Novel Pt metal group catalysts and process for the preparation thereof

The present invention relates to novel Pt metal group catalysts and a process for the preparation thereof. More particularly, the invention relates to novel nanoparticles of metals of the Pt metal group and aqueous dispersions thereof; to novel structured catalysts, e.g., woven fabrics, with such nanoparticles distributed thereon, to the use of such carriers as catalysts; and to a process for the preparation of the novel structured carriers.

Metal nanoparticles have attracted a special attention due to their use in catalysis. Microemulsions are widely used for the preparation of nano-particles with narrow size distribution by the precipitation/reduction of metal salts in the dispersed phase. From the technological point of view, the use of the supported catalysts is preferable. Methods for the preparation of the supported catalysts via microemulsions can be found in Applied Catalysis A: General 265 (2004) 207-219.

Usually, powdered supports/monolith or support precursors are mixed with microemulsion containing nanoparticles followed by washing with a solvent to remove the excess of the surfactant. Then the catalysts are calcined at 200-600°C for 2-12 hours in an air or hydrogen flow. However, the support impregnation should be carried out with the amount of microemulsion necessary only to fill the pore volume in order to avoid an excess of solvent. This leads to a high metal concentration in microemulsion since to attain ~1 wt. % of metal deposited, its concentration in the water core should be -0.5 M (see Catalysis Letters 64 (2000) 179-184). This limits an application of microemulsions for the preparation of the supported catalyst, since an increase of the metal concentration in a water core results in a bigger metal nanoparticle diameter (see Journal of Colloid and Interface Science 210 (1999) 123-129). Moreover, calcination leads to metal particles sintering. Thus, there is still a need for a new method of the preparation of supported catalysts with increased metal loading, recovering and

recycling both the liquid phase and surfactant, and excluding energy/time consuming calcination step.

Thus, in one aspect, the present invention relates to a process for the preparation of nanoparticles of Pt-group metals which comprises the steps of

(a) preparing a reverse microemulsion of a Pt metal in a water-in-hydrocarbon system;

(b) evaporating at least part of the solvent of said microemulsion, preferably until a precipitate is formed, more preferably under reduced pressure;

(c) adding a C ₁ -C^aIcOhOl, preferably methanol, to the residue to form a precipitate of Pt metal nanoparticles; and, optionally,

(d) separating the precipitate and re-dispersing the precipate in water to obtain a dispersion of Pt metal nanoparticles.

The preparation of a reverse microemulsion of a Pt metal in a water-in-hydrocarbon system can be accomplished in a manner know per se. For example, a solution of a Pt metal salt selected from chloride, bromide, iodide, cyanide or triflate in water is reacted with ammonia or an aliphatic amine to form a Pt metal complex solution, whereupon to said aqueous Pt metal complex solution an aqueous solution of a reducing agent and a solution of an emulsifier in a hydrocarbon is added under stirring.

Examples of Pt metals for use in the present invention are platinum and, particularly, palladium. In one particular embodiment of the invention, PdCI ₂ is reacted with aqueous ammonia to obtain a solution of Pd(NHa) ₄ CI ₂ . lncertain embodiments of the invention instead of ammonia organic amines may be used to form a Pt metal complex.

Examples of reducing agents for reducing the Pt metal complex to elementary Pt metal include hydrazine, hydrazine hydrate and sodium or potassium borohydrate, particularly hydrazine hydrate. The hydrocarbon may be any aliphatic linear or branched chain, or cycloaliphatic hydrocarbon which is liquid at room temperature and atmospheric pressure and can be readily evaporated, such as hexane, cyclohexane, n-

heptane, octane and, particularly, isooctane. The microemulsion is suitably stabilized by a surfactant which may be of any type (i.e., cationic, anionic, non-ionic). Examples of cationic surfactants are CTAB (cetyitrimethylammonium bromide) and CTAC (cetyltrimethylammonium chloride); an examplary cationic surfactants is Aerosol OT (AOT, sodium bis(2-ethylhexyl) sulphosuccinate; and exemplary non-ionic surfactants for use in the present invention are Berol 02 (nonylphenolethoxylate); Berol 050 (pentaethyleneglycol dodecyl ether PEGDE), and NP-X(poly(oxyethylene)nonylphenol ether). CTAB and CTAC are suitably used with a co-surfactant such as n-hexanol, (10 vol.% to hydrocarbon). The preferred surfactant is AOT, especially when used with isooctane.

In step (a) of the process of the present invention the mixture of hydrocarbon, surfactant, Pt metal complex solution and reducing agent is stirred, suitably at room temperature, until a transparent microemulsion is obtained. The molar ratio of water to surfactant is suitably from about 0.1 to about 100, preferably from 1 to 10 and more preferably about 3. The ratio of hydrocarbon to aqueous phase is suitably from about 0.1 to about 100, preferably from 1 to 10.

In one embodiment of step (a) the reducing agent and the Pt metal complex solution are added separately to the hydrocarbon/emulsifier mixture and stirred until transparent emulsions are obtained, whereupon the so-obtained emulsions are mixed immediately to provide a reverse microemulsion.

In step (b) the reverse microemulsion obtained in step (a) is evaporated to remove substantially all of the solvents, i.e., hydrocarbon and water. The evaporation is carried out in a manner to retain the stability of the emulsion and thus, the size of the metal nanoparticles. While the parameters to ascertain stability of the emulsion may depend on the particular components of the emulsion involved, the evaporation is, in general, carried out at a temperature not exceeding about 100 ⁰ C , preferably not more than about 75 ⁰ C, more preferably not more than about 50 ⁰ C. The evaporation can be performed under reduced pressure. In certain embodiments it can be useful to perform the evaporation at room temperature under reduced pressure.

In step (c) the evaporation residue obtained in step (b) comprising the metal nanoparticles is suspended in and washed with C ₁ to C ₄ alcohols, including methanol ethanol. propanol, isopropano!, butanol and isobutanoi, preferably methanol to flocculate the metal nanoparticles which are, finally, redispersed in water (step (d) for further use.

In a further aspect, the invention relates to structured catalysts comprising nanoparticles of the Pt group as obtainable by the process disclosed earlier herein distributed thereon; and to a process for the preparation of such structured catalysts.

The term "structured catalyst" as used herein refers to catalysts wherein the spatial position of the catalyst is controlled. Structured catalysts are known in the art, see, e.g., Chimia 56(4), 2002, 159-163. Examples of structured catalysts are ceramic carrier constructions and fibrous structures, especially filamentous woven fabrics. All types of filamentous woven fabrics can be used for use in the present invention. The fibers may be from organic or inorganic matter. Examples are fabrics from activated carbon fibers, acrylonitril fibers, glass fibers, ceramic fibers, metal fibers or fleece composite oxides of activated carbon fibers. Of particular interest are activated carbon fibers. The individual fibers of the filamentous woven fabrics preferably have a diameter of about 2 μm to about 100 μm, especially a diameter of no more than about 20 μm. The fabrics are suitably be woven from threads consisting of a boundle of individual fibers, providing a porous size of the woven cloth of less than about 1 mm. They may be chemically treated , e.g., with nitric acid to modify the specific surface and may have a coating, e.g. of metals such as Al, Ti or Pb to improve selectivity.

The structured catalysts in accordance with the invention can be prepared by impregnating an appropriate structured carrier, e.g., a ceramic carrier or a woven fabric with an aqueous dispersion of Pt metal nanoparticles as obtainable by the process disclosed earlier herein, and drying the impregnated structured carrier, e.g. a woven fabric, at a temperature up to about 150 degrees C.

In a still further embodiment, the invention relates to the use structured carriers, particularly woven fabrics, having Pt metal nanoparticles deposited thereon as obtainable by the process disclosed earlier herein.

The novel process allows recovering the liquid phase of the microemulsion and does not need tetrahydrofuran; it is characterized by a lower E-factor (waste/catalyst mass ratio (see Chemistry 3(2000) 541-551 ). Further The novel process allows obtaining catalysts with higher metal loading at low metal concentration in the micelle aqueous core, and retaining the nanoparticles monodispersity by avoiding the calcinations. Activity and selectivity of the catalysts obtainable in accordance with the invention were observed to be stable from the second run and no Pd leaching occurred during repeated catalyst reuse. The catalysts do not require any conditioning between runs and showed better catalytic performance than fresh Lindlar catalyst both in solvent- assisted and solvent-free hydrogenations.

The invention is illustrated further by the Examples.

Example 1

A 0.05 M Pd(NH ₃ ) ₄ C! ₂ solution was prepared as described elsewhere [Chen et at., Journal of Colloid and Interface Science 210 (1999), 123-129] by dissolving PdCI ₂ in 0.5N HCI solution and then adjusting pH of 8.5 with ammonia. A 1 M hydrazine hydrate solution was prepared before use. A Pd-nanoparticle containing microemulsion with the water-to-surfactant molar ratio of 3 was prepared according to the reference Chen et al., supra. A 0.35 M solution of AOT in 200 ml. of isooctane was divided in half. 1.88 ml_ of the palladium precursor and reductant solutions were injected separately to AOT/isooctane and stirred until transparent microemulsions were obtained, which were mixed immediately.

The reverse microemulsion obtained was stirred at room temperature for 1 h. Then it was placed in a rotary evaporator at 323 K for 20 min under vacuum. To the resulting foam, methanol was added at room temperature to dissolve AOT and to flocculate the palladium nanoparticles. After centrifugation at 8,000 rpm (20 min), the supernatant was decanted and the washing was repeated. The precipitate was redispersed in 12 mL of water by ultrasonic treatment for 20 min. to give a homogeneous black Pd suspension.

Example 2

The Pd suspension obtained in Example 1 was used immediately to impregnate 0.5 g of activated carbon fiber (ACF) fabric. KoTHmex® Activated Carbon Fiber Fabrics AW- 1101 (BET specific surface area of 880 m ² /g, average pore diameter of 2 nm [37]) as obtained from Taiwan Carbon Technology Co., Ltd. was used after pretreatment in a boiling aqueous solution of 6.5 wt.% HNO ₃ for 1 h . Then the support was rinsed with distilled water, air-dried for 12 h at room temperature and for 5 h at 393 K. BET specific surface area of the ACF was 950 m ² /g The impregnated support was air-dried at 393 K for 40 min, followed by a second impregnation step. Finally, the catalyst was air-dried overnight at 393 K.

Microemulsion-Methanol (ME-Met) catalysts with different Pd loading (0.4í1.2 wt.% Pd) were prepared by variation of the amount of suspension. Some catalysts were

repeatedly washed by heptane between the impregnation steps. As hydrazine is known to promote AOT dimerization , the ME-Met catalyst was also prepared using 5 M hydrazine hydrate solution (instead of 1 M).

Catalyst ME-Mix ("MicroEmulsion-MIXing") was prepared by mixing the microemulsion and the support. The reverse microemulsion was stirred for 30 min to form the nanoparticles , then 0.5 g of ACF were immersed into the microemulsion and stirred for 1 h , followed by repeatedly washing in heptane and air-drying at 393 K overnight.

Catalyst ME-THF ("MicroEmulsion-TetraHydroFuran") was prepared as a Catalyst ME- Mix but with tetrahydrofuran added drop-wise during the mixing of the microemulsion and support for destabilization.

Example 3

Catalyst characterization : The Pd amount was determined by atomic absorption spectroscopy (AAS) at 247.6 nm via Shimadzu AA-6650 spectrometer with an air- acetylene flame. The catalysts (fresh and spent) were heated in air at 970 K for 3 h to burn out carbon, the residuals were dissolved in a mixture of concentrated acids (HCI:HNO ₃ =3:1 by vol.) and aqueous HF solution. To determine the amount of Pd leached during the catalytic experiments, the reaction mixtures were placed in a rotary evaporator under vacuum, and then the retort was washed with acids.

Pd particle sizes in the microemulsion were determined by high-resolution transmission electron microscopy (HRTEM) using a Philips EM 430 instrument with a resolution of 0.23 nm, at 300 kV. The sample for TEM analysis was prepared by placing a drop of the aqueous suspension obtained in step (d) of the preparation of the catalyst ME-Met onto the carbon film on the copper grid and evaporation the solvent at room temperature. Different parts of the grid were examined. The instrument was equipped with an energy-dispersive X-ray (EDX) analyzer.

Fig. 1 shows TEM image of Pd nanoparticles. Monodispersed particles of 8 nm diameter were observed indicating that no coagulation takes place during the catalyst preparation. Evaporation of the water/ AOT/n-heptane microemulsion has been shown

not to lead to Pd aggregation. The methanol added dissolves the AOT and destroys the surfactant shell around the nanoparticles causing flocculation. Agglomerates (-100 nm) consisting of small particles (~2 nm) may be easily redispersed in any hydrophobic solvent.

Example 4

Hydrogenations using structured catalysts of the present invention were carried out in a batch stainless steel reactor (150 ml. autoclave, Buchi AG, Uster, Switzerland) equipped with a heating jacket and a hydrogen supply system. The structured Pd/ACF catalyst was placed between two metal gauzes (2 x 4 cm) fixed on the self-gassing hollow shaft stirrer, see Chem. Eng. Sci. 57 (2002) 343. For the reaction with the powdered Lindlar catalyst, 8-blade disk turbine impeller was used. At the working temperature the reactor filled with the reaction mixture and the catalyst, flushed with Ar (0.8 MPa) was kept for 5 min under stirring to equalize the temperature. Then the reactor was flushed with hydrogen and pressurized. During the course of the reaction, the pressure in the reactor was maintained constant.

Typical experiments were carried out at 303 K and 1.3 MPa H ₂ pressure at intensive stirring of 1500 rpm avoiding external diffusion limitations. π-Heptane was used as a reaction medium containing 0.5 kmol/m ³ of 1-hexyne in total volume of 100 ml_. Substrate-to-palladium molar ratio for the solvent-assisted reactions was between 20,000 - 30,000. The reaction kinetics was studied at temperatures 293-323 K and hydrogen pressure variation between 0.4-1.7 MPa.

In the reuse experiments, the catalyst was air-dried at room temperature between the reaction runs. In the solvent-free experiments, 101.3 ml. of 1-hexyne that corresponds to 0.9 mole was hydrogenated.

The samples of the reaction mixtures were periodically withdrawn from the reactor via a sampling tube and analyzed by GC. The GC analysis was performed using Auto System XL (Perkin Elmer) equipped with a 100 m Petrocol DH 0.25 mm capillary column with a 0.5 μ coating at the oven temperature of 333 K and the carrier gas (He)

pressure of 280 kPa. Injector and FID temperature were 493 K. n-Octane was used as the internal standard.

The initial reaction rate, r ₀ [kmol _H 2-kgp _d ^"1 -s ^~1 ], was used as a characteristic of catalytic activity. Activities in the succesive runs were calculated per initial Pd loading. Selectivity to 1-hexene was calculated as a ratio of its concentration to a total concentration of all products in the reaction mixture. As by-products, n-hexane, cis- and trans-2-hexenes were found.

Data on the hydrogenation activity and selectivity of ME-Met, ME-Mix and ME-THF are presented in Table 1.

Table 1. Catalytic performance of different Pd/ACF catalysts in 1-hexyne partial hydrogenation

Reaction conditions: 0.5 kmol/rrv 1-hexyne in n-heptane (100 mL); substrate-to-Pd molar ratio of 20O00; 1.3 MPa H ₂ pressure; 303 K; 1500 rpm

Example 5

Reuse of ME-Met (Pd/ACF) catalyst. Comparison with Lindlar catalyst

The ME-Met catalyst batches prepared without washing in heptane between the impregnation steps (such as 1.2 wt% Pd) showed progressive Pd leaching during the successive reaction runs with the same catalyst. Washing resulted in lower Pd content attained (0.45±0.07 wt.%) but it preservs strongly adsorbed catalytic species on the support surface. Negligible leaching ( <10%within the experimental error) was observed in this case at the catalyst reuse.

Fig. 2 shows the initial reaction rate and the selectivity at the 1-hexyne conversion of 90% for the washed ME-Met catalyst (0.4 wt% Pd). No treatment of the catalyst between the reactions was applied. After some activity drop in the second reaction run,

it stabilized at 0.085±0.008 kmol _H 2 kgp _d ^'1 s ^'1 , while selectivity to 1-hexene was 94±1%. Kinetic curves were also found to be identical up to the 6 runs. This is shown in Fig. 3a for the 6 ^th run along with the curves over fresh Lindiar catalyst. The loading of Lindlar catalyst was adjusted to obtain the same Pd amount as in the fresh ME-Met catalyst. The plot of selectivity vs. conversion is presented in Fig. 3b. Spent ME-Met catalyst exhibits a higher selectivity than fresh Lindlar catalyst, e.g., 94±1% vs. 89±2%, respectively, at 90% conversion. The higher yield of 1-hexene was with the used ME- Met catalyst (87±2% vs. 82±3%) in a 1.3-fold shorter reaction time. Higher catalyst activity and selectivity may be attributed to Pd size and monodispersity, as alkyne hydrogenation is considered as a structure-sensitive reaction. Thus, both activity and selectivity of the developed catalyst were stable from the second run on, and were superior in comparison with the fresh Lindlar catalyst.

Example 6

Solvent-free hydrogenation over the used ME-Met catalyst

Catalyst ME-Met (0.4 wt.% Pd) was tested also in a solvent-free hydrogenation after 6 runs in heptane and drying under ambient conditions. Fresh Lindlar catalyst was used for a comparison. Concentration-time profiles are presented in Fig. 4a, and data on selectivity are shown on Fig. 4b. As in solvent-assisted reactions, the used ME-Met catalyst shows a higher selectivity than fresh Lindlar catalyst; higher maximum yield of 1-hexene was achieved (92% vs. 90% for Lindlar catalyst) for 1.2-fold lower reaction time. No noticeable Pd leaching was detected. The total time-on-stream of the ME-Met catalyst (0.4 wt% Pd) was ~21 hrs. This allows us to consider the catalyst as stable during long-term operation.