ES2785798B2 - Ni-BASED CATALYSTS ON INORGANIC SUPPORTS AND THEIR USE IN THE SELECTIVE OXIDATION OF PROPYLENE IN THE GAS PHASE - Google Patents

Description

DESCRIPTION

Ni-BASED CATALYSTS ON INORGANIC SUPPORTS AND THEIR USE IN THE SELECTIVE OXIDATION OF PROPYLENE IN THE GAS PHASE

FIELD OF THE INVENTION

The present invention falls within the general field of chemical engineering and, in particular, refers to a noble metal-free catalyst comprising an inorganic support and nickel nanoparticles, to the process for obtaining said catalyst and to the use thereof in the Selective oxidation of propylene in the gas phase.

STATE OF THE ART

Propylene oxide (PO) is a compound that has a high reactivity and chemical specificity towards the formation of polymers. These properties make this compound used as a prepolymer for the synthesis of polyurethanes, polyethers, polyols and other polymers. This fact has caused an increase in the world production of OP in the last two decades, estimating that in 2020 a production of more than 13 million tons of this chemical product can be reached (Huang, J. et al. "Gas-phase propene epoxidation over coinage metal catalysts. ”Res. Chem. Intermed. 2012, 38, 1-24.

At present, the synthesis of OP on an industrial scale is carried out using propylene as raw material. The current methodology for the synthesis of OP on an industrial scale from propylene is based on non-catalyzed reactions in the liquid phase using dangerous and highly polluting agents such as Ch and H2O2 (Nijhuis, TA et al. "The Production of Propene Oxide: Catalytic Processes and Recent Developments ”. Ing. Eng. Chem. Res. 2006, 45, 3447 3459). Some problems associated with these processes are the toxicity and danger of some reagents, such as chlorine. In addition, these processes are produce large amounts of by-products in the reaction. The amount of by-products is so large that the name of the route used for the synthesis of OP derives from the majority by-product obtained in it. For example, in the case of the process where uses H2O2 as a reagent, by-products such as styrene are produced, which are called, as well as “the styrene route.” These by-products reduce the efficiency of the process and significantly increase the cost of the target product (Khatib, S. et al. "Direct Oxidation of Propylene to Propylene Oxide with Molecular Oxygen: A Review ”. Catal. Rev. 2015, 57, 306-344).

Therefore, in recent years the scientific community has been focusing on developing new, more sustainable alternatives for the industrial production of OP, specifically it is focusing on the development of catalysts that form the least amount of by-products of the reaction, both in phase liquid and gas phase (Prieto, A. et al. “Propylene epoxidation with in situ generated H2O2 in supercritical conditions.” Catal. Today 2014, 227, 87-95). In this regard, N2O has been used for the selective oxidation of propylene due to its great oxidizing power and reactivity, although with this reagent it is not possible to reduce the danger with respect to those mentioned above (Wang, X. “Iron-catalysed propylene epoxidation by nitrous oxide: dramatic shift of allylic oxidation to epoxidation by the modification with alkali metal salts ". Chem. Commun. 2004, 12, 1396-1397). On the other hand, interest is also growing in performing the selective oxidation of propylene to generate OP in the gas phase, because the reagents used to carry out this reaction in the gas phase are less toxic and dangerous (eg O2 or H2 / O2) (Nijhuis, TA et al. In Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis 2008, 339-354.) (Haruta, M. In Mechanisms in Homogeneous and Heterogeneous Epoxidation Catalysis, 2008, 297-313).

In the latter case, there are several works in the literature with new heterogeneous catalysts that have the best catalytic properties (propylene conversion and selectivity towards OP) for the selective oxidation reaction of propylene to OP in the gas phase. The catalysts based on gold nanoparticles deposited on titanosilicates (Au / Ti-SiO2) stand out above all. These systems (Au / Ti-SiO2) were initially studied by Haruta et al. (Hayashi, T. “Selective Vapor-Phase Epoxidation of Propylene over Au / TiO2 Catalysts in the Presence of Oxygen and Hydrogen.” J. Catal. 1998, 178, 566-575) (Sinha, AK “Catalysis by Gold Nanoparticles: Epoxidation of Propene. ”Topics in Catalysis 2004, 29, 95 102) and have been developed by the scientific community until reaching a selectivity towards propylene oxide of 90% at relatively moderate temperatures (140-200 ° C) and using H2 / O2 mixtures as reaction gases. Maintaining selectivities around 90 % is an essential requirement because these catalysts present conversions below 10% (Lu, J. et al. "Effect of composition and promoters in Au / TS-1 catalysts for direct propylene epoxidation using H2 and O2 ”. Catal. Today 2009, 147, 186-195.). However, these catalysts have several drawbacks. One of the main drawbacks is the use of noble metals such as gold, due to their high cost (Chowdhury, B. et al. "Trimethylamine as a Gas-Phase Promoter: Highly Efficient Epoxidation of Propylene over Supported Gold Catalysts." Angew. Chem. Int. Ed. 2006, 45, 412-415) (Sinha, A. et al. “A Three-Dimensional Mesoporous Titanosilicate Support for Gold Nanoparticles: Vapor-Phase Epoxidation of Propene with High Conversion.” Angew. Chemie Int. 2004, 43, 1546-1548.) In this sense, the scientific community has studied the use of catalysts based on Ag nanoparticles, significantly cheaper than those of Au, in addition to having already been used for the ethylene epoxidation reaction using as a molecular oxygen reagent. Schüth et al. they deposited Ag nanoparticles in TiO2 (Degussa P25) and carried out the reaction with a mixture of O2, H2 and N2, obtaining a selectivity of 90% towards the OP; however, a very low conversion of around 0.36% was obtained (Lange de Oliveira, A. et al. “Highly selective propene epoxidation with hydrogen / oxygen mixtures over titaniasupported silver catalysts.” Catal. Lett. 2001, 73, 157-160). Focusing on this topic of study, Guo et al. reported that the Ag deposited in TS-1 exhibited a selectivity towards PO formation of 93.5% and a propylene conversion of 1.4%. In addition, the authors observed that the Ag supported on TiO2 by this synthesis procedure does not present activity for the synthesis reaction of OP in gas phase, which is in contradiction with the results published by other authors (Wang, R. et al. " Effects of preparation conditions and reaction conditions on the epoxidation of propylene with molecular oxygen over Ag / TS-1 in the presence of hydrogen. ”Appl. Catal. A: Gen. 2004, 261, 7-13.) (Guo, X. et al. "Effects of preparation method and precipitator on the propylene epoxidation over Ag / TS-1 in the gas phase". Catal. Today. 2004, 93-95, 211-216). Oyama et al. used a catalyst of Ag nanoparticles supported on CaCO3, for this reaction using molecular oxygen as an oxidizing agent. Under these conditions the catalysts produced a conversion of propylene of 30 % and a selectivity of 10 % for the formation of the OP (Lu, J. et al. "In situ UV-vis studies of the effect of particle size on the epoxidation of ethylene and propylene on supported silver catalysts with molecular oxygen. ”J. Catal. 2005, 232, 85-95). In general, the works found in the bibliography show that Ag nanoparticles deposited on different supports show significantly worse results compared to Au-based catalytic systems (Barteau MA et al. "Low-pressure oxidation mechanism and reactivity of propylene on silver (110) and relation to gas-phase acidity. ”J. Am. Chem. Soc.

1983, 105, 344-349) (Roberts J.T. et al. "The Rate-Limiting Step for Olefin Combustion on Silver: Experiment Compared to Theory". J. Catal. 1993,141, 300-307).

Due to the drawbacks of using the catalysts discussed above for the selective oxidation reaction of propylene, scientific efforts are focusing on developing catalysts that do not contain noble metals and that only use O2 as an oxidizing agent (Zhu, W. et al. " Cu (I) -Catalyzed Epoxidation of Propylene by Molecular Oxygen ”. Journal of Phys. Chem. C 2008, 112, 7731-7734.) (García-Aguilar J. et al." One step-synthesis of highly dispersed iron species into silica for propylene epoxidation with dioxygen. ”J. Catal.

2016, 338, 154-167) (Lei, Y. "Enhanced catalytic performance in the gas-phase epoxidation of propylene over Ti-modified MoO3-Bi2SiO5 / SiO2 catalysts". J. Catal. 2015, 321, 100-112).

It should be noted that none of the works described in the literature have been able to obtain selectivities as high as catalysts based on gold nanoparticles (Au / Ti-SiO2).

There is thus a need to provide improved catalysts that allow the oxidation of propylene to propylene oxide which during use do not generate a large amount of by-products and are capable of giving a high yield without the need to include noble metals.

BRIEF DESCRIPTION OF THE INVENTION

The present invention solves the problems described in the state of the art since it provides a catalytic system free of noble metals based on nickel nanoparticles. deposited on an inorganic support (titanosilicate or silica) for use in the selective oxidation reaction of propylene to propylene oxide (PO) in the gas phase, using mainly H2 / O2 mixtures that have catalytic properties (in terms of conversion of propylene, selectivity towards OP and H2 efficiency), superior to catalysts based on noble metals such as Au.

Thus, in a first aspect, the present invention refers to a catalytic system (hereinafter, the catalytic system of the present invention), comprising:

- a silica-based inorganic support and

- nickel nanoparticles dispersed on the inorganic silica-based support,

where the catalytic system lacks noble metals and nickel is in a proportion between 0.5-5% by weight with respect to the total weight of the catalytic system, preferably nickel is in a proportion between 0.5-2.5% by weight with With respect to the total weight of the catalytic system, more preferably, nickel is in a proportion comprised between 0.5-1.5% by weight with respect to the total weight of the catalytic system.

In the present invention catalyst system and catalyst are used interchangeably.

In the present invention by inorganic silica-based support refers to any inorganic support that comprises a percentage of silica comprised between 95-100% by weight.

In the present invention, "nickel nanoparticles" refers to nickel particles and / or nickel oxides having at least one dimension between 1 and 100 nm.

In a particular embodiment, the silica-based inorganic support is selected from silica and a mixture of silica-titania. More in particular, it is selected from silica and titanosilicate.

In another aspect, the present invention refers to a process for the preparation of the catalytic system of the present invention (hereinafter the process of the present invention), comprising the following steps:

a) impregnating an inorganic support with a nickel precursor dissolved in water,

b) add to the solution obtained in step a) an alkaline substance or solution until obtaining a pH between 9-11,

c) washing and filtering the suspension obtained in step b) until obtaining a neutral pH, d) drying the product obtained in step c).

In a particular embodiment of the present invention, the nickel precursor is nickel nitrate in water (Ni (NO3) 2-6H2O).

In another particular embodiment of the present invention, the inorganic support is in powder form.

In another particular embodiment of the present invention, the inorganic support of stage a) is obtained by the following stages:

i. add a silica-based precursor to a previous solution of urea and surfactant with acetic acid,

ii. heating the product obtained in stage i at a temperature between 30 - 45 ° C, for 15-25 h,

iii. remove urea,

iv. calcine the product obtained in iii.

Preferably, the silica-based precursor is silicon oxide or silicon oxide and titanium oxide.

In a particular embodiment, stage iv of calcination is carried out at a temperature between 500-600 ° C for 5-7 hours. Preferably between 525-580 ° C.

In a particular embodiment, the product obtained in stage iv is ground.

In another aspect, the present invention relates to the use of the catalytic system of the present invention in selective oxidation reactions of propylene to propylene oxide in the gas phase where the gas phase comprises between 70-78% by volume of helium, and between 2 -10% by volume of hydrogen and the reaction temperature is between 100-250 ° C, preferably between 180-225 ° C, more preferably between 185-215 ° C.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1. UV-Vis spectrum of the TÍ-SÍO2 and Ni / Ti-SiO2 catalytic system with 1% Ni by weight before pretreating and after pretreating according to the treatment described in Example 5 of the present invention.

Figure 2. SEM image of the Ni / Ti-SiO2 catalytic system with 1% Ni by weight described in Example 5 of the present invention.

Figure 3. SEM image of the Ni / Ti-SiO2 catalytic system with 5% Ni by weight described in Example 7 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Initially, the titanosilicate (T i ^ d -S ^) and silica (SiO2) were synthesized, which will serve as support for the Ni nanoparticles. The synthesis of both materials was prepared by adapting a protocol previously described in the literature (García-Aguilar J. et al. "Enhanced ammoniaborane decomposition by synergistic catalysis using CoPd nanoparticles supported on titanosilicates". RSC Adv., 2016,6, 91768-91772 ) the main difference between the two materials being the incorporation of the titanium precursor in the synthesis of the titanosilicate. Therefore, in this section only the synthesis of titanosilicate will be described due to its similarity with the synthesis of silica. The synthesis of Tb. d-SO consisted of dissolving 0.45 g of urea (Merck, 99%) and 0.4 g of the surfactant Pluronic F127 (Sigma-Aldrich) in an aqueous solution of acetic acid (Sigma-Aldrich, 99%) (0, 01M) by stirring for 80 min. The final pH of the solution should be around 4. The solution was then cooled in an ice bath to 0 ° C and the appropriate amount of the oxide precursors was added to the mixture. titanium (titanium ethoxide (IV), Sigma-Aldrich) and silicon oxide (Tetramethyl orthosilicate, Sigma-Aldrich) in order to obtain a molar ratio of 0.01 Ti / Si. This solution was kept under stirring for 40 min at 0 ° C. Finally, the sol was introduced into a Teflon autoclave (40 ml capacity) and the container was heated at 40 ° C for 20 h to age the sun, the pH after this step should be kept around 4. It was then made hydrothermal treatment at 120 ° C for 6 h to decompose urea and thus generate hierarchical porosity in the titanosilicate. The final pH of the supernatant should be around 9-10. After this step, the supernatant liquid was removed and the monolith obtained was calcined at 550 ° C for 6h in order to completely remove the surfactant. The monolith obtained after calcination was ground. The synthesis of the titanosilicate (T i ^ m -S ^) was carried out in order to obtain the metal (Ti) well dispersed in the silica network.

The impregnation of the Ni nanoparticles on the surface of the titanosilicate described above consisted of dissolving a certain amount of the Ni precursor (Ni (NO3) 2-6H2O, Sigma-Aldrich, 99.99%) in distilled water with the purpose of obtain a Ni loading by weight (%) in the range of 0.5 to 5% on the Ti-SiO2 support. Then 1 g of the Ti-SiO2, described above, was added to the aqueous nickel solution. Then an aqueous solution of NaOH (Sigma-Aldrich, 99.99%) (2M) was added dropwise until reaching a pH of 10.5 causing a color change to a greenish hue. This suspension was stirred at room temperature for 2 h, then filtered and washed with water until reaching neutral pH. Finally, the catalysts were dried at 100 ° C. The catalysts made in this invention have Ni loading by weight (%) in the range of 0.5 to 5%.

The general procedure for the conversion of propylene to propylene oxide was carried out by the following methodology. Prior to the catalytic tests, all the catalysts described in this invention were pretreated by a heat treatment at 500 ° C with a heating ramp of 10 ° C / min, passing a gas stream (30 ml / min) composed of 5% by volume of H2 (99.99%) in He (99.99%) in order to reduce the Ni (OH) 2 nanoparticles formed on the titanosilicate. After this pretreatment, the catalysts presented in this invention were tested in the propylene oxidation reaction for at least 4 h under steady state conditions at a constant temperature of 200 ° C (although other temperatures have also been studied). For this analysis, standard conditions were used where the space velocity was 10,000 m lg_1h_1 and the gas composition was 10% C3H6 (99.99%), 10% H2 (99.99%), 10% O2 (99.99%) and 70% He (99.99%) by volume (although other compositions have also been studied). The composition of the flue gases was analyzed by a mass spectrophotometer (Pfeiffer Vacuum, Thermostar) and a gas chromatograph (Agilent 7820A) equipped with two columns, PoraBond Q (Agilent) and CTR-I (Alltech), the first separates organic compounds (propylene, propane, propylene oxide, acetaldehyde and acetone) and the second separates inorganic compounds (mainly O2 and CO2). The propylene conversion, the OP yield, the OP selectivity and the H2 efficiency were determined by the equations presented below and that have already been published previously (Lu, J. et al. “Effect of composition and promoters in Au / TS-1 catalysts for direct propylene epoxidation using H2 and O2. ”Catal. Today 2009, 147, 186-195) (García-Aguilar J. et al.“ K- and Ca-promoted ferrosilicates for the gasphase epoxidation of propylene with O2 ”Appl. Catal. A: Gen. 2017, 538, 139-147).

EXAMPLES

The process for the preparation of the catalytic system of the present invention comprised the following steps:

One of the stages of the procedure consisted in the synthesis of the support; in this case, the synthesis of the titanosilicate (Ti0.01-SiO2) was carried out. The synthesis of this material (T b ^ -S ^) was prepared by adapting a protocol previously described in the bibliography (García-Aguilar J. and cabbage. "Enhanced ammonia-borane decomposition by synergistic catalysis using CoPd nanoparticles supported on titano-silicates". RSC Adv., 2016,6, 91768-91772). The synthesis of Tiüi01-SiO2 consisted of dissolving urea and a surfactant in an aqueous solution of acetic acid (0.01M) with stirring for 80 min. The solution was then cooled in an ice bath to 0 ° C and the mixture of the precursors of titanium oxide and silica was added. This solution was kept in stirring for 40 min at 0 ° C. The generated sol was introduced into a Teflon autoclave and heated at 40 ° C for 20 h to age the sol, then a hydrothermal treatment was carried out at 120 ° C for 6 h to decompose urea The final product was calcined at 550 ° C for 6 h and ground to obtain the final powder.

To complete the synthesis of the Ni / Ti-SiO2 catalyst, the nanoparticles of oxidized Ni species were deposited on the surface of the titanosilicate described above. The impregnation consisted of initially dissolving a certain amount of the Ni precursor (Ni (NO3) 2-6H2O) in water. Next, 1g of the Ti-SiO2 synthesized above in the aqueous nickel solution was added. Then an aqueous NaOH solution (2M) was added dropwise until reaching a pH of 10.5. This suspension was stirred at room temperature for 2 h, then filtered and washed with water until neutral pH was reached. Finally, the catalysts were dried at 100 ° C. The catalysts manufactured in this invention have a Ni loading by weight (%) between 0.5 and 5%.

Example 1: preparation of the support YES ² .

0.45 g of urea (Merck, 99%) and 0.4 g of the surfactant Pluronic F127 (Sigma-Aldrich) were dissolved in an aqueous solution of acetic acid (Sigma-Aldrich, 99%) (0.01M) by stirring for 80 min. The final pH of the solution should be around 4. The solution was then cooled in an ice bath to 0 ° C and 2.03 g of Tetramethyl Orthosilicate (Sigma-Aldrich) were added. This solution was kept under stirring for 40 min at 0 ° C. Finally, the sol was introduced into a Teflon autoclave (40 ml capacity) and heated at 40 ° C for 20 h. Subsequently, a hydrothermal treatment was carried out at 120 ° C for 6 h. The final pH of the supernatant should be around 9-10. After this step, the supernatant liquid and the monolith obtained was calcined at 550 ° C for 6h. The monolith obtained after calcination was ground.

Example 2: preparation of the NÍ / SÍO ² catalyst containing 2% Ni by weight.

0.099 g of Ni (NO3) 2-6H2O (Sigma-Aldrich, 99.99%) was dissolved in distilled water. Subsequently, 1g of SiO2, described in Example 1, was added to the aqueous nickel solution. Then an aqueous solution of NaOH (Sigma-Aldrich, 99.99%) (2M) was added dropwise until reaching a pH of 10.5. This suspension was stirred at room temperature for 2 h, then filtered and washed with water until neutral pH was reached. Finally, the catalysts were dried at 100 ° C.

Example 3: preparation of the Ti-SiO ^{2 support}

0.45 g of urea (Merck, 99%) and 0.4 g of the surfactant Pluronic F127 (Sigma-Aldrich) were dissolved in an aqueous solution of acetic acid (Sigma-Aldrich, 99%) (0.01M) by stirring for 80 min. The final pH of the solution should be around 4. The solution was then cooled in an ice bath to 0 ° C and 0.056 g of titanium (IV) ethoxide (Sigma-Aldrich) and 2.03 g of Tetramethyl orthosilicate (Sigma-Aldrich) to obtain a 0.01 Ti / Si molar ratio. This solution was kept under stirring for 40 min at 0 ° C. Finally, the sol was introduced into a Teflon autoclave (40 ml capacity) and heated at 40 ° C for 20 h. Subsequently, a hydrothermal treatment was carried out at 120 ° C for 6 h. The final pH of the supernatant should be around 9-10. After this step, the supernatant liquid was removed and the monolith obtained was calcined at 550 ° C for 6h. The monolith obtained after calcination was ground. The solid was characterized by UV-Visible spectrometry (see Figure 1). It was observed that Ti was incorporated into the silica lattice mainly in a tetrahedral coordination, but a small fraction of Ti was in an octahedral coordination.

Example 4: preparation of the Ni / Ti-SiO2 catalyst containing 0.5% Ni by weight.

0.024 g of Ni (NO3) 2-6H2O (Sigma-Aldrich, 99.99%) was dissolved in distilled water. Subsequently, 1g of Ti-SiO2 was added to the aqueous nickel solution. Then an aqueous solution of NaOH (Sigma-Aldrich, 99.99%) (2M) was added dropwise until reaching a pH of 10.5. This suspension was stirred at room temperature for 2 h, then filtered and washed with water until neutral pH was reached. Finally, the catalysts were dried at 100 ° C.

Example 5: preparation of the Ni / Ti-SiO2 catalyst containing 1% Ni by weight.

0.049 g of Ni (NO3) 2-6H2O (Sigma-Aldrich, 99.99%) was dissolved in distilled water. Subsequently, 1g of Ti-SiO2 was added to the aqueous nickel solution. Then an aqueous solution of NaOH (Sigma-Aldrich, 99.99%) (2M) was added dropwise until reaching a pH of 10.5. This suspension was stirred at room temperature for 2 h, then filtered and washed with water until neutral pH was reached. Finally, the catalysts were dried at 100 ° C.

The solid was characterized by X-ray energy dispersion spectrometry (EDS), observing the presence of Ni in the catalyst. Furthermore, by means of this characterization technique, about 1% Ni by weight was obtained in the catalyst described in this example, being in agreement with the theoretical amount incorporated in the synthesis. The solid was also characterized by UV-Visible spectrometry. It was observed that Ti was incorporated into the silica lattice mainly in a tetrahedral coordination, but a small fraction of Ti was in an octahedral coordination. It was also observed that the presence of Ni metallic after pretreatment modifies the absorbance range of the catalyst in the UV-Visible spectrum (see Figure 1). This fact indicated the presence of Ni metallic nanoparticles after the pretreatment described in the section "Detailed description of the invention".

Example 6: Preparation of the Ni / Ti-SiO2 catalyst containing 2% Ni by weight.

0.099 g of Ni (NO3) 2-6H2O (Sigma-Aldrich, 99.99%) was dissolved in distilled water. Subsequently, 1g of Ti-SiO2 was added to the aqueous nickel solution. Then an aqueous solution of NaOH (Sigma-Aldrich, 99.99%) (2M) was added dropwise until reaching a pH of 10.5. This suspension was stirred at room temperature for 2 h, then filtered and washed with water until neutral pH was reached. Finally, the catalysts were dried at 100 ° C.

Example 7: Preparation of the Ni / Ti-SiO2 catalyst containing 5% Ni by weight.

0.2478 g of Ni (NO3) 2-6H2O (Sigma-Aldrich, 99.99%) was dissolved in distilled water. Subsequently, 1g of Ti-SiO2 was added to the aqueous nickel solution. Then an aqueous solution of NaOH (Sigma-Aldrich, 99.99%) (2M) was added dropwise until reaching a pH of 10.5. This suspension was stirred at room temperature for 2 h, then filtered and washed with water until neutral pH was reached. Finally, the catalysts were dried at 100 ° C.

The solid was characterized by transmission electron microscopy (TEM). The presence of a large amount of Ni nanoparticles was observed in the solid (see Figure 2). The presence of Ni nanoparticles was also observed in all the examples shown in this invention. Samples with 0.5 and 1% Ni by weight (example 1 and 2) show good dispersion of the Ni nanoparticles. On the other hand, for the samples with 2 and 5% Ni by weight (example 3 and 4), the presence of a large amount of Ni nanoparticles in the solid was observed (see Figure 2)

Example 8: Catalytic test of the materials described in examples 1 to 7 in the selective oxidation reaction of propylene.

The catalytic properties of the materials described in this invention are studied by reacting propylene to propylene oxide (PO) in the gas phase using the methodology detailed in the section "detailed description of the invention". The supports for the catalysts (example 1 (SiO2) and 3 (Ti-SiO2)) did not show activity in the selective oxidation reaction of propylene under the conditions described. The SiO2 material with the presence of Ni (example 2) presented a very low activity in the reaction studied, these activity values being lower than those published in the bibliography for catalysts based on Au (Au / Ti-SiO2). However, the materials containing 0.5 and 1% Ni by weight (examples 4 and 5) deposited on the support containing titanium in its structure, present a catalytic activity similar to those published in the literature for catalysts based on Au (Au / Ti-SiO2). Regarding the selectivity values, the catalysts containing 0.5 and 1% Ni by weight (examples 4 and 5) presented similar values to the Au-based catalysts, the latter being the ones with the highest values described in the bibliography to date. However, when increasing the amount of Ni in the catalyst, a negative effect on selectivity was observed (examples 6 and 7), since it decreased with increasing Ni load (see Table 1). These results show that the optimal catalyst for this application is the one containing 1% Ni by weight, obtaining conversion and selectivity values similar to those based on Au for the selective oxidation reaction of propylene, with the advantage of using a cheaper metal like Ni. This invention opens the door to the use of cheaper metals than Au such as Ni for the synthesis of catalysts for this reaction. This achieves a reduction in the cost of the catalyst and therefore in the overall cost of the production of propylene oxide from propylene, obtaining conversions and selectivities similar to those obtained with noble metals, specifically Au.

Table 1. Catalytic results for the selective oxidation reaction of propylene in the gas phase under stationary conditions at 200 ° C.

Catalysts Conversion Generation Efficiency Selectivity CaHa (%) PO (%) H2 (%) PO (%)

SiÜ2 0 0 0 0

2% Ni / SiÜ2 2 0.7 7 28

Ti-SiÜ2 0 0 0 0

0.5% Ni / Ti-SiÜ2 6.3 5.4 36.9 85

1% Ni / Ti-SiÜ2 7.5 6.9 16.3 91

2% Ni / Ti-SiÜ2 5.8 3.0 10.6 52

5% Ni / Ti-SiÜ2 9.3 1.4 1.5 15

0.02 Au / TS-1_170a 2.3 - - 92

Titanosilicate 6.8 - 35 93

Au-Ba (NÜ3) 2 b

Catalyst 3 4.1 - 31.8 96

(based on gold) c

^to Lu, J. et al. "Effect of composition and promoters in Au / TS-1 catalysts for direct propylene epoxidation using H ² and O ² ". Catal. Today 2009, 147, 186-195.

^b Chowdhury, B. et al. "Trimethylamine as a Gas-Phase Promoter: Highly Efficient Epoxidation of Propylene over Supported Gold Catalysts". Angew. Chem. Int. Ed. 2006, 45, 412-415.

^c Sinha, A. et al. "A Three-Dimensional Mesoporous Titanosilicate Support for Gold Nanoparticles: Vapor-Phase Epoxidation of Propene with High Conversion." Angew. Chemie Int. 2004, 43, 1546-1548.

Example 9: Effect of temperature in the reaction of the selective oxidation of propylene in the gas phase under stationary conditions and for the material described in example 5.

The catalytic properties of the 1 % nickel catalyst (example 5) were affected by varying the temperature (see Table 2). It was observed that at low temperatures, in the range of 100 to 150 ° C, this material exhibited poor catalytic activity. Surprisingly, at a reaction temperature of 200 ° C the catalyst exhibited an activity similar to the Au-based catalysts. However, when the reaction temperature was increased to 250 ° C, an increase in propylene conversion was observed, but with a drop in the generation of PO and, therefore, in the selectivity towards PO. These results show that the optimum temperature for this reaction using Ni-based catalysts is 200 ° C.

Table 2. Catalytic results for the selective oxidation reaction of propylene in the gas phase under stationary conditions of the Ni / Ti-SiO2 catalyst containing 1% Ni by weight (example 5) at different temperatures.

Temperature Conversion Generation Selectivity

C3H6 (%) PO (%) PO (%)

(° C)

100 1.2 0.3 28

150 2.6 0.9 30

200 7.5 6.9 90

250 8.2 3 41

Example 10: Effect of the H and He ratio ( %) in the gas stream for the selective oxidation reaction of propylene in the gas phase under stationary conditions and for the material described in Example 4.

The catalytic properties of the catalyst with 0.5 % nickel (example 4) were affected by varying the proportion of H2 and He (%) in the gas stream (see Table 3). By increasing the percentage of H2 in the gas stream using Example 4 as a catalyst, an increase was observed in the catalytic properties with respect to propylene conversion, PO generation and selectivity towards PO. However, in terms of the efficiency of1H2, it was observed that when increasing the proportion of H2 from 2% to 5% in the gas stream, this value decreased.

Table 3. Catalytic results for the selective oxidation reaction of propylene in the gas phase under stationary conditions of the Ni / Ti-SiO2 catalyst containing 0.5% Ni by weight (example 4) with different proportion of H2 and He (%) in the gaseous stream.

Gas composition Conversion Generation Efficiency Selectivity in volume (%) CaHa (%) PO (%) H2 (%) PO (%)

10% CaHa / 2% H2 / 3, at 2.6 40 65

10% O2 / 78% He

10% CaHa / 5% H2 / 4.9 3.9 34 78

10% O2 / 75% He

10% CaHa / 10% H2 / 6.3 5.4 37 85

10% O2 / 70% He

Claims (5)

1. Procedure for the preparation of a catalytic system comprised of: - a silica-based inorganic support and - nickel nanoparticles dispersed on the inorganic silica-based support, where the catalytic system lacks noble metals and nickel is in a proportion comprised between 0.5-2.5% by weight with respect to the total weight of the catalyst, which comprises the following stages: a) impregnating an inorganic support with a nickel precursor dissolved in water, b) add to the solution obtained in step a) an alkaline substance until obtaining a pH between 9-11, c) wash and filter the suspension obtained in step b) until a neutral pH is obtained, d) drying the product obtained in step c). where the inorganic support of step a) is obtained by the following steps: i. add a silica-based precursor to a previous solution of urea and surfactant with acetic acid, ii. heating the product obtained in stage i at a temperature between 30-45 ° C, for 15-25 h, iii. remove urea, iv. calcine the product obtained in iii, and where the calcination stage iv is carried out at a temperature comprised between 500-600 ° C for 5-7 hours. 2. Process according to claim 1, wherein the nickel precursor is nickel nitrate in water. 3. Process according to any of claims 1-2, wherein the inorganic support is in powder form. 4. Process according to any of claims 1-3, wherein the silica-based precursor is silicon oxide or silicon oxide and titanium oxide. 5. Use of a catalytic system obtained by the process according to claims 1-4, in selective oxidation reactions of propylene to propylene oxide in the gas phase characterized in that the gas phase comprises between 70-78% by volume of helium, and between 2-10% by volume of hydrogen and the reaction temperature is between 100-250 ° C.