of TPAOH used during the impregnation step of the method. If the TPAOH amount is below 0.4 mmol per gram of zeolite (0.2 mmol for 0.5 g of zeolite) a lower amount of BAS is found compared to the untreated zeolite (BAS of pristine zeolite: 261 µmol.g-1). At TPAOH amounts between 0.4 and 2.4 mmol per gram of zeolite (0.2 to 1.2 mmol for 0.5 g of zeolite) a BAS concentration higher than 350 µmol.g-1is achieved. LAS concentration is higher than 110 µmol.g-1for TPAOH amounts higher than 0.4 mmol per gram of zeolite (0.2 mmol for 0.5 g of zeolite). 2.2 / Measurement of crystallinity X-ray powder diffraction (XRD) patterns are collected using a PANalytical Empyrean X-ray diffractometer using CuKα radiation (1.54059 Å) from 5° to 50° 2θ. The XRD patterns are shown in figure 2, for each amount (in mmol) of TPAOH used in the method for treating 0.5 g of the zeolite, and for the untreated USY zeolite (“USY”). Crystallinity reduction is observed for amounts of TPAOH of less than 0.2 mmol per gram of zeolite (0.1 mmol for 0.5 g of zeolite). For amounts of TPAOH between 0.2 and 0.8 mmol per gram of zeolite (0.1 to 0.4 mmol for 0.5 g of zeolite) a high crystallinity is observed. For amounts of TPAOH higher than 0.8 mmol per gram of zeolite (0.4 mmol for 0.5 g of zeolite) partial transformation into MFI zeolite is observed. 2.3 / Measurement of pore volume and specific surface area The volume of the pores of the zeolites is measured by the nitrogen physisorption technique, using a Micromeritics 3Flex at 77 K. Approximately 50 mg of each sample are outgassed at 300 °C for 12 h before the measurement. Microporous volumes and specific surface areas are calculated through BET and t-plot method. The results obtained are shown in figure 3 for the micropore volume and in figure 4 for the pores specific surface area. Upon the modification of the zeolite by the method of the invention, the fraction of larger micropores is removed for a large range of TPAOH amount. Overall, these results demonstrate that after treatment with a method according to the invention the zeolites present modified textural and acidic properties, which can be controlled by varying the amount of TPAOH used in the impregnating step. Example 3 – Variation of the tetraalkyl ammonium hydroxide The same experiment as in Example 1 is carried out for tetraalkyl ammonium hydroxides having different lengths for their alkyl chain: tetramethyl ammonium hydroxide (TMAOH), tetraethyl ammonium hydroxide (TEAOH), tetrapropyl ammonium hydroxide (TPAOH) and tetrabutyl ammonium hydroxide (TBAOH). The amount of each tetraalkyl ammonium hydroxide implemented in the impregnating step of the method is 0.4 mmol for 0.5 g of the USY zeolite. The amount of Brønsted acid sites and Lewis acid sites in the zeolites obtained at the end of the method are determined as described in Example 2, section 2.1 / . The results obtained are shown in figure 5, in A / for the Brønsted acid sites (BAS) and in B / for the Lewis acid sites (LAS). For all the tetraalkyl ammonium hydroxides the BAS concentrations and the LAS concentrations are higher than those of the untreated zeolite. The highest increase in both BAS and LAS concentrations is observed with TPAOH. Example 4 – Catalytic properties The catalytic properties for cumene cracking of the zeolite obtained in Example 1, using 0.4 mmol of TPAOH for 0.5 g of USY zeolite, are assessed in this example. The experiment is carried out in a tubular glass fixed-bed microreactor under plug flow conditions using, as the catalyst forming the fixed-bed, 50 mg of untreated zeolite or 50 mg of the zeolite treated by the method of the invention. Prior to the cracking reaction, the catalysts are pretreated for 4 h at 300 °C under a flow of dry nitrogen. Nitrogen saturated with cumene which has been vaporized at 25 °C is then passed through the reactor (flow rate = 16 cm3.min-1) at the reaction temperature of 300 °C. The reaction products are sampled during 240 min in a 10-port valve, then analyzed by a gas chromatograph (GC) equipped with a Flame Ionization Detector (FID), with a 50 m Cp-Al2O3 / Na2SO4 capillary column of 0.32 mm internal diameter and a 60 m BR wax column of 0.25 internal diameter. The operating parameters are the following ones: run from 35 to 200 °C with a heating rate of 5°C / min; upon reaching 200 °C the temperature is maintained for 10 min. The % of conversion of cumene, the benzene selectivity and the C3-C6 selectivity are determined based on the data thus obtained using the following equations: ^^^^ ^^^^^^= ^^^^^^^^^^^^ ^^^^^^^^^ 100 (2) where: CIPB is the cumene conversion, IPBc is the amount of cumene consumed at each point, and IPBir is the sum of reaction products, Sprod is the selectivity with respect to a specific product, Prodf is the amount of this reaction product and nHydrocarbon products is the total amount of all reaction products. The results are shown in figure 6, respectively in A / , B / and C / . It is observed that the zeolite treated according to the invention (“0.4 mmol TPAOH” on the graphs) presents a higher cumene conversion compared to the untreated zeolite (maximal % of conversion: 98% and 96%, respectively). The selectivity for benzene as a product of the catalytic reaction is also globally higher. The selectivity for C3-C6 olefins as products of the catalytic reaction is slightly lower. These results demonstrate that after treatment with a method according to the invention the zeolites present modified catalytic properties. These properties can advantageously be controlled by appropriate selection of the operating conditions of the method of the invention. Example 5 – Impact of the method of the invention on USY zeolites of varying framework Si / Al ratio The zeolites used in this example are commercial USY zeolites. Their initial framework Si / Al ratio, calculated as described in Example 1, are respectively: 3.8, 8.4, 10.5, 15.9. 5.1 / Treatment of the USY zeolites by a method according to the invention The method according to the invention implemented in this example comprises the 4 following steps: a / Impregnation step: 1 g of zeolite is combined in a vessel with 8 mL of an aqueous solution of TPAOH at a given concentration determined so as to obtain the desired amount of TPAOH per gram of zeolite: 0.197 mmol / g, 0.788 mmol / g, 3.941 mmol / g, 7.882 mmol / g. The mixture is stirred using a stirring bar at 500 rpm for 30 min. b / Drying step: the vessel is transferred to an oven, where the mixture is dried at 80 °C until complete evaporation of solvent. c / Water vapor treatment step: the powder obtained in step b / is transferred to a ceramic cup and placed inside an autoclave (of larger diameter than the ceramic cup) of total volume of 100 mL.5 mL of deionized water is added at the bottom of the autoclave (the water not being in direct contact with the dried powder). The autoclave is sealed and then placed in an oven at 180 °C for 24 h. After this time the autoclave is quenched to room temperature and the powder recovered. d / Calcination step: the power is transferred to an oven and calcined at 550 °C for 6 h under air. 5.2 / Analysis of the treated zeolites The zeolites obtained at the end of the method are characterized for their Brønsted acid site concentration (µmol / g) according to the protocol described in Example 2. The results are shown in table 1. Initial framework Without 0.197 0.788 3.941 7.882 Si / Al ratio of USY treatment mmol mmol mmol mmol TPAOH / g TPAOH / g TPAOH / g TPAOH / g zeolite zeolite zeolite zeolite 3.8 749 690 741 745 732 8.4 481 - 365 709 642 10.5 369 275 366 391 434 15.9 264 241 319 327 306 Table 1 - Brønsted acid site concentration (µmol / g) of unmodified and modified USY zeolites The Brønsted acid site concentration is impacted as a function of the TPAOH amount and is lower with respect to untreated USY samples for low TPAOH amounts (e.g., 0.197 mmol / g) and higher for higher TPAOH amounts (e.g., 3.941 mmol / g). 5.3 / Microactivity test Methods Three samples of zeolitic materials were evaluated in the conversion of a standard vacuum gas oil (VGO). The experiments were conducted based on the MAT ASTM technique (ASTM D-3907 / 03) at a 500°C reaction temperature with average mass CATOIL of 2.3 and time on stream of 60 s. Catalysts (a commercial USY zeolite with a framework Si / Al ratio of 10.5 and two zeolites obtained by treatment of this zeolite according to the protocol described in section 5.1 / above, with 0.197 mmol or 0.788 mmol of TPAOH per gram of zeolite) were pelletized and grounded to 117-400 µm. Reaction products in the gas and liquid phases were analyzed separately in a HP 6890 Plus gas chromatograph using a 50 m length, 0.25 mm i.d., and 0.25 μm phase HP-1 column with FID detection. The individual hydrocarbon yields were determined according to the chromatographic report, assuming a unit response factor for all the hydrocarbons. Coke yield was determined by means of a temperature- programmed oxidation procedure on the exhausted catalysts; carbon oxides from the combustion were transformed into methane and quantified with the help of an FID detector. Total conversion was calculated as follows: ^(%) = Σ^^ where ^^ are the individual hydrocarbon yields (percent), which were calculated as ^^ = ^ / (Σ !) x 100 where i is the mass of individual hydrocarbons and j is each product mass, summed over all individual hydrocarbons (including coke). The group yields were defined as the addition of the individual hydrocarbon yields taking part of, respectively, Dry gas (C1-C2), LPG (C3-C4), Gasoline (C5- 216 ºC), LCO (216-343 ºC) and HCO (343 ºC+). The conversion of desired products was calculated by the equation: ^Desired(%) = ^Σz^ / 100 where z^ is the yield fraction (in %) of each desired hydrocarbon relative to all hydrocarbons and ^ the total conversion (in %). The mass balance for each experiment (recovery) was calculated as follows: "#$$ %&'()&%^ (%) = ( *#$+ +^,-^.+ '( / & 012) ^ 100 where *#$, +^,-^. and '( / & are the observed masses of product gases, liquid and coke, respectively, after the experiment, and 012 is the mass of reactant VGO injected to the reactor. The commercial VGO feedstock was provided by a running refinery. It can be considered a typical feedstock of the FCC process. Table 2 shows the characterization of the VGO. Density 20 / 4 ºC (g / cm3) 0.9162 º API 22.94 CCR (% wt.)a0.11 Distillation curve (ºC)bInitial 199 10 % vol. 345 30 % vol. 405 50 % vol. 438 70 % vol. 465 90 % vol. 495 Final 512 Nickel (ppm) 0.10 Vanadium (ppm) 0.73 Sodium (ppm) 0.38 Iron (ppm) 2.36 Copper (ppm) < 0.02 Sulphur (% wt.) 0.39 SARA fractions (% wt.)cSaturated 68 Aromatics 20 Resins 11 Asphaltenes 1 Table 2 – VGO properties -aASTM D-4530;bASTM D-1160;cASTM D-2007 The textural and acidic properties of the tested samples (starting untreated USY zeolite and treated zeolites), determined as described above (in Example 2 regarding the pore volume), are shown in table 3. Concentration of Micropore Mesopore Brønsted acid sites volume volume (BAS) (µmol / g) (cm3 / g) (cm3 / g) Untreated USY 369 0.26 0.19 USY treated with 275 0.19 0.24 0.197 mmol TPAOH USY treated with 366 0.21 0.22 0.788 mmol TPAOH Table 3 - Properties of untreated USY (with framework Si / Al = 10.5) and the same zeolite modified with 0.197 or 0.788 mmol TPAOH / g Results The results of the MAT test are shown in table 4. Untreated USY treated with USY treated with USY 0.197 mmol 0.788 mmol TPAOH / g TPAOH / g Conversion (wt%) 90.6 85.6 88.2 Dry gas (wt%) 4.97 4.10 4.59 LPG (wt%) 32.61 31.81 37.81 o / p of LPGa0.097 0.164 0.121 Gasoline (wt%) 27.91 30.73 28.59 LCO (wt%) 6.64 10.10 7.75 HCO (wt%) 2.76 4.28 4.07 Coke (wt%) 25.11 19.02 17.19 Desired products (wt%) 33.8 38.6 35.7 Table 4 – Results of the MAT test -a= olefin to paraffine ratio of LPG In MAT the desired product fractions are gasoline, LCO and HCO. The undesired products are LPG, dry gas and LPG. Though LPG is undesired fraction, its commercial value increases with increasing olefinicity (i.e., fraction of olefins vs paraffins). From table 4 it can be observed that for both treated USY zeolites the fraction of desired products increases with respect to the starting USY of framework Si / Al = 10.5. The olefinicity of LPG is also higher for the treated USY samples. Example 6 – Comparative method devoid of steaming step c / The method described in section 5.1 / was applied to a USY zeolite with initial framework Si / Al = 10.5 using 0.788 mmol TPAOH for 1 g of zeolite, but omitting the steaming step (step c / ). Negligible variations in acidic and textural properties were observed. This indicates that steaming step is essential in the method of the invention. Example 7 – Treatment of mordenite (MOR) A commercial MOR zeolite from Clariant, named MOR13, was used in this example. MOR13, presents a global Si / Al ratio of 6.74. This starting MOR zeolite was subjected to the method described in section 5.1 / above using 0.788 mmol TPAOH per 1 g of MOR zeolite. XRD analysis was performed on the starting and treated zeolite as described in Example 1 above, and unit cell parameters were determined from XRD patterns. The results obtained are shown in table 5. a (Å) b (Å) c (Å) Starting MOR13 18.0622 20.2916 7.4725 Treated MOR13 18.0486 20.2746 7.4514 Table 5 - Unit cell parameters of starting MOR and treated MOR sample calculated from XRD patterns It can be observed that all unit cell parameters are reduced for the treated sample when comparing them with the starting MOR. This reduction indicates a decrease in the Brønsted acid site concentration of the zeolite.

Claims

CLAIMS 1. Method of treating a microporous crystalline zeolite which is an Ultrastable Zeolite Y, a mordenite, a Beta zeolite, a ZSM-5 zeolite or a ferrierite zeolite, for modifying textural and / or acidic properties thereof, said method comprising successive steps of: - a / impregnating said zeolite with a solution containing a tetraalkyl ammonium hydroxide in a liquid vehicle, - b / drying the impregnated zeolite obtained in step a / , and being characterized in that it comprises a subsequent step of: - c / contacting the dried zeolite obtained in step b / with water vapor at a temperature of between 100 and 200 °C.

2. Method according to claim 1, wherein said impregnating step a / is carried out in the absence of any alkali hydroxide.

3. Method according to any one of claims 1 or 2, which does not comprise contacting said dried zeolite obtained in step b / with liquid water before or during said contacting step c / .

4. Method according to any one of claims 1 to 3, wherein the alkyl group of said tetraalkyl ammonium hydroxide comprises from 1 to 4 carbon atoms.

5. Method according to claim 4, wherein said tetraalkyl ammonium hydroxide is tetrapropyl ammonium hydroxide.

6. Method according to any one of claims 1 to 5, wherein said solution contains 0.01 to 1 mol / l of tetraalkyl ammonium hydroxide(s).

7. Method according to any one of claims 1 to 6, wherein said impregnating step a / uses 0.01 to 8 mmol of tetraalkyl ammonium hydroxide(s) per gram of said zeolite.

8. Method according to claim 7, wherein said impregnating step a / uses 0.1 to 1 mmol of tetraalkyl ammonium hydroxide(s) per gram of said zeolite.

9. Method according to claim 7, wherein said impregnating step a / uses 1.2 to 8 mmol of tetraalkyl ammonium hydroxide(s) per gram of said zeolite.

10. Method according to any one of claims 1 to 9, wherein said liquid vehiclecomprises water.

11. Method according to any one of claims 1 to 10, wherein said drying step b / is carried out at a temperature of between 30 and 100 °C.

12. Method according to any one of claims 1 to 11, wherein said contacting step c / is carried out for a period of between 2 and 48 hours.

13. Method according to any one of claims 1 to 12, comprising a final step d / of calcinating the zeolite obtained at the end of the contacting step c / , preferably at a temperature of between 500 and 650 °C.

14. Method according to claim 13, wherein said final step d / of calcinating the zeolite obtained at the end of the contacting step c / is carried out for a period of between 0.5 and 8 hours.

15. Method according to any one of claims 1 to 14, wherein said zeolite has been previously subjected to a dealumination treatment.

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