TECHNICAL FIELD

The present invention relates to a new method for preparing the silicoaluminate form of the AEI zeolite structure, containing copper atoms introduced therein, by means of a direct synthesis methodology. This new methodology requires the combination of a copper organometallic complex and an organic molecule capable of directing the crystallisation of the AEI zeolite structure as organic co-structure-directing agents (OSDAs). The present invention also relates to the application of said Cu-containing silicoaluminate materials with the AEI zeolite structure as catalysts in the selective catalytic reduction (SCR) of NO_x, amongst others.

BACKGROUND

Zeolites are microporous materials formed by TO₄tetrahedra (T=Si, Al, P, Ti, Ge, Sn, etc.) interconnected by oxygen atoms creating pores and cavities of uniform size and shape within the molecular range (3-15 Å).

These microporous crystalline materials may be used as catalysts in numerous chemical processes. The use of a zeolite with specific physico-chemical properties in a given chemical process is directly dependent on the nature of the reagents and products involved in the process (such as size, shape, hydrophobicity, etc.), as well as the reaction conditions. On the one hand, the nature of the reagents and products will affect the diffusion of these molecules in the pores and cavities of the zeolite and, consequently, the choice of a zeolite with a suitable pore topology for the products involved in the reaction is essential. On the other hand, the zeolite must be chemically and structurally stable under the required reaction conditions.

The formation of nitrogen oxides (NOx) during the combustion of fossil fuels has become a problem for society, since they are amongst the main air pollutants. The selective catalytic reduction (SCR) of NOx using ammonia as the reducing agent has become an efficient method for controlling said emissions (Brandenberger, et al. Catal. Rev. Sci. Eng., 2008, 50, 492).

Recently, it has been disclosed that silicoaluminates with the AEI structure and Cu atoms introduced therein present high catalytic activity and hydrothermal stability in the SCR reduction of NO_x(Moliner et al. WO2013159825; Moliner et al. Chem. Commun., 2012, 2012, 48, 8264).

The AEI zeolite structure presents a tri-directional system of small pores (<4 Å) interconnected by large cavities, and also double six-membered rings (DA6) as secondary building units (Wagner, et al. J. Am. Chem. Soc., 2000, 122, 263).

The silicoaluminate form of the AEI zeolite structure can be synthesised using cyclic ammonium cations with alkyl substituents (Zones et al. U.S. Pat. No. 5,958,370; Cao et al. WO 2005/063624; Moliner et al. WO2013159825) or tetraalkylphosphonium cations (Sano et al. WO/2015/005369) as OSDAs.

In order to prepare the copper-containing silicoaluminate form of the AEI zeolite structure, the incorporation of the copper species is preferably performed by means of post-synthetic metal ion exchange processes on the previously synthesised and calcined AEI material (Moliner et al. WO2013159825; Sonoda, et al. J. Mater. Chem. A., 2015, 3, 857). When using this methodology, several steps are required to obtain the final material, including the hydrothermal synthesis of the silicoaluminate, calcination in order to eliminate the OSDA, transformation into the ammonium form, metal ion exchange and, finally, calcination, to obtain the material in the desired Cu-silicoaluminate form. All these steps contribute to increase the total cost of the material preparation process.

Therefore, the possibility of directly synthesising the material with the copper-containing silicoaluminate form of the AEI zeolite structure may considerably decrease the costs associated with the preparation thereof, since it would avoid most of the steps described above, making these directly prepared materials very attractive for industry.

DESCRIPTION OF THE INVENTION

The main object of the present invention is to provide a new method for preparing the copper-containing silicoaluminate form of the AEI zeolite structure by means of a direct synthesis methodology. This new process involves combining a copper organometallic complex with an additional organic molecule capable of directing the crystallisation of the silicoaluminate form of the AEI zeolite structure as organic structure-directing agents (OSDAs). The additional organic molecule may be, amongst others, any cyclic ammonium cation with alkyl substituents, such as N,N-dimethyl-3,5-dimethylpiperidinium.

Following this synthesis process, it is possible to synthesise the copper-containing silicoaluminate form of the AEI zeolite structure directly, thus avoiding the steps required to obtain said material by means of the traditional post-synthetic metal ion exchange processes.

The present invention also relates to the use as catalysts of the materials with the copper-containing silicoaluminate form of the AEI zeolite structure obtained according to the present methodology.

Therefore, the present invention relates to a process for the direct synthesis of the material with the copper-containing silicoaluminate form of the AEI zeolite structure with high synthesis yields, which comprises, at least, the following steps:

- (i) Preparation of a mixture containing, at least, one source of water, one source of copper, one polyamine to form the Cu organometallic complex, one source of tetravalent element Y, one source of trivalent element X, one cyclic ammonium cation with alkyl substituents as the OSDA and one source of alkaline or alkaline-earth cations (A), where the synthesis mixture has the following molar composition:

YO₂:aX₂O₃:b OSDA:cA:dH₂O:eCu:f Polyamine

- where
  - a ranges between 0.001 and 0.2, preferably between 0.005 and 0.1, and, more preferably, between 0.01 and 0.07,
  - b ranges between 0.01 and 2; preferably between 0.1 and 1, and, more preferably, between 0.1 and 0.6;
  - c ranges between 0 and 2; preferably between 0.001 and 1, and, more preferably, between 0.01 and 0.8;
  - d ranges between 1 and 200; preferably between 1 and 50, and, more preferably, between 2 and 20;
  - e ranges between 0.001 and 1; preferably between 0.001 and 0.6, and, more preferably, between 0.001 and 0.5;
  - f ranges between 0.001 and 1; preferably between 0.001 and 0.6, and, more preferably, between 0.001 and 0.5.
- (ii) Crystallisation of the mixture obtained in (i) in a reactor.
- (iii) Recovery of the crystalline material obtained in (ii).

According to the present invention, Y is a tetravalent element that may be preferably selected from Si, Sn, Ti, Ge and combinations thereof; more preferably, it is Si.

The source of Si used may be selected from silicon oxide, silicon halide, colloidal silica, fumed silica, tetraalkyl orthosilicate, silicate, silicic acid, a previously synthesised crystalline material, a previously synthesised amorphous material and combinations thereof, and, more preferably, it is a material selected from a previously synthesised crystalline material, a previously synthesised amorphous material and combinations thereof; more preferably, it is a previously synthesised crystalline material.

According to the present invention, X is a trivalent element that may be preferably selected from Al, B, Fe, In, Ga and combinations thereof; more preferably, it is Al.

The source of Al used may be selected from any aluminum salt, any hydrated aluminum oxide, any aluminum alkoxide, a previously synthesised crystalline material, a previously synthesised amorphous material and combinations thereof, and, more preferably, it is a material selected from a previously synthesised crystalline material, a previously synthesised amorphous material and combinations thereof; more preferably, it is a previously synthesised crystalline material.

According to a particular embodiment of the present invention, the crystalline material with the FAU zeolite structure may be used in (i) as the only source of Y and X, preferably silicon and aluminum, and may preferably present a Si/AI ratio greater than 7.

Therefore, according to a particular embodiment of the present invention, Y is Si and X is Al, for which reason the process for the direct synthesis of the material with the copper-containing silicoaluminate form of the AEI zeolite structure with high synthesis yields would comprise, at least, the following steps:

- (i) Preparation of a mixture containing, at least, one source of water, one source of copper, one polyamine to form the Cu organometallic complex, one zeolite with the FAU crystal structure, such as zeolite Y, as the only source of silicon and aluminum, one cyclic ammonium cation with alkyl substituents as the OSDA and one source of alkaline or alkaline-earth cations (A), where the synthesis mixture has the following molar composition:

SiO₂:aAl₂O₃:b OSDA:cA:dH₂O:eCu:f Polyamine

- where
  - a ranges between 0.001 and 0.2, preferably between 0.005 and 0.1, and, more preferably, between 0.01 and 0.07;
  - b ranges between 0.01 and 2; preferably between 0.1 and 1, and, more preferably, between 0.1 and 0.6;
  - c ranges between 0 and 2; preferably between 0.001 and 1, and, more preferably, between 0.01 and 0.8;
  - d ranges between 1 and 200; preferably between 1 and 50, and, more preferably, between 2 and 20;
  - e ranges between 0.001 and 1; preferably between 0.001 and 0.6, and, more preferably, between 0.001 a 0.5;
  - f ranges between 0.001 and 1; preferably between 0.001 and 0.6, and, more preferably, between 0.001 and 0.5.
- (ii) Crystallisation of the mixture obtained in (i) in a reactor.
- (iii) Recovery of the crystalline material obtained in (ii).

According to the present invention, any source of Cu may be used in (i). Preferably, the source of copper may be selected from nitrate, sulfate and oxalate salts, and combinations thereof, amongst others.

According to the present invention, the mixture formed in (i) is free from any source of phosphorous.

According to a preferred embodiment of the present invention, the mixture formed in (i) may be free from any source of fluorine.

According to a preferred embodiment of the present invention, the source of alkaline or alkaline-earth cations may be any source of these elements, and may be preferably selected from a source of Na, K, and combinations thereof.

According to the present invention, the OSDA required in step (i) may be any cyclic ammonium cation with an alkyl substituent, preferably a quaternary ammonium selected from N,N-dimethyl-3,5-dimethylpiperidinium (DMDMP), N,N-diethyl-2,6-dimethylpiperidinium (DEDMP), N,N-dimethyl-2,6-dimethylpiperidinium, N-ethyl-N-methyl-2,6-dimethylpiperidinium and combinations thereof, preferably N,N-dimethyl-3,5-dimethylpiperidinium.

According to a particular embodiment, the process of the present invention may further comprise another OSDA, called co-operative OSDA, which may also be present in step (i), and may be selected from any cyclic quaternary ammonium or any other organic molecule, such as, for example, any amine or quaternary ammonium.

According to the present invention, any polyamine or mixture of different polyamines capable of forming a copper complex may be used in (i), regardless of the form (cyclic, linear, branched, etc.), and regardless of the nature of the amine (primary, secondary or tertiary). Preferably, said polyamine may be selected from tetraethylenepentamine, triethylenetetramine, 1,4,8,11-tetraazacyclotetradecane, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, and combinations thereof, amongst others. Preferably, the polyamine is tetraethylenepentamine.

According to the present invention, the crystallisation process described in (ii) is preferably performed in autoclaves, under static or dynamic conditions (for example, by stirring the mixture) at a temperature selected from 100° C. and 200° C., preferably between 130° C. and 200° C., and, more preferably, between 130° C. and 175° C.; with a crystallisation time that may range between 6 hours and 50 days, preferably between 1 and 20 days, and, more preferably, between 2 and 15 days. It must be borne in mind that the components of the synthesis mixture may originate from different sources, which may modify the crystallisation conditions described.

According to a particular embodiment of the process of the present invention, it is possible to add AEI crystals to the synthesis mixture, which act as seeds that favour the synthesis described, in a quantity of up to 25% by weight with respect to the total quantity of oxides. These crystals may be added before or during the crystallisation process.

According to the process described, following the crystallisation described in (ii), the resulting solid is separated from the mother liquors and recovered. The recovery step (iii) may be performed by means of any well-known separation technique, such as, for example, decantation, filtration, ultrafiltration, centrifugation or any other solid-liquid separation technique, and combinations thereof.

The process of the present invention may further comprise the elimination of the organic content retained inside the material by means of an extraction process.

According to a particular embodiment, the elimination of the organic compound retained inside the material may be performed by means of a heat treatment at temperatures greater than 25° C., preferably between 100° C. and 1000° C., for a period of time preferably ranging between 2 minutes and 25 hours.

According to a particular embodiment of the present invention, in the process for obtaining the material described above, at least one metal may be further introduced by means of post-synthetic processes, such as impregnation, ion exchange or combinations thereof. These metals are preferably selected from precious metals and, more preferably, from Pt, Pd and combinations thereof, and they are preferably located at extra-lattice positions.

According to another particular embodiment of the present invention, during the process for obtaining the material described above, any metal oxide may be further introduced which contains, at least, one precious metal, preferably selected from Pt, Pd, and combinations thereof.

According to another particular embodiment, the material produced according to the present invention may be pelletised using any well-known technique.

According to a preferred embodiment, the material obtained according to the present invention may be calcined. Therefore, the zeolite material with the AEI structure may have the following molar composition after being calcined:

YO₂:oX₂O₃:pA:rCu

- where o ranges between 0.001 and 0.2, preferably between 0.005 and 0.1, and, more preferably, between 0.01 and 0.07;
- where p ranges between 0 and 2, preferably between 0.001 and 1, and, more preferably, between 0.01 and 0.8;
- where r ranges between 0.001 and 1, preferably between 0.001 and 0.6, and, more preferably, between 0.001 and 0.5.

According to a particular embodiment, Y is Si and X is Al; therefore, the zeolite material with the AEI structure may present the following molar composition after being calcined:

SiO₂:o Al₂O₃:p A:r Cu

- where o ranges between 0.001 and 0.2, preferably between 0.005 and 0.1, and, more preferably, between 0.01 and 0.07;
- where p ranges between 0 and 2, preferably between 0.001 and 1, and, more preferably, between 0.01 a 0.8;
- where r ranges between 0.001 and 1, preferably between 0.001 and 0.6, and, more preferably, between 0.001 and 0.5.

According to a preferred embodiment, the material obtained is Cu-SSZ-39.

According to a particular embodiment of the present invention, the zeolite material with the AEI structure obtained may further comprise a precious metal, preferably selected from Pd, Pt and combinations thereof.

The present invention also relates to the use of the materials described above, obtained according to the process of the present invention, as catalysts in the conversion of feeds formed by organic compounds into higher-added-value products, or as molecular sieves for the elimination/separation of streams (for example, gas mixtures), by placing the feeds in contact with the material obtained.

According to a preferred embodiment, the material obtained in the present invention may be used as a catalyst in the selective catalytic reduction (SCR) of NOx (nitrogen oxides) in a gas stream. In particular, the SCR of NOx will be performed in the presence of reducing agents, preferably selected from ammonium, urea, hydrocarbons, and combinations thereof. According to this particular embodiment, the selective catalytic reduction (SCR) of NOx (nitrogen oxides) may be performed using a monolith as the substrate, and applying a layer of the zeolite material obtained according to the present invention thereto, such that the gas stream may go through it to perform the desired reaction. Likewise, a layer of the zeolite material obtained according to the present invention may be applied to other substrates, such as, for example, a filter through which the gas stream may pass.

According to another particular embodiment of the present invention, the material synthesised according to the present invention, which contains a precious metal, such as Pt or Pd, may be used as a catalyst in the selective oxidation of ammonia to nitrogen. According to this particular embodiment, the selective catalytic oxidation of ammonia to nitrogen may be performed using a monolith as the substrate, and applying a layer of the zeolite material obtained according to the present invention thereto, such that the gas stream may go through it to perform the desired reaction. Likewise, a layer of the zeolite material obtained according to the present invention may be applied to other substrates, such as, for example, a filter, amongst others, through which the gas stream may pass.

According to another particular embodiment, the material described according to the present invention may be used in the conversion of methane into methanol (Wulfers, et al. Chem. Commun. 2015, 51, 4447).

Throughout the description and the claims, the word “comprises” and variants thereof are not intended to exclude other technical characteristics, additives, components or steps. For persons skilled in the art, other objects, advantages and characteristics of the invention will arise, partly from the description and partly from the practice of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: PXRD patterns of the Cu-silicoaluminate materials with the AEI structure synthesised according to the present invention.

FIG. 2: UV-Vis spectrum of the Cu-silicoaluminate material with the AEI structure synthesised according to Example 2 of the present invention.

EXAMPLES
Example 1: Synthesis of N,N-dimethyl-3,5-dimethylpiperidinium (DMDMP)

10 g of 3,5-dimethylpiperidine (Sigma-Aldrich, ≥96% by weight) is mixed with 19.51 g of potassium bicarbonate (KHCO₃, Sigma-Aldrich; 99.7% by weight), and dissolved in 140 ml of methanol. Subsequently, 54 ml of methyl iodide (CH₃I, Sigma-Aldrich, ≥99% by weight) is added, and the resulting mixture is kept under stirring for 5 days at room temperature. Once this time has elapsed, the reaction mixture is filtered in order to eliminate the potassium bicarbonate. The filtrated solution is partially concentrated by means of a rotary evaporator. Once the methanol has been partially evaporated, the solution is washed with chloroform several times and magnesium sulfate is added (MgSO₄, Sigma-Aldrich, ≥99.5% by weight). Subsequently, the mixture is filtered in order to eliminate the magnesium sulfate. The ammonium salt is obtained by precipitation with diethyl ether and subsequent filtration. The final yield of N,N-dimethyl-3,5-dimethylpiperidinium iodide is 85%.

In order to prepare the hydroxide form of the preceding organic salt: 10.13 g of the organic salt is dissolved in 75.3 g of water. Subsequently, 37.6 g of an anion-exchange resin (Dower SBR) is added, and the resulting mixture is kept under stirring for 24 hours. Finally, the solution is filtered, to obtain N,N-dimethyl-3,5-dimethylpiperidinium hydroxide (with a 94% exchange).

Example 2: Direct Synthesis of the Cu-Silicoaluminate with the AEI Structure

154.0 mg of a 20% by weight aqueous solution of copper sulfate (II) (CuSO₄, Alfa Aesar, 98%) is mixed with 31.2 mg of tetraethylenepentamine (TEPA, 98%, Sigma Aldrich), in order to prepare the organometallic copper-complex in situ, keeping the resulting mixture under stirring for 2 hours. Once this time has elapsed, 3216.0 mg of a 7.4% by weight aqueous solution of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide and 163.1 mg of a 20% by weight aqueous solution of sodium hydroxide are added, keeping the resulting mixture under stirring for 15 minutes. Finally, 235.3 mg of a zeolite with the FAU structure (CBV-720, SiO₂/Al₂O₃molar ratio=21) is introduced into the synthesis mixture and kept under stirring for the period of time required to evaporate the excess water and achieve the desired gel concentration. The final composition of the gel is SiO₂: 0.047 Al₂O₃: 0.046 Cu(TEPA)²⁺: 0.2 DMDMP: 0.2 NaOH: 23 H₂O. The resulting gel is transferred to a teflon-lined autoclave. The crystallisation is performed at 135° C. for 7 days under static conditions. The solid product is filtered, washed abundantly with water, dried at 100° C. and, finally, calcined in air at 550° C. for 4 h in order to eliminate the organic remainders. The yield of the solid obtained is greater than 90% (without taking the organic remainders into account).

The solid is characterised by means of powder X-ray diffraction, and the characteristic peaks of the AEI structure are obtained (see FIG. 1). Chemical analyses of the sample indicate a Si/AI ratio of 9.95 and a copper content of 3.3% by weight.

The uncalcined crystalline material obtained is characterised by UV-VIS spectroscopy in order to study the stability of the molecules of the organometallic copper-complex after the crystallisation of the zeolite. As can be observed in FIG. 2, the UV-VIS spectrum shows a single band centred at ˜265 nm, which has been assigned to the presence of the intact Cu-TEPA complex inside the zeolite structure (Franco, et al. 2013/159828, 2012).

Example 3: Direct Synthesis of the Cu-Silicoaluminate with the AEI Structure

75.1 mg of a 20% by weight aqueous solution of copper sulfate (II) (CuSO₄, Alfa Aesar, 98%) is mixed with 18.0 mg of tetraethylenepentamine (TEPA, 98%, Sigma Aldrich), in order to prepare the organometallic copper-complex in situ, keeping the resulting mixture under stirring for 2 hours. Once this time has elapsed, 4049.0 mg of a 5.9% by weight aqueous solution of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide and 159.1 mg of a 20% by weight aqueous solution of sodium hydroxide are added, keeping the resulting mixture under stirring for 15 minutes. Finally, 285.2 mg of a zeolite with the FAU structure (CBV-720, SiO₂/Al₂O₃molar ratio=21) is introduced into the synthesis mixture, and kept under stirring for the period of time required to evaporate the excess water and achieve the desired gel concentration. The final composition of the gel is SiO₂: 0.047 Al₂O₃: 0.019 Cu(TEPA)²⁺: 0.3 DMDMP: 0.2 NaOH: 18 H₂O. The resulting gel is transferred to a teflon-lined autoclave. The crystallisation is performed at 135° C. for 7 days under static conditions. The solid product is filtered, washed abundantly with water, dried at 100° C. and, finally, calcined in air at 550° C. for 4 h in order to eliminate the organic remainders. The yield of the solid obtained is greater than 90% (without taking the organic remainders into account). The solid is characterised by means of powder X-ray diffraction, and the characteristic peaks of the AEI structure are obtained (see FIG. 1).

Example 4: Direct Synthesis of the Cu-Silicoaluminate with the AEI Structure

112.2 mg of a 20% by weight aqueous solution of copper sulfate (II) (CuSO₄, Alfa Aesar, 98%) is mixed with 27.0 mg of tetraethylenepentamine (TEPA, 98%, Sigma Aldrich), in order to prepare the organometallic copper-complex in situ, keeping the resulting mixture under stirring for 2 hours. Once this time has elapsed, 2416.0 mg of a 7.4% by weight aqueous solution of N,N-dimethyl-3,5-dimethylpiperidinium hydroxide and 66.2 mg of a 20% by weight aqueous solution of sodium hydroxide are added, keeping the resulting mixture under stirring for 15 minutes. Finally, 196.2 mg of a zeolite with the FAU structure (CBV-720, SiO₂/Al₂O₃molar ratio=21) is introduced into the synthesis mixture, and kept under stirring for the period of time required to evaporate the excess water and achieve the desired gel concentration. The final composition of the gel is SiO₂: 0.047 Al₂O₃: 0.041 Cu(TEPA)²⁺: 0.3 DMDMP: 0.1 NaOH: 21 H₂O. The resulting gel is transferred to a teflon-lined autoclave. The crystallisation is performed at 135° C. for 7 days under static conditions. The solid product is filtered, washed abundantly with water, dried at 100° C. and, finally, calcined in air at 550° C. for 4 h in order to eliminate the organic remainders. The yield of the solid obtained is greater than 90% (without taking the organic remainders into account). The solid is characterised by means of powder X-ray diffraction, and the characteristic peaks of the AEI structure are obtained.

Example 5: Catalytic Assay of the SCR of NO_x

The catalytic activity for the selective catalytic reduction of NOx is studied using a quartz fixed-bed tubular reactor 1.2 cm in diameter and 20 cm in length. In a typical experiment, the catalyst is compacted into particles with a size ranging between 0.25-0.42 mm, which are introduced into the reactor, and the temperature is increased to 550° C. (see the reaction conditions in Table 1); subsequently, this temperature is maintained for one hour under a flow of nitrogen. Once the desired temperature has been reached, the reaction mixture is fed. The SCR of NOx is studied using NH₃as the reducing agent. The NOx present at the reactor gas outlet is continuously analysed by means of a chemiluminescence detector (Thermo 62 C). The catalytic results are summarised in Table 2.

TABLE 1

Reaction conditions for the SCR of NOx.

Total gas flow (ml/min)
300

Catalyst loading (mg)
40

NO concentration (ppm)
500

NH₃concentration (ppm)
530

O₂concentration (%)
7

H₂O concentration (%)
5

Temperature interval studied (° C.)
170-550

TABLE 2

Conversion (%) of NOx at different temperatures (200, 250, 300, 350,

400, 450, 500) using the Cu-AEI catalyst synthesised following the

methodology described in the present invention.

Conversion (%) of NOx at different temperatures

450°
500°

200° C.
250° C.
300° C.
350° C.
400° C.
C.
C.

Example 2
71.7
98.4
99.6
99.8
97.1
96.9
85.1