WO2011086343A2

WO2011086343A2 - Improved catalyst

Info

Publication number: WO2011086343A2
Application number: PCT/GB2011/000018
Authority: WO
Inventors: Lynne Macaskie; Kevin Deplanche
Original assignee: The University Of Birmingham
Priority date: 2010-01-15
Filing date: 2011-01-10
Publication date: 2011-07-21
Also published as: WO2011086343A3; GB201000638D0

Abstract

The present invention relates to a bio-catalyst for use in chemical reactions, wherein said bio-catalyst comprises palladium and gold adsorbed onto a biological support. In certain embodiments the palladium and gold are in the form of nanoparticles and the particles have a core-shell structure. The invention also relates to methods of making such catalysts and their use in selective chemical reactions.

Description

IMPROVED CATALYST

The present invention relates to a catalyst for use in chemical reactions, and more particularly but not exclusively hydrogenation and oxidation reactions.

It is well known that hydrogenation and oxidation reactions are important in the synthesis of chemicals and intermediates. Such hydrogenation and oxidation reactions can be carried out using stoichiometric reagents such as permanganate or chromate, but these reagents are toxic and expensive.

An alternative to using stoichiometric reagents in such reactions is to use a catalyst. Metal nano-particles on an inorganic support have been recently shown to be effective catalysts in oxidation reactions which use air or O2 as the oxidant under mild conditions. Gold nanocrystals have been shown to be effective for the selective oxidation of alcohols to aldehydes in the gas phase but this catalyst is not suitable for use in the aqueous phase as the selectivity is lost leading to production of the monoacid. Hydroxyapatite-supported palladium nanoclusters have also been reported to be effective in the oxidation of aromatic alcohols but show limited activity for the oxidation of primary alcohols.

The use of Palladium nanoparticles in catalyst systems is widely known, for example Dominguez-Quintero et al ("Silica-supported palladium

nanoparticles show remarkable hydrogenation catalytic activity", Journal of Molecular Catalysis A: Chemical 197 (2003) 185-191) describe their use in hydrogenation reactions when stabilised on a silica support.

Titanium dioxide-supported palladium and palladium-gold catalysts have been reported to have improved activity in the selective oxidation of benzyl alcohol to benzaldehyde in solvent-free conditions. However the reactions can produce undesirable by-products such as toluene, benzene and benzyl benzoate. A further drawback is that a significant proportion of the substrate is also fully oxidised to benzoic acid.

The combination of palladium with gold as a bimetallic nanoparticle gives synergistic effects and is reported to have superior catalytic performance by Nutt et al ("Improved Pd-on Au bimetallic nanoparticle catalysts for aqueous- phase trichloroethene hydrodechlorination", Applied Catalysis B:

Environmental 69 (2006) 115-125).

Methods of producing palladium-gold catalysts are also known. US6022823 teaches a process for the preparation of an improved supported palladium- gold catalyst for the vapour phase production of vinyl acetate, wherein an inert support impregnated with palladium and gold salts is calcined prior to reduction. WO2007/094905 describes a method for preparing a supported palladium-gold catalyst for use in acetoxylation comprising sulphating a titanium dioxide support, calcining the sulphated support and impregnating the calcined support with a palladium salt, a gold salt, and an alkali metal or ammonium compound. The impregnated support is calcined and then reduced to form the supported palladium-gold catalyst.

Core shell structures of Au/Pd have been shown to be better in some reactions, for example vinyl acetate monomer (VAM) synthesis and selective oxidations. Such structures are reported by Ejima et al ("Preparation of AU-PD Core-shell Nanoparticles Supported T1O2 Photocatalyst with

Sonochemical Technique", Nagasaki Symposium on Nano-Dynamics 2009 (NSND2026) p.34-35, 2009) where the thickness of the palladium layer was found to strongly affect the activity of H2 formation. Au/Pd core-shell structures are currently produced by co-deposition or sequential deposition. However, it is difficult to control the size of catalyst nanoparticles, making it difficult to control their behaviour during chemical reactions, and Au/Pd core-shell nanoparticle catalyst performance is damaged by nanoparticle aggregation and the difficulty of controlling the core-shell structure of the end product.

It is an object of one aspect of the present invention to provide an improved catalyst for use in chemical reactions to mitigate one or more problems associated with the prior art catalysts.

According to a first aspect of the present invention there is provided a bio- catalyst for use in chemical reactions, wherein said bio-catalyst comprises palladium and gold adsorbed onto a biological support.

Examples of suitable biological supports include cells (in suspension or immobilised), cell fragments, membranes, membrane fragments which may contain protiens or purified protein templates such as flagella, bacterial S- layers or enzymes. In certain embodiments the biological support includes bacterial cells, for example Escherichia coli or Rhodobacter spp. cells.

In certain embodiments the palladium and gold are in the form of

nanoparticles. The nanoparticle size may be less than 50nm, less than 20nm, or less than lOnm in size. In one embodiment the nanoparticles are from 5nm to lOnm in size.

In a specific embodiment, the palladium and gold nanoparticles comprise a core-shell structure, for example a gold core and palladium shell. The cells (pre or post adsorption) can be processed in a number of ways, for example by thermal treatment and/or grinding.

According to a second aspect of the present invention there is provided a method for the preparation of a bio-catalyst in accordance with the first aspect of the present invention, comprising the steps of:

(i) treating a biological support with palladium such that

palladium is adsorbed onto the biological support;

(ii) after palladium adsorption, treating the biological support with gold such that gold is adsorbed onto the biological support; and

(iii) carrying out at least one reduction step such that the

palladium and gold present in the bio-catalyst are in the zero oxidation state.

Where more than one reduction step is performed, step (iii) may be carried out after step (i) and again after both steps (i) and (ii) have been performed.

Examples of suitable biological supports include cells, or purified protein templates such as flagella, bacterial S-layers or enzymes. In certain embodiments the biological support includes bacterial cells, for example

Escherichia coli or Rhodobacter spp.

In those embodiments where the biological support comprises cells, the method may include a step prior to step (i) of growing the cells in the absence of oxygen.

In various embodiments, step (i) and/or step (ii) and/or step (iii) or all of steps (i) to (iii) of said method are carried out in the absence of oxygen. The amount of palladium and gold to be added to the biological support is calculated based on the mass of the biological support and the desired final metal loading of the bio-catalyst. The amount of palladium in said bio- catalyst may be in the range from 0.25wt% to 6wt%, in the range from 0.5wt% to 4wt%, or in the range from 0.75wt% to 3wt%. The amount of gold in the bio-catalyst may be in the range from 0.25wt% to 6wt , in the range from 0.5wt% to 4wt%, or in the range from 0.75wt% to 3wt% .

Specific examples of Au/Pd loadings are 5wt%/5wt%, 2.5wt%/2.5wt%, lwt%/lwt% and 0.5wt%/0.5wt% .

In certain embodiments the ratio of the amounts of palladium and gold added to the biological support are in the range from 1 :2 to 2: 1. It will be appreciated that the optimum ratio may be different for different chemical reactions for which the catalyst is suitable. The ratio of palladium and gold added to the biological support may be in the range 0.8: 1 to 1 :0.8 and for some reactions the amounts of palladium and gold added to the biological support will be the same.

In one embodiment said at least one reduction step is carried out using H gas. This H2 gas can be added directly as gas or generated in situ by catalytic splitting of an organic molecule such as formate (HCOOH), by palladium in its added form as captured on the bacteria, which may be Pd(II), Pd(I) or small amounts of Pd(0) generated by endogenous reduction in the absence of added electron donor.

In one embodiment said palladium is added to the biological support in the form of a palladium (II) salt. Said palladium (II) salt may includes at least one halide anion, for example a chloride ion. In one specific embodiment the palladium (II) salt is Na2PdCk

Said gold may be added to the biological support in the form of a gold (III) salt.

In specific embodiments said gold (III) salt is HAuCU or NaAuCU.

In one embodiment the Pd seeds formed after the second reduction step may be partially re-oxidised to Pd(II) and present as such in the bio-catalyst.

In a specific embodiment of the method E. coli cells grown in the absence of oxygen are firstly mixed with palladium in the form of a palladium(II) salt followed by reduction of the palladium (II) to palladium (0) in the presence of H2 gas, and are secondly mixed with gold in the form of a gold (III) salt followed by reduction of the gold (III) to gold (0) in the presence of H2 gas wherein all steps are carried out in the absence of oxygen, and wherein the final metal loading of the cells is the same for each metal.

It has been found that generating the Pd nanoparticle seeds onto the biological support gives an arrangement of nanoparticles on the surface of the biological support. The biological support surface maintains a spacing between the Pd nanoparticles to give distinct particles without the agglomeration that is seen with other methods of nanoparticle manufacture. It is thought that

aggregation of the Pd seeds is prevented by the presence of biological residues at the surface of the Pd seeds.

Minimising the nanoparticle aggregation gives better control over the final Au/Pd core-shell structure and hence improved catalyst performance. The Pd nanoparticle seeds form primarily on the cell surface and the periplasmic space, however, some seeds will also be formed in the intracellular space as well as in areas immediately surrounding the cells.

The Pd seeds are reacted with Au, for example Au(III) ions under conditions for the formation of Au/Pd core-shell structure nanoparticles.

More than 80%, more than 95 % or more than 98% of the Pd seeds may be reacted with Au to form the bimetallic nanoparticles.

According to a third aspect of the present invention there is provided a method for the selective formation of products during chemical reactions, said method comprising contacting a reaction substrate with a biocatalyst in accordance with the first aspect of the present invention.

Examples of said chemical reactions for which the bio-catalyst is suitable are reduction/oxidation (REDOX) reactions, selective hydrogenations and selective oxidations.

"Selective formation of products" refers to steering a reaction towards a specifically desired product from a number of possible reaction products. Reactions involving selective product formation include organic and inorganic reactions. Examples of organic selective product formation reactions include the selective oxidation of alcohols to aldehydes (rather than carboxylic acids) for the production of flavourings and fragrances, the selective oxidation of glycerol to glyceric acid (an important intermediate in flavourings and fragrances), and the production of vinyl acetate monomer (VAM) . Examples of inorganic selective product formation reactions include the formation of H2O2 (over H2O) and increased activity in proton exchange membrane fuel cell reactions. In certain embodiments, the bio-catalyst catalyses said chemical reactions (e.g. oxidation) with greater than 90% selectivity, greater than 95%

selectivity, greater than 98% selectivity or with 100% selectivity, selectivity being defined as the percentage of desired product relative to the total of all possible products.

The H2O2 reaction presents particular challenges as it is an indirect synthesis and there are a number of competing reactions, such as the formation of H2O that can take place in preference to the formation of H2O2 and some of the reaction products can cause the mixture to become explosive. A biocatalyst in accordance with the first aspect of the invention gives a direct synthesis of H2O2 as it selectively favours the formation of this product over H2O at low H2 and O2 concentrations and low temperatures ( ~ 2°C).

In the case of an oxidation reaction, the substrate may be an alcohol, such as a primary alcohol. A suitable primary alcohol substrate is benzyl alcohol.

Said oxidation reactions may be carried out in the presence of O2 gas or air.

The oxidation reaction may be carried out at a temperature of between 20 °C and 200°C, between 40°C and 140°C or between 60°C and 120°C.

Said method may comprise contacting the substrate with the bio-catalyst in the presence or absence of a solvent. Where a solvent is used, it is not particularly limited and may be chosen by the skilled person using their ordinary skill and knowledge bearing in mind the nature of the reaction and substrates. Suitable solvents include water (including sub- and super-critical water) and polar or non-polar organic solvents and mixtures of miscible solvents. Embodiments of the invention will now be described by way of example with reference to the accompanying figures, in which:

Figure 1 (a) and (b) are electron micrographs showing native E. coli MC4100 cells and the bio-catalyst according to the present invention formed from those cells respectively,

Figure 2 (a) and (b) are micrographs of a biogenic Pd/Au nanoparticle formed by E. coli,

Figures 2(c) to (f) are EDX mappings of two Pd-Au particles showing the Au and Pd distributions: (c) HAADF image; (d) X-ray signal intensity from the characteristic La transitions of Au; (e) the characteristic La

transitions of Pd, (f) superimposition of the Pd and Au signals

Figure 3 shows the rate of conversion and selectivity for the benzyl alcohol to benzaldehyde reaction in the presence of the bio-catalyst of the present invention,

Figure 4 shows the impact of subjecting the biocatalyst to reducing conditions (prior to use) on the rate of conversion for a benzyl alcohol to benzaldehyde reaction.

Figure 5 shows the rate of conversion of 2-octanol to 2-octanone in the presence of the bio-catalyst of the present invention.

Figure 6 shows the products of glycerol oxidation reactions.

The electron micrograph of Figure 1(a) shows a transmission electron micrograph (TEM) of native E.coli MC4100. The TEM of Figure 1(b) shows cells of E.coli MC4100 after sequential reduction of Pd(II) and Au(III) at proportions of 5 %/5 % on biomass (weight %).

The high resolution TEM image of Figure 2(a) shows a biogenic Pd/Au nanoparticle formed by E.coli. The Au core and Pd shell structure can be seen in the high angular annular dark field (HAADF) microscopy image of Figure 2(b) as the intensity of light emitted is a function of atomic number (Z), the Au core gives a brighter region in the image than the Pd shell.

Figure 2(c) is similar to Figure 2(b) and shows an HAADF image for two Pd-Au particles. Figures 2(d) to (f) show respectively the X-ray signal intensity from the characteristic La transitions of Au, Pd, and the

superimposition of the Pd and Au signals.

METHODOLOGY

The bio-catalyst was prepared using Escherichia coli MC4100. The bacterial cells were grown in the absence of oxygen and harvested by centrifugation. A concentrate of cells dispersed in 50 mis of buffer (MOPS) was obtained and stored under oxygen-free nitrogen until use.

The cell concentration determined using OD/dry weight conversion for an active catalyst neds to be in the range 5 to 150 mg/ml, preferably in the range 10 to 100 mg/ml. Cell concentrations greater than 150 mg/ml have been found to give a catalyst that is not active.

The dry weight of the cells was determined and the mass of palladium required to achieve a final metal loading of 2.5 % was calculated. The corresponding volume of 2mM Na2PdCk, pH 2.3, was degassed for 30 minutes using a vacuum pump and then mixed with the bacterial cells under oxygen free nitrogen (OFN). The mixture is pressurised with OFN to maintain a positive pressure to stop air coming into contact with the mixture and incubated for 30 minutes at 30°C with occasional shaking. H2 was then bubbled through the suspension for at least 10 mins to give Pd(0) on the cell surface. Complete removal/reduction of the palladium was confirmed by spectrophotometric assay of the spent solution.

It is thought that the Pd(II) ions complex with biochemical ligands and bacterial cell groups in the vicinity of the cell surface and periplasmic hydrogenases when the Na2PdCU solution and bacterial cells are mixed.

When H2 is fed into the mixture of Pd (II) ions and bacterial cells the hydrogenases perform their physiological function, splitting H2 into 2H⁺ and 2e . The e^~ pool generated reduces Pd(II) to Pd(0), generating small discrete seeds of Pd(0).

A 1 mM solution of gold(III) (HAuCk), pH 2.3, was degassed for 30 minutes under vacuum and saturated with H2 by bubbling H2 through the solution for a minimum of 10 mins and then added in the absence of oxygen to the cell-Pd suspension and mixed. The mixture is put under positive pressure oxygen free atmosphere such as H2 and left overnight at 37°C.

The H2 used to saturate the Au(III) solution is not enough to bring noticeable reduction to Au(0). When the mixture of cells and Pd(0) is added to the Au(III) solution it is thought that a redox reaction occurs between the couples Pd(0)/Pd(II) and Au(0)/Au(III) according to the reaction:

2 Au^{3 +} + 3 Pd(0) > > > 2 Au(0) + 3 Pd^{2 +}

The Au(0) particles are then formed. Because the Au(III) solution was initially saturated by H2, the formed Pd(II) ions are thought to be rapidly reduced back to Pd(0), predominantly at the Au nanoparticle surface where the Pd(II) ions are most likely located. This results in an Au core and Pd shell where the order of addition of the metal salts suggest that we should get nanoparticles with a reversed Pd/Au core-shell configuration. Nanoparticles with a reversed Pd/Au core-shell configuration are noticeably difficult to make because of the difference of EV potential between Pd(II)/Pd and Au(III)/Au couples, hence the redox reaction is favoured.

The Pd/Au loaded cells settled from the solution and were harvested by centrifugation, washed twice with distilled H2O and once with acetone, resuspended in acetone and dried in air. The dried powder was then ground to give the bio-catalyst in the form of a powder.

Example 1. Oxidation of benzyl alcohol to benzaldehyde.

The bio-catalyst prepared above was first subjected to reducing conditions at 120°C for 3 hours under a 50ml/min flow of 10%H2/Ar to eliminate any possible metal oxide that might be occupying the active sites. This reduction in hydrogen for cleaning the catalyst has been found to give only marginal benefit over using the catalyst without the reduction (and may therefore be omitted for catalysts of this invention). This cleaning is a step that is currently required with known Pd, Au and Pd/Au catalysts so a catalyst that is already clean enough to give the required selectivity is an advantage over the existing state of the art.

40ml benzyl alcohol was mixed with 0.15g dried bio-catalyst and stirred at 90°C, 1 bar pressure, with 200ml/min air flow. The oxidation reaction of benzyl alcohol to benzaldehyde was monitored using gas chromatography. The rate of conversion is shown in the graph of Figure 3 for the Pd/Au biocatalyst along with the conversion rate for Pd and Au monometallic catalysts for comparison purposes. Reaction selectivity is also shown in Figure 3 for each of these catalyst systems. Figure 4, plot (a) shows the conversion of benzyl alcohol to benzaldehyde using the bio-catalyst that had been subjected to the reducing conditions above prior to the oxidation reaction. Figure 4 plot (b) shows the conversion of benzyl alcohol to benzaldehyde using the bio-catalyst that had not been subjected to reducing conditions prior to the oxidation reaction.

The oxidation of benzyl alcohol shown in Figure 4 plot (a) (using the bio- catalyst that was subjected to reducing conditions) proceeded to 18 % conversion over 6 hours. The reaction shown in plot (b) (no additional reduction of the bio-catalyst) proceeded to approximately 15% conversion over 6 hours. In both cases no undesired by-products such as toluene, benzene or benzoic acid were detected, therefore the selectivity of the reaction for benzaldehyde was 100%.

After the reaction the bio-catalyst was removed by gentle centrifugation or centrifugation, washed with demineralised water and dried. The biocatalyst could then be reused a number of times without any deterioration in performance.

Example 2. Oxidation of 2-octanol to 2-octanone.

Using the same conditions as in Example 1 except that the bio-catalyst was not subjected to reducing conditions prior to use, oxidation of 2-octanol to 2- octanone was carried out using the bio-catalyst. The reaction was monitored using gas chromatography. The rate of conversion is shown in Figure 5.

As can be seen from Figure 5, the conversion of 2-octanol proceeded at a constant rate to 18% over 5 hours. No undesired by-products were detected. Example 3. Formation of H2O2

The biocatalyst prepared above was not subject to reducing conditions prior to use. H2 and O2 at low pressures and maintained at a temperature of 2°C are reacted in the presence of the biocatalysts to give a direct synthesis of

Example 4. Oxidation of glycerol to glyceric acid

The biocatalyst prepared as in Example 1 above (non reducing conditions) was used to catalyse the glycerol to glyceric acid reaction. This reaction is known to be particularly difficult as there are many different intermediate and alternative reaction products that can be formed (see Figure 6), however, the reaction is widely used, for example in the production of flavourings and fragrances.

Using the Pd/Au biocatalyst of the invention glycerol conversion to glyceric acid was 72 % after 3 hours, a significant improvement over conventional catalyst materials. Selectivity was good with Glyceric acid being the main reaction product and only small traces of other reaction products present.

Claims

1. A bio-catalyst for use in chemical reactions, wherein said bio-catalyst comprises palladium and gold adsorbed onto a biological support.

2. The bio-catalyst according to claim 1 , wherein the biological support comprises bacterial cells.

3. The bio-catalyst according to claim 2, wherein the bacterial cells are Escherichia coli or Rhodobacter spp.

4. The bio-catalyst according to any preceding claim, wherein the palladium and gold are in the form of nanoparticles.

5. The bio-catalyst according to claim 4, wherein the nanoparticle size is less than 50nm.

6. The bio-catalyst according to claim 4 or claim 5, wherein the nanoparticle size is from 5nm to lOnm.

7. The bio-catalyst according to any one of claims 4 to 6, wherein the palladium and gold nanoparticles comprise a core-shell structure.

8. A method for the preparation of a bio-catalyst according to any one of claims 1 to 7, comprising the steps of:

(i) Treating a biological support with palladium such that palladium is adsorbed onto the biological support;

(ii) Treating the biological support with gold such that gold is adsorbed onto the biological support; and

(iii) Carrying out at least one reduction step such that the palladium and gold present in the bio-catalyst are in the zero oxidation state.

9. The method according to claim 8, wherein the biological support comprises bacterial cells.

10. The method according to claim 8 or claim 9, wherein at least one of the steps is carried out in the absence of oxygen.

1 1. The method according to any one of claims 8 to 10, wherein the ratio of the amounts of palladium and gold added to the biological support is in the range of from 1 :2 to 2: 1.

12. The method according to any one of claims 8 to 1 1 , wherein the at least one reduction step is carried out using hydrogen gas.

13. The method according to any one of claims 8 to 12, wherein the palladium is added to the biological support in the form of a palladium (II) salt, preferably Na₂PdCl₄.

14. The method according to any one of claims 8 to 13, wherein the gold is added to the biological support in the form of a gold (III) salt, preferably HAuCl₄ or NaAuCl₄.

15. The method according to any one of claims 8 to 14, wherein in step (i) palladium nanoparticle seeds are formed on the biological support.

16. A method for the selective formation of products during chemical reactions, said method comprising contacting a reaction substrate with a bio-catalyst according to any one of claims 1 to 7.

17. The method according to claim 16, wherein the bio-catalyst catalyses said chemical reactions with greater than 90% selectivity.

18. The method according to claim 16 or claim 17, wherein the substrate is an alcohol.

19. The method according to claim 16 or claim 17, wherein the chemical reactions are reduction/oxidation (REDOX) reactions.

20. The method according to claim 19, wherein the reaction is an oxidation reaction.

21. The method according to any one of claims 16 to 20, wherein the substrate is contacted with the bio-catalyst in the absence of a solvent.