WO2018164989A1

WO2018164989A1 - Titanosilicate zeolite catalyst with encapsulated gold nanoparticles, method of synthesis and use

Info

Publication number: WO2018164989A1
Application number: PCT/US2018/020849
Authority: WO
Inventors: Ihab Nizar Odeh; Yuming Xie; C. William GUNDLACH; Alexander Nijhuis
Original assignee: SABIC Global Technologies BV
Current assignee: SABIC Global Technologies BV
Priority date: 2017-03-06
Filing date: 2018-03-05
Publication date: 2018-09-13
Anticipated expiration: 2019-09-06

Abstract

A catalyst composition of a silica-titania (SiO₂/TiO₂) zeolite having a tetrahedral structure contains gold (Au) nanoparticles wherein the gold (Au) particles are encapsulated within and dispersed throughout void spaces of the zeolite. The catalyst may be prepared by forming an aqueous mixture of a silicon (Si) source, a titanium (Ti) source, a gold (Au) source, a gold complexing agent, and a structure-directing agent. The mixture is hydrothermally treated to provide a silica-titania (SiO₂/TiO₂) zeolite having a tetrahedral structure wherein the gold (Au) particles are encapsulated within and dispersed throughout void spaces of the zeolite. The catalyst composition can be contacted with a reaction feed under reaction conditions suitable for producing the reaction product.

Description

TITANOSILICATE ZEOLITE CATALYST WITH ENCAPSULATED GOLD NANOPARTICLES, METHOD OF SYNTHESIS AND USE

TECHNICAL FIELD

[0001] The invention relates to zeolite catalysts, their synthesis and use. BACKGROUND

[0002] Propylene oxide is a valuable chemical intermediate used in the production of valuable commercial products, such as propylene glycols and polyethers, which are then used for making products such as polyurethane. Propylene oxide may be formed by the oxidation of propylene with organic peroxides. These processes, however, form organic co-products that must be separated from the propylene oxide. Recently, commercial production of propylene oxide has been achieved by reacting propylene with hydrogen peroxide (H₂O₂) to form propylene oxide. In this process, water (¾0) is the only co-product produced.

[0003] In the commercial process where propylene is reacted with hydrogen peroxide (H₂O₂) to form propylene oxide, titanium silicalite-1 (TS-1) is used as the catalyst. Highly dispersed titanium oxide in a tetrahedral structure on silica has been confirmed to form active sites to promote formation of hydrogen-peroxy species on the surface of the catalysts. TS-1 zeolites naturally provide tetrahedral T1O₂ and have been proven to be excellent catalyst supports for propylene epoxidation using H₂O₂.

[0004] While hydrogen peroxide may be used as a reactant for reactions in commercial processes, it is relatively expensive and must be handled with care due to its high reactivity. Therefore, processes where hydrogen peroxide necessary for reaction may be produced in situ from other reactants, such as ¾ and O₂, would be beneficial. There has been little success in preparing such catalysts that can be used commercially that have these capabilities, however.

[0005] Gold (Au) is typically a chemically inert metal. But when Au is formed into extremely small particles of less than 5 nm, it has been found to be a very effective catalyst. In particular, highly divided nanoscale Au (< 5 nm) particles are known for activation of O₂ at low temperature. With such nanoscale Au particles, in the presence of O₂ and ¾, hydrogen can be oxidized to form hydrogen peroxide. H₂O₂ is short lived in the presence of these gold particles, however, so that it cannot react with other reactants. When TS-1 zeolites are used in combination with Au nanoparticles, however, the tetrahedral titanium oxide tends to hold the disassociated hydrogen peroxide that is formed with the oxidation of hydrogen by the gold particles so that it can react with other reactants, such as propylene to form propylene oxide.

[0006] In conventional precipitation-deposition processes for forming gold-containing TS-1 catalysts, however, gold nanoparticles are only deposited on the outer surface of the TS-1 support. Although TS-1 catalysts with gold deposited on the outer surface show excellent initial performance, the long term stability of the catalyst is sub-optimal. This is due to the high mobility of Au° particles at reaction temperatures. In such conditions, the small gold particles will begin to migrate so that neighboring gold particles will contact one another and aggregate to form larger particles. To remain active, the gold particles must remain small (< 5 nm). As the size of the gold particles increases, the catalytic activity of the gold decreases. To overcome this in catalysts with gold deposited on the surface, Au loading is typically limited to avoid the formation of large gold aggregates. Therefore, the overall catalytic conversion efficiency is limited so that these types of catalysts are seldom used commercially.

[0007] Accordingly, a need exists to provide catalysts that overcome the shortcomings of the aforementioned catalysts.

SUMMARY

[0008] A catalyst composition comprises a silica-titania (Si0₂/Ti0₂) zeolite having a tetrahedral structure. The zeolite contains gold (Au) particles encapsulated within and dispersed throughout void spaces of the zeolite.

[0009] The catalyst composition may have several variations. In specific embodiments, the silica-titania zeolite has a Ti0₂ content of from 0.1 wt.% to 5 wt.% by total weight of the silica-titania zeolite. In certain instances, the gold particles may have a particle size of 5 nm or less. In others, the gold particles may have a particle size of 2 nm or less. The silica- titania zeolite may have an average pore size of from 1 nm or less. For some embodiments, the gold may be present within the zeolite in an amount of from 0.01 wt.% to 5 wt.% by total weight of the gold-containing zeolite. In specific embodiments, the silica-titania (Si0₂/Ti0₂) zeolite is at least one of a TS-1 zeolite, Ti-mordenite, and a Ti-beta zeolite.

[0010] A method of forming a catalyst composition may be carried out by forming an aqueous mixture comprising a silicon (Si) source, a titanium (Ti) source, a gold (Au) source, a gold complexing agent and a structure-directing agent. The mixture is hydrothermally treated to provide silica-titania (Si0₂/Ti0₂) zeolite having a tetrahedral structure wherein the gold (Au) particles are encapsulated within and dispersed throughout void spaces of the zeolite.

The method may have several variations. The silicon source and titanium source may be used in the mixture in an amount to provide a Si:Ti molar ratio of from 30: 1 or more for the zeolite. In certain instances, the gold source may provide a gold content of from 0.01 wt.% to 5 wt.% by total weight of the gold-containing zeolite. The titanium source may be at least one of a titanium alkoxide, a titanium halide, a titanium nitrate, a titanium sulfate, a titanium acetoacetonate, a titanium oxyhalide, a titanium formate, and a titanium acetate. The gold complexing agent may be at least one of a silicon-containing and titanium-containing compound containing at least one of phosphorus (P) and sulfur (S). In some embodiments, alcohol produced from hydrolysis of the mixture is removed from the mixture prior to hydrothermally treating the mixture. The structure-directing agent may be at least one of a tetraalkylammonium hydroxide, a tetraalkylammonium halide, tetraalkylammonium nitrate, a tetraalkylammonium sulfate, and an organoamine. The silicon source may be at least one of a tetraalkyl orthosilicate, a haloalkoxysilane, an alkaline silicate, a colloidal silica, and a fumed silica. The gold source may be at least one of auric chloride, chlorauric acid, gold nitrate, and gold sulfate. In particular embodiments, the gold-containing zeolite may be calcined. [0011] A method of forming a reaction product is carried out by contacting a catalyst composition with a reaction feed under reaction conditions suitable for producing the reaction product. The catalyst composition is a silica-titania (SiOi/TiOi) zeolite having a tetrahedral structure. The zeolite contains gold (Au) particles encapsulated within and dispersed throughout void spaces of the zeolite. In a particular embodiment, the reaction feed is oxygen and at least one of an alkene, an aromatic compound, and hydrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] For a more complete understanding of the disclosure, reference is now made to the following descriptions taken in conjunction with the accompanying figures, in which:

[0013] FIG. 1 shows a 3-dimensional schematic representation of TS-1 zeolite containing gold (Au) particles dispersed within void spaces of the Si/Ti tetrahedral framework of the zeolite;

[0014] FIG. 2 is a scanning electron microscope (SEM) image of a prepared gold-containing TS-1 zeolite;

[0015] FIG. 3 is a SEM image at lower magnification of a prepared gold-containing TS-1 zeolite observed with backscatter electron detector to image elemental contrast;

[0016] FIG. 4 is an XRD plot of the gold-containing TS-1 catalyst of Sample 6 from Table 1 of Example 1, showing that it has a TS-1 zeolite structure.

[0017] FIG. 5 is an XRD plot of the gold-containing TS-1 catalyst of Sample 9 from Table 1 of Example 1, showing that it has a TS-1 zeolite structure.

[0018] FIG. 6 is a plot of propene oxide yield, propene conversion, and hydrogen efficiency for the gold-containing TS-1 catalyst of Sample 6 from Table 1 of Example 1 used in the propene epoxidation of Example 2; and

[0019] FIG. 7 is a plot of propene oxide yield, propene conversion, and hydrogen efficiency for gold-containing TS-1 catalyst of Sample 9 from Table 1 of Example 1 used in the propene epoxidation of Example 2. DETAILED DESCRIPTION

[0021] Titanosilicate materials, such as TS-1 zeolite, Ti-mordenite, and Ti-beta zeolite, have small pore sizes of 1 nm or less. For TS-1 zeolites, the pore size is typically at 0.55 nm. For Ti-mordenite, the pore sizes are typically at 0.65 nm X 0.65 nm. For Ti-beta zeolite the pore sizes are typically 0.55 nm X 0.70 nm. Furthermore, when gold is deposited on the outer surface of such titania- silica support materials it cannot enter the small pore channels, but instead remains solely on the outer surface of the zeolite. Even with ionic exchange treatment, gold ions typically do not enter and become deposited within the pore channels of the titania- silica support materials. This is because titanosilicate materials, such as TS-1, Ti- mordenite and Ti-beta, do not contain ion exchange sites as do silica/alumina zeolite materials. Gold ions (e.g., Au⁺ or Au³⁺) therefore do not enter or migrate into the pore channels through ion exchange mechanisms, as may occur in metal ion treatments of silica/alumina materials that do contain such ion exchange sites.

[0022] It has been discovered, however, that gold can be incorporated into the zeolite itself and not just upon the outer surface of the titanosilicate zeolite material through the hydrothermal synthesis of a titanosilicate zeolite in the presence of Au ions. Upon calcination of the produced zeolite in air or post-synthesis reduction with H₂, the Au³⁺ is reduced to form Au° nanoclusters of a size beneficial for use as an oxidative catalyst. These Au nanoclusters or particles are encapsulated and dispersed throughout the void spaces of the zeolite. FIG. 1 shows a 3-dimensional schematic of such a structure for a TS-1 zeolite containing gold particles. The nanocluster sizes appear to be limited by the zeolite channel dimensions and the resulting nanoclusters of Au° are protected against coalescence and poisoning by their uniform dispersion and encapsulation within the void spaces of the zeolite.

[0023] The catalyst composition can be prepared through hydrothermal synthesis wherein an aqueous mixture comprising a silicon (Si) source, a titanium (Ti) source, a gold (Au) source, a gold complexing agent and a structure-directing agent are combined and then hydrothermally treated to provide a silica-titania (Si0₂/Ti0₂) zeolite wherein the gold (Au) particles are encapsulated within and dispersed throughout void spaces of the zeolite.

[0024] In preparing the titanosilicate zeolite, the titanium (Ti) is integrated into the framework of the zeolite as highly dispersed titanium oxide in a tetrahedral geometry. Titanium is difficult to build into the framework, however. Typically, no more than a 1:50 molar ratio of Ti:Si, which is equivalent to approximately 3 wt.% Ti0₂ or 1.8 wt.% Ti by weight of the titanosilicate structure can be included. The use of higher amounts of titanium during synthesis can result in different phase materials from the desired Si0₂/ i0₂ zeolite structure.

[0025] During the hydrothermal synthesis of the Si0₂/Ti0₂ zeolite, methods such as those described in Serrano, D.P., et al., Synthesis of TS-1 by Wetness Impregnation of Amorphous Si0₂-Ti0₂ Solids Prepared by the Sol-Gel Method, Microporous Materials 4, 1995, pp. 273- 282, which is hereby incorporated by reference, may be used. The titanium source may be a hydrolyzable titanium compound that provides a source of titanium oxide. This is in contrast with crystalline or amorphous Ti0₂ as the titanium source, which has been shown to produce uneven titanium distribution in the product zeolite and therefore its use as a titanium source may be limited or eliminated entirely. The hydrolyzable titanium compound used for the titanium source may include a titanium alkoxide, such as tetramethyl orthotitanate, tetraethyl orthotitanate (TEOT), tetrapropyl orthotitanate (TPOT), and tetrabutyl orthotitanate (TBOT). Other titanium sources may include titanium halides (e.g., TiF₄, TiF₃, TiCl_4i TiCl₃, TiCl₂, TiBr₄, TiL , titanium oxyhalides (e.g., TiOCl₂), titanium tetranitrates, titanium sulfates, titanium acetoacetonate, titanium formate, and titanium acetate. Combinations of these materials may also be used. These materials may be used to provide a Si:Ti molar ratio of from 30: 1 to 40: 1 or more of the synthesized titanosilicate zeolite. In particular instances, the titanium may be used to provide a Ti0₂ content of from 0.1 wt.% to 5 wt.% by total weight of the silica-titania zeolite or from 0.06 wt.% to 3.0 wt.% of Ti by weight of the titanosilicate structure. Typically the Si:Ti molar ratio will be from 50: 1 or higher. In certain embodiments, the titanium may be used to provide a Ti0₂ content of from 1.0 wt. % to 4 wt.% by total weight of the titanosilicate zeolite or from 0.6 wt.% to 2.5 wt.% of Ti by weight of the titanosilicate structure. Where the Si:Ti molar ratio is lower than 50: 1, some amounts of Ti may be present as an amorphous or crystalline Ti0₂ as a secondary phase.

[0026] It should be noted in the description, if a numerical value, concentration or range is presented, each numerical value should be read once as modified by the term "about" (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the description, it should be understood that an amount range listed or described as being useful, suitable, or the like, is intended that any and every value within the range, including the end points, is to be considered as having been stated. For example, "a range of from 1 to 10" is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific points within the range, or even no point within the range, are explicitly identified or referred to, it is to be understood that the inventor appreciates and understands that any and all points within the range are to be considered to have been specified, and that inventor possesses the entire range and all points within the range.

[0027] The silicon source may be a hydrolyzable or non-hydrolyzable silicon compound that is used to provide the zeolite primarily composed of Si0₂ with Ti atoms in tetrahedral geometry and dispersed throughout the silicate structure. The amount of silicon source used is determined by the relative ratio of Si:Ti or Si0₂:Ti0₂ discussed previously. Non-limiting examples of the silicon source include at least one of a tetraalkyl orthosilicate, an alkaline silicate, a colloidal silica, or a fumed silica. Commercial examples of suitable colloidal silica may include those marketed under the name LUDOX^®, available from Sigma-Aldrich, which may be in the form of aqueous silica suspensions. In particular embodiments, the silicon source may include tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS).

[0028] A templating or structure-directing agent (SDA) is used in the synthesis to aid in forming the crystalline Si0₂/Ti0₂ structure. This is typically a nitrogen-containing organic base, such as quaternary ammoniums, which direct the formation of the MFI zeolite structure. Non-limiting examples of such structure-directing agents include tetraalkylammonium hydroxides, tetraalkylammonium halides, and organoamines. In particular embodiments, tetrapropylammonium hydroxide (TPAOH) may be used as the structure-directing agent. The amount of structure-directing agent used is sufficient to stabilize the desired zeolite framework and broadly includes a SDA:Si molar ratio of greater than 0 to 0.20. In particular embodiments, the structure-directing agent may be used in an amount of from 0.01 to 0.10 SDA: Si molar ratio.

[0029] The gold source used in the synthesis may be that which provides gold ions in solution, which may be Au³⁺ cations, during the synthesis. Non-limiting examples of suitable gold sources include auric halide such as fluoride, chloride, bromide, iodide, auric cyanide, chloroauric acid, bromoauric acid, gold hydroxide, gold nitrate, and gold sulfate. The gold source is used in an amount to provide from 0.01 wt.%, 0.02 wt.%, 0.03 wt.%, 0.04 wt.%, 0.05 wt.%, 0.06 wt.%, 0.07 wt.%, 0.08 wt.%, 0.09 wt.%, 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, 0.5 wt.%, 0.6 wt.%, 0.7 wt.%, 0.8 wt.%, 0.9 wt.%, or 1 wt.% to 1.5 wt.%, 2 wt.%, 2.5 wt.%, 3 wt.%, 3.5 wt.%, 4 wt.%, 4.5 wt.%, or 5 wt.% of gold by total weight of the gold- containing titanosilicate zeolite catalyst composition. In some applications, the gold source may be used in an amount to provide from 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, 0.5 wt.%, 0.6 wt.%, 0.7 wt.%, 0.8 wt.%, 0.9 wt.%, or 1 wt.% to 1.5 wt.%, 2 wt.%, 2.5 wt.%, 3 wt.%, 3.5 wt.%, 4 wt.%, 4.5 wt.%, or 5 wt.% of gold by total weight of the gold-containing titanosilicate zeolite catalyst composition. [0030] In order for the gold nanoparticles to be dispersed throughout the final zeolite, a gold complexing agent is used to facilitate dispersing the gold ions during synthesis throughout the synthesis gel. One portion of the complexing agent may contain a silicon or titanium moiety that can be incorporated into the crystalline Si0₂/Ti0₂ structure either as-is or after a reaction, such as hydrolysis. The gold complexing agent should also not release chemicals that are not compatible with zeolite formation. A preferred moiety for incorporation the Si0₂/Ti0₂ structure is a trialkoxysilane.

[0031] The gold complexing agent also has or contains a functional group(s) that attracts, binds or otherwise complexes with the gold ions so that they are dispersed and drawn into the synthesis gel and into the resulting zeolite structure. This functionality may be provided by sulfur (S) and/or phosphorus (P) atoms, which may be in the form of thiol (mercapto) or phosphine moieties. Non-limiting examples of suitable compounds for use as the gold complexing agent include mercaptoalkyl trialkyloxysilanes, such as 3-mercaptopropyl trimethyloxysilane (3-MPTS), 4-mercapto-n-butyltrimethoxysilane, and 3-mercaptopropyl triethoxysilane.

[0032] The gold complexing agent is typically used in an amount to complex with all the gold so that it is fully dispersed. This may be in an amount so that the gold complexing functional groups of the gold complexing agent are used in at least a 1: 1 molar ratio or more with respect to the gold used.

[0033] It should also be noted that because the silicon or titanium of the gold complexing agent is incorporated into the silicate structure of the zeolite, it also functions as a silicon or titanium source and the amount of silicon or titanium from the gold complexing agent should be taken into account with respect to the relative amounts of Si and Ti and Si0₂ and Ti0₂, as was discussed earlier.

[0034] It is desirable that the titanium and silicon sources, as well as other compounds used in the synthesis, be free or substantially free from impurities. In many instances, however, this is not practical, as commercial sources of catalyst precursor materials often contain such impurities in minor amounts. These impurities may include minor amounts of Al, Fe, Na, Mg, Ca, etc., as well as residual materials from the synthesis precursors, such as S or P from the gold complexing agent that are not completely removed during the catalyst formation. Typically, these materials will be present in the final catalyst composition in amounts of less than 0.1 wt. % by weight of the catalyst composition.

[0035] In preparing the gold-containing Si0₂/Ti0₂ zeolite catalyst composition, an aqueous mixture of the catalyst precursors is prepared. This may be carried out in sequential steps. In one method of catalyst synthesis, an aqueous solution of the structure-directing agent, such as TPAOH, is combined or admixed with the gold complexing agent, such as 3-MPTS. The structure-directing agent, which may be a nitrogen-containing organic base, serves as a base to hydrolyze the silane functional groups of the gold complexing agent to form the silicon- containing oligomer or polymer chain structure with the -Si-O-Si- linkages and gold complexing functional groups, as discussed previously. Therefore, such structure-directing agent may be used in excess (e.g., > 50 wt. %) to that needed in synthesizing the Si0₂/Ti0₂ structure to promote the hydrolysis of the gold complexing agent. To this solution may be admixed the gold source, which may be gold cations in an aqueous solution. The gold source may be added gradually to this solution to prevent local large concentrations of gold from forming in solution that could tend to form larger gold particles. This step is typically operated at or near room temperature. The amount of gold solution added is kept at or near a 1 : 1 ratio of gold to the gold complexing agent to ensure all gold is complexed with the gold complexing agent. This forms a first solution of the gold with the gold complexing agent.

[0036] In a separate second solution, the titanium source may be combined or admixed at or near 0 °C with an aqueous solution of a structure-directing agent, such as TPAOH, to form a titanium sol. Such structure-directing agent may be used in excess (e.g., > 50 wt. %) in this second solution to that needed in synthesizing the Si0₂/Ti0₂ structure. The titanium source may be added gradually to prevent high local Ti concentrations so that the Ti remains isolated and to prevent the precipitation of titanium hydroxide or titanium oxide, which tends to readily precipitate. The temperature is typically maintained below 4 °C. The amount of the titanium source used is determined by the targeted zeolite Si:Ti molar composition. The titanium sol may then be admixed with the first solution of gold with the gold complexing agent to form a third solution containing the titanium sol gel and gold with the gold complexing agent.

[0037] The silicon source, such as TEOS, is immediately added to this third solution at or near room temperature. This may be added with additional water being added to the third solution or the silicon source may be added as an aqueous solution to better facilitate zeolite synthesis. The total water volume is controlled to be about twice of volume of TEOS added. This forms a fourth solution of a synthesis gel, containing a silica sol gel and titanium sol gel with the gold and the gold complexing agent. This solution is then mixed at or near room temperature for a sufficient period of time (e.g., 2 hrs to 48 hrs or more) to ensure thorough dispersal of the titanium and gold throughout the final synthesis gel. Such mixing may occur at ambient conditions. [0038] During the formation of the synthesis gels, alkyl alcohols may be formed, such as methanol from the hydrolysis of 3-MTPS, ethanol from the hydrolysis of TEOS, and butanol from the hydrolysis of TBOT. Such residual alcohols can act as reductants of the Au cations, particular at higher temperatures, so that removal of these alcohols may be desired prior to the hydrothermal treatment.

[0039] The alcohols may be removed at various stages during the reaction steps and prior to crystallization by evaporative separation where the various solutions are subjected to heat and/or vacuum to drive off these alcohols. In one such method, the alcohols are driven off after all the precursors are combined to form the final synthesis gel. This may occur by heating the final mixture to a temperature of 100 °C or less. Alternatively, the alcohols may be removed by evaporation under reduced pressure.

[0040] In certain embodiments, the final synthesis gel may be heated to dryness to ensure all or substantially all the alcohol is removed. In such instances, the resulting solid may be ground to a fine powder to ensure homogeneity and then reconstituted by suspending it in an aqueous solution containing a structure-directing agent. The amount of structure-directing agent may be from greater than 0 to 0.20 SDA:Si molar ratio, more particularly from 0.01 to 0.10 SDA:Si molar ratio. Additional water added is several times (e.g., 6-8) the weight of the dried gel weight.

[0041] In an alternate embodiment, the gold source is not added to the initial solution of structure-directing agent and gold complexing agent. In such case, all the other precursors are combined, however, as describe previously. This solution containing the synthesis gels absent the gold source is then subjected to heating to a temperature of 100 °C or less with or without vacuum to dryness to remove alcohols. This may be done in oxygen-free conditions to eliminate oxygen exposure that can cause oxidation of thiols to disulfides, which may hinder Au cation complexation. After drying, the resulting solid is ground to a fine powder and then reconstituted by suspending the powder with additional structure-directing agent and water, in the same manner as was described previously. The gold source is then admixed with the suspension for a sufficient period of time (e.g., 2 hrs to 48 hrs or more) to ensure thorough dispersal of the titanium and gold throughout the synthesis gel.

[0042] In still another alternate embodiment, the gold source and the gold complexing agent are not added to the initial solution of structure-directing agent. In such case, all the other precursors of the titanium and silicon sources are combined in solution with the structure- directing agent, as describe previously. This solution containing the synthesis gels, absent the gold source and gold complexing agent, is then subjected to heating to a temperature of 100 °C or less with or without vacuum to dryness to remove alcohols. After drying, the resulting solid is ground to a fine powder and then reconstituted by suspending the powder with additional structure-directing agent and water, in the same manner as was described earlier. The gold source and gold complexing agent are then admixed with the suspension for a sufficient period of time (e.g., 2 hrs to 48 hrs or more) to ensure thorough dispersal of the titanium and gold throughout the synthesis gel.

[0043] The final synthesis gel, as formed in solution or reconstituted, as described above, is then hydrothermally treated to facilitate crystallization. This may be conducted in an autoclave with agitation to facilitate mixing within the liquid suspension and to avoid local concentration differences. Heating temperatures may range from 120 °C to 180 °C, more particularly from 160 °C to 180 °C. Pressures encountered during the hydrothermal treatment may range from 800 kPa to 1200 kPa. The hydrothermal treatment may be continued for a sufficient period of time to ensure complete or substantially complete crystallization. This may be for several hours to several days (e.g., 2 days to 1 week or more).

[0044] After the hydrothermal treatment, the product is cooled and the liquids are separated from the solids. The solids are washed, such as with deionized water, and the resulting crystals are dried. Drying temperatures may range from 80 °C to 120 °C.

[0045] Following drying, the zeolite crystals may be calcined in dry air or oxygen wherein the zeolite crystals are heated through stepwise or ramped heating to the final calcination temperatures. Such stepwise or ramped heating prevents high concentrations of steam being formed during calcination. High steam concentrations formed during calcination can damage the zeolite. Final calcination temperatures may range from 400 °C to 450 °C, which may be maintained from 1 hr to 4 hrs or more.

[0046] While calcination will typically reduce the Au cations to form Au° nanoclusters in the resulting zeolite, a further reduction step may be used to reduce any non-reduced Au cations remaining in the crystals. In such method, after calcination, the zeolite crystals may be cooled to less than 100 °C and then purged using a flow of an inert gas, such as nitrogen, to remove oxygen. The calcined crystals may then be subjected to hydrogen gas (H₂) with stepwise or ramped heating (e.g., 1 °C/min to 5 °C/min) to a final temperature of from 400 °C to 450 °C for 1 or more hours. Thereafter, the zeolite crystals may be cooled and subjected to purging with an inert gas, such as nitrogen.

[0047] The resulting composition formed is a silica-titania (Si0₂/Ti0₂) zeolite having a tetrahedral structure with gold (Au°) nanoparticles encapsulated within and dispersed throughout void spaces of the formed zeolite crystals. These gold nanoparticles may have a particle size of from 5 nm or less. In certain embodiments, the gold nanoparticles may have a particle size of 2 nm or less. In particular embodiments, the gold nanoparticles may have a particle size of from 0.5 nm, 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, or 1 nm to 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm, or 2 nm. Gold particles size may be determined through transmission electron microscopy (TEM) or extended X-ray absorption fine structure (EXAFS) measurement techniques. These gold particles are isolated within the zeolite so that they cannot migrate, aggregate and grow into larger particles, as with titanosilicate zeolites that have gold deposited on the outer surface. It is believed that gold particles may exist within the interstices where the pore channels intersect. Such areas constitute a supercage that is larger than the pore channel size so that the gold nanoparticles can be larger than the pore channel size of the zeolite. The nanoparticles can reside within these void spaces and be dispersed throughout the zeolite. In particular embodiments, a TS-1 zeolite is the silica-titania (Si0₂/Ti0₂) zeolite that is formed. In other embodiments, a Ti- mordenite or Ti-beta zeolite is the silica-titania (Si0₂/Ti0₂) zeolite that is formed. Such zeolites typically have average pore sizes of less than 1 nm, however, the gold particles can be greater than such pore channel size due some Si-O-Si or Si-O-Ti bonds being locally broken, while the overall zeolite structure is still maintained.

[0048] The gold nanoparticles of less than 5 nm of the formed catalyst facilitate the activation of 0₂ at low reaction temperatures. Such low reaction temperatures may range from 50 °C to 300°C, more particularly from 100 °C to 250 °C, and still more particularly from 150 °C to 225 °C. With such nanoscale Au particles, in the presence of 0₂ and H₂, hydrogen can be oxidized to form hydrogen peroxide so that hydrogen peroxide does not have to be used as an added reactant but is instead created in situ during the reaction. Furthermore, the tetrahedral titanium oxide of the silica-titania zeolite structure tends to hold the disassociated hydrogen peroxide that is formed with the oxidation of hydrogen by the gold particles so that it can react with other reactants, such as propylene to form propylene oxide. Such catalyst may also be used for similar selective oxidation of other hydrocarbons in the presence of oxygen and hydrogen. In such reactions, the reaction feed may include oxygen and at least one of an alkene and/or an aromatic compound, and hydrogen. Examples of these reactions may include propylene to propylene oxide, butene to butene oxide, pentene to pentene oxide, benzene to phenol, etc. In certain instances, an additional co-feed of carbon monoxide (CO) may be used in small amounts (e.g., <5 vol. %) in the epoxidation reactions. This may facilitate reducing the amount of unwanted alkanes that may be produced as side reactions.

[0049] The following examples serve to further illustrate various embodiments and applications.

EXAMPLES

EXAMPLE 1

[0050] Various gold-containing TS-1 zeolites were prepared in the following manner: Formation of TS-1 Zeolite Precursors

[0051] Step 1. Tetrapropyl ammonium hydroxide (TPAOH) at 8.25 ml as a 2M TPAOH solution diluted with 8.25 ml of water was combined with 3-mercaptopropyltrimethoxysilane in various amounts as presented in Table 1 below. These were mixed in a 100 ml polypropylene reaction bottle and stirred for 8 hrs at ambient conditions to make Solution 1.

[0052] Step 2. HAuCl₄ in 1.88 ml of deionized water in the amounts listed in Table 1 below was added dropwise to the solution of Step 1 to make Solution 2.

[0053] Step 3. Tetrabutylorthotitanate (TBOT) in the amounts listed in Table 1 below was added dropwise to an 8.25 ml as a 2M TPAOH solution diluted with 8.25 ml of water at 0 °C for 60 minutes to form Solution 3.

[0054] Step 4. Solution 2 from Step 2 was added to Solution 3 of Step 3 to form Solution 4.

[0055] Step 5. Tetraethylorthosilicate (TEOS) at 17.3 g was added to Solution 4 of Step 4 and additional water was added, as set forth in Table 1 below.

[0056] Step 6. The reaction bottle was capped and stirred for 13 hrs or more at ambient conditions.

Table 1

Formula (wt.%) Composition

1M

HAuCI₄-3H₂0 3MPTS TEOS TBOT TPAOH H₂0

Au SiO TiO⁺ (g) (g) (g) (g) (mL) (mL)

Sample 196.97 60.08 79.87 393.83 196.34 208.33 340.32 203.36 18.02

1 0.5 100 0.5 0.050 0.150 17.3 0.141 29.21 6.5

2 1 100 0.5 0.100 0.300 17.3 0.141 29.21 6.5

3 2 100 0.5 0.200 0.600 17.3 0.141 29.21 6.5

4 0.5 100 1 0.050 0.151 17.3 0.283 29.36 6.3

5 1 100 1 0.101 0.301 17.3 0.283 29.36 6.3

6 2 100 1 0.202 0.603 17.3 0.283 29.36 6.3

7 0.5 100 2 0.051 0.152 17.3 0.565 29.65 6.1

8 1 100 2 0.102 0.304 17.3 0.565 29.65 6.1

9 2 100 2 0.203 0.609 17.3 0.565 29.65 6.1

TS-1 Zeolite Synthesis

[0057] Step 7. The product mixture from Step 6 was uncapped and heated in a water bath at 60 °C for 2 hrs in a fume hood to reduce alcohol concentration.

[0058] Step 8. The resulting solution from Step 7 was transferred to PTFE-lined autoclaves, sealed, and then heated to 170 °C with end-over-end tumbling and held at temperature for 3 days.

[0059] Step 9. The product from Step 8 was cooled to ambient temperature and was centrifuged and decanted to separate liquids and solids of the formed zeolite crystals. The solids were washed three times with deionized water and the collected zeolite crystals were dried at 90 °C overnight.

[0060] Step 10. The dried zeolite was heated in dry air flow (100 seem) to 273 °C at 1 °C/min and held at that temperature for 2 hrs. The temperature was raised to 375 °C at 1 °C/min and held for 2 hrs. This was followed by an increase of temperature to 450 °C at 1 °C/min, which was held for 4 hrs.

[0061] Step 11. The calcined product from Step 10 was cooled to less than 100 °C and subjected to nitrogen gas (N₂) flow so that the product was nitrogen purging from >15 min. The gas flow was then switched to H₂ gas (100 seem) flow and the samples were heated to 273 °C at 1 °C/min and held at temperature for 2 hrs. After cooling, the samples were purged with flowing N₂. [0062] Catalysts 6 and 9 from the above Table 1 were analyzed for their chemical compositions and surface area. The results are presented in Table 2 below.

Table 2

[0063] FIG. 2 shows the typical SEM result of the prepared Au-containing TS-1 zeolites. As can be seen, the particles are in the size range of 200-400 nm with well defined crystals. FIG. 3 shows a lower SEM magnification observed with the backscatter electron detector to image elemental contrast. As can be seen, some larger particles of gold (in the tens of nm size) were visible due to alcohol reduction during the zeolite synthesis. Large gold particles are catalytically inactive. The activity of the prepared catalyst is attributed to the SEM-invisible nanosized Au encapsulated inside of zeolite pore channels.

[0064] FIGS. 4-5 show XRD diffraction plots for catalyst Samples 6 and 9, respectively, each showing that the zeolite possesses the TS-1 structure.

EXAMPLE 2

Catalyst Testing

[0065] The resulting gold-containing TS-1 zeolite catalysts from Example 1 were tested in a packed bed flow reactor at various reaction conditions in a single experimental run. A catalyst quantity of 300 mg was loaded into a reactor, diluted with 500 mg of silicon carbide to assure good heat transfer and even temperature within the catalyst bed. The standard reaction condition was repeated throughout the catalytic experiment to determine the catalyst stability. The initial reactant feed rate was 50 seem of a blend of 10 vol.% 0₂, 10 vol.% H₂, 10 vol.% propene, and the balance N₂ at a temperature of 120 °C and 700 kPa pressure. During catalyst testing, the pressure was varied from 400 kPa to 1200 kPa, the reaction temperature was varied from 120 °C to 250 °C and the propene content was varied from 10 vol.% to 40 vol.%.

[0066] At each reaction condition, the catalyst performance was monitored for 8 hrs, during which all relevant reaction products were analyzed. Typically, each catalyst was tested at 20- 25 different conditions, during which the standard reaction condition was repeated 5 times. Before the first catalytic condition and between the different reaction conditions, the catalysts were activated/regenerated at 300°C in 10 vol. % oxygen in N₂ at 31 seem for 3 hrs. Irreversible deactivation was the main indication of sintering of the gold particles

[0067] FIGS. 6-7 show the propene oxide yield, propene conversion, and hydrogen efficiency for the gold-containing TS-1 zeolite catalyst of Sample Nos. 6 and 9, respectively, from Table 1. As shown in these figures, each segment represents the catalytic performance of the catalyst at 120 °C and 700 kPa pressure in an 8 hour run. Samples of the gas leaving the reactor were taken once per hour. The plots are not continuous so the vertical lines represent breaks where data at other conditions is not presented.

[0068] Between the 1st and 2nd segments of the plots, a regeneration of the catalyst was performed. All the regenerations were for 2 hours at 300 °C in 10 vol. % oxygen in nitrogen. Three more experiments at different temperatures and pressures were conducted, each followed by a regeneration. In the second segment "standard" testing conditions of 120 °C and 700 kPa were used. Thereafter two experiments at different temperatures and pressure (each followed by regeneration) were conducted. In the 3rd segment shown, another test at the standard condition was made. Between the 3rd and 4th segment, we ran 8 experiments at different temperatures and pressures (again each followed by a regeneration). The 4th experiment was run at the standard conditions of 120 °C and 700 kPa.

[0069] While the invention has been shown in some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes and modifications without departing from the scope of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.

Claims

CLAIMS We claim:

1. A catalyst composition comprising a silica-titania (Si0₂/Ti0₂) zeolite having a tetrahedral structure, the zeolite having gold (Au) particles encapsulated within and dispersed throughout void spaces of the zeolite.

2. The composition of claim 1, wherein: the silica-titania zeolite has a Ti0₂ content of from 0.1 wt.% to 5 wt.% by total weight of the silica-titania zeolite.

3. The composition of claim 1, wherein: the gold particles have a particle size of 5 nm or less.

4. The composition of claim 1, wherein: the gold particles have a particle size of 2 nm or less.

5. The composition of claim 1, wherein: the silica-titania zeolite has an average pore size of from 1 nm or less.

6. The composition of claim 1, wherein: the catalyst composition consists essentially of a silica-titania (Si0₂/Ti0₂) zeolite having a tetrahedral coordination, the zeolite having gold (Au) particles encapsulated within and dispersed throughout void spaces of the zeolite.

7. The composition of claim 1, wherein: the gold is present within the zeolite in an amount of from 0.01 wt.% to 5 wt.% by total weight of the gold-containing zeolite.

8. The composition of claim 1, wherein: the silica-titania (Si0₂/Ti0₂) zeolite is at least one of a TS-1 zeolite, Ti-mordenite, and a Ti-beta zeolite.

9. A method of forming a catalyst composition comprising: forming an aqueous mixture comprising a silicon (Si) source, a titanium (Ti) source, a gold (Au) source, a gold complexing agent and a structure-directing agent; and hydrothermally treating the mixture to provide silica-titania (Si0₂/Ti0₂) zeolite having a tetrahedral structure wherein the gold (Au) particles are encapsulated within and dispersed throughout void spaces of the zeolite.

10. The method of claim 9, wherein: the silicon source and titanium source are used in the mixture in an amount to provide a Si:Ti molar ratio of from 30: 1 or more for the zeolite.

11. The method of claim 9, wherein: the gold source provides a gold content of from 0.01 wt.% to 5 wt.% by total weight of the gold-containing zeolite.

12. The method of claim 9, wherein: the titanium source is at least one of a titanium alkoxide, a titanium halide, a titanium nitrate, a titanium oxyhalide, a titanium sulfate, a titanium acetoacetonate, a titanium formate, and a titanium acetate

13. The method of claim 9, wherein: the gold complexing agent is at least one of a silicon-containing and titanium- containing compound containing at least one of phosphorus (P) and sulfur (S).

14. The method of claim 9, wherein: alcohol produced from hydrolysis of the mixture is removed from the mixture prior to hydro thermally treating the mixture.

15. The method of claim 9, wherein: the structure-directing agent comprises at least one of a tetraalkylammonium hydroxide, a tetraalkylammonium halide, a tetraalkylammonium nitrate, a tetraalkylammonim sulfate, and an organoamine.

16. The method of claim 9, wherein: the silicon source is at least one of a tetraalkyl orthosilicate, a haloalkoxysilane, an alkaline silicate, a colloidal silica, and a fumed silica.

17. The method of claim 9, wherein: the gold source is at least one of auric chloride, chlorauric acid, gold nitrate, and gold sulfate.

18. The method of claim 9, further comprising: calcining the gold-containing zeolite.

19. A method of forming a reaction product comprising: contacting a catalyst composition with a reaction feed under reaction conditions suitable for producing the reaction product, the catalyst composition comprising a silica- titania (Si0₂/ i0₂) zeolite having a tetrahedral structure, the zeolite having gold (Au) particles encapsulated within and dispersed throughout void spaces of the zeolite.

20. The method of claim 18, wherein: the reaction feed comprises oxygen and at least one of an alkene, an aromatic compound, and hydrogen.