WO2015126327A1

WO2015126327A1 - Supported nanowire catalysts

Info

Publication number: WO2015126327A1
Application number: PCT/SG2015/000049
Authority: WO
Inventors: Hongyu Chen; Jiating HE; Yawen Wang; Bin Liu
Original assignee: Nanyang Technological University
Priority date: 2014-02-21
Filing date: 2015-02-16
Publication date: 2015-08-27
Also published as: SG11201606554VA

Abstract

The invention relates to supported nanowire catalysts. In particular, the catalyst comprises a fiber substrate and a plurality of nanowires attached to the fiber substrate, wherein the plurality of nanowires are comprised of at least one noble metal. The invention also relates to a method of performing a chemical reaction, comprising reacting a mixture of reactants in the presence of the catalyst.

Description

SUPPORTED NANOWIRE CATALYSTS

CROSS-REFERENCE TO RELATED APPLICATION

[001] This application claims the benefit of priority of United States of America Provisional Patent Application No. 61/943,051 , filed February 21 , 2014, the contents of which being hereby incorporated by reference in its entirety for all purposes.

TECHNICAL FIELD

[002] The invention relates to supported nanowire catalysts. In particular, the catalyst comprises a fiber substrate and a plurality of nanowires attached to the fiber substrate, wherein the plurality of nanowires are comprised of at least one noble metal. The invention also relates to a method of performing a chemical reaction, comprising reacting a mixture of reactants in the presence of the catalyst.

BACKGROUND

[003] For a long time, nanoparticles have been extensively explored for use in catalysis. Being small, they offer many advantages over conventional bulk catalysts. They have large surface area and possibly unusual surface facets; both of which can greatly improve catalytic performance. However, the disadvantage of using nanostructure for catalysis is the extra effort to maintain their small size. For. any materials, small size means large surface energy. As a result, the particles tend to aggregate and coalesce together to reduce their surface to volume (S/V) ratio.

[004] Nanoparticles can be directly employed in colloidal form in catalytic reactions. They are homogeneously suspended in the solution but the catalytic reactions on their surface are heterogeneous in nature. The capping ligands on the nanoparticles can suppress the surface reactions but yet they are essential for the colloidal stability of the nanoparticles. The presence of salt and/or ligands (reactants or products) can cause the aggregation of the colloidal nanoparticles. After the catalysis, the colloidal nanoparticles are difficult and tedious to separate, often leading to their aggregation during the centrifugation/filtration step which lowers their potential for recycling.

[005] By anchoring nanoparticles on catalyst support surface, the recyclability can be greatly improved. But the process of anchoring nanoparticles is non-trivial. The nanoparticles need to be spaced out to avoid aggregation, and must be securely anchored/adsorbed (typically via strong/charged ligands) to reduce the loss of catalysts. At the same time, the strong ligands should not block the catalyst surface. In fixed-bed catalysis, catalysts are attached to an immobilized solid support and the reaction mixture passed through, providing a methodology to avoid the catalyst-product separation step and promote recyclability. In this kind of catalysis, the support particles (usually silica particles) should be small so to provide a large surface area for anchoring the catalyst nanoparticles. However, this could present another major hurdle in its applications, as the small nature of the crevices among support particles pose high resistance for the reaction mixture to pass through. To get around this problem, high pressure conditions are often necessary.

[006] Another major problem in using nanoparticles for catalysis is the difficulty in increasing the catalyst loading. More catalysts means a more efficient catalysis, but it is difficult to get a large number of nanoparticles together without aggregation. In both colloidal and anchored catalysis, high concentration of nanoparticle can easily cause their aggregation, compromising the available surface for catalysis. For industrial applications, catalyst loading should be kept sufficiently high to keep the reactor volume low. [007] Therefore, there remains a need to provide for a more efficient catalyst system that overcomes, or at least alleviates, the above drawbacks.

SUMMARY

[008] According to a first aspect of the invention, there is disclosed a catalyst comprising a fiber substrate and a plurality of nanowires attached to the fiber substrate. The plurality of nanowires are comprised of at least one noble metal selected from the group consisting of gold (Au), silver (Ag), palladium (Pd), platinum (Pt), iridium (Ir), osmium (Os), rhodium (Rh), and ruthenium (Ru).

[009] According to a second aspect of the invention, there is disclosed a method for forming the catalyst of the first aspect.

[010] According to a third aspect of the invention, there is disclosed a method for performing a chemical reaction, wherein a mixture of reactants is reacted in the presence of the catalyst of the first aspect. The method is particularly useful in continuous flow system, such as in fixed bed catalytic system.

BRIEF DESCRIPTION OF THE DRAWINGS

[011] In the drawings, like reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.

[012] Figure 1 shows (a) photograph of the catalytic fibers; (b, c) SEM images showing the low- and high-magnification of sample a; (d) reaction scheme of the catalysis; (e, f) photographs showing the setup and the complete reduction of 6 ml. of 4-nitrophenol (20 mM) in 40 s. The total Au in the column is 13 mg.

[013] Figure 2 shows photographs showing the color change of the reaction mixture (2.5 mM of 4-nitrophenol and 250 mM of NaBH₄) (a) before and (b) after the addition of 1.0 mg catalytic fibers (2.2 wt% Au); (c) successive UV-vis spectra recorded with 2 min interval.

[014] Figure 3 shows schematics illustrating the difference of pores among (a) loosely packed silica spheres and (b) glass fibers (the pores are indicated as shown); schematics illustrating the difference in terms of available catalytic surface for (c) AuNPs and (d) AuNWs loaded on a support surface. With the same surface density, AuNWs would have 200 times the catalytic surface area of AuNPs.

[015] Figure 4 shows a graph showing the total turnover number versus time for the reduction of 4-nitrophenol (10 mM) with NaBH₄ (1 M) by using 195 mg of catalytic fibers (total 3.1 mg of Au).

[016] Figure 5 shows SEM images showing the low- and high-magnification of vertical AuNWs on glass fibers.

[017] Figure 6 shows TEM image of 3-5 nm citrate-stabilized AuNPs.

[018] Figure 7 shows successive UV-vis spectra recording the reduction of 2.5 mM of 4- nitrophenol by using 830 mM of NaBH₄ in the presence of 1.0 mg catalytic fibers (2.2 wt% Au) at 1 min interval.

[019] Figure 8 shows photographs showing the color change of the reaction mixture (2,5 mM of 4-nitrophenol and 250 mM of NaBH₄) (a) before and (b) after the addition of citrate-stabilized 3-5 nm AuNPs; (c) successive UV-vis spectra recorded with 0.5 min interval. [020] Figure 9 shows HPLC spectra of (a) the reactant solution and (b) exiting solution of the catalytic reaction in Figure 1f, indicating the fully conversion of 4-nitropheno|.

[021] Figure 10 shows (a) a full ¹H NMR spectrum and (b) magnified portion of the product in the catalytic reaction in Figure 1f.

[022] Figure 11 shows photographs showing the catalysis using 600 mg of glass fibers adsorbed with Au seeds. These fibers were packed in a column for the reduction of 4- nitrophenol (1 mM) with 0.1 M NaBH₄ (a) before and (b) 20 s after the reaction solution was allowed to flow through. It is shown that the reaction was incomplete after flowing through this column.

[023] Figure 12 shows photographs of (a) catalytic fibers packed in the column with 3 cm in height; (b, c) AuNP-adsorbed silica spheres (of = 60 μηη) packed in a column with (b) 0.3 cm and (c) 4.5 cm in height.

[024] Figure 13 shows (a, b) SEM images of low- and high-magnification of a sample with .6 wt% of Au loaded on glass fiber. This sample was used for studying the catalytic performance over a long period, as shown in Figure 4.

[025] Figure 14 shows photographs showing the setup for catalytic test over an extended period (as shown in Figure 4). A column with 195 mg of catalytic fibers and lower catalyst loading (1.6 wt% Au) was used for the reduction of 4-nitrophenol (10 mM) with NaBH₄ (1 M).

[026] Figure 15 shows SEM images of low- and high-magnification of catalytic fibers (a, b) before and (c-f) after heated in an aqueous solution for 2h: (c, d) at 60 °C and (e, f) at 100 °C.

^■ DESCRIPTION

[027] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practised. These embodiments are described in sufficient detail to enable those skilled in the art to practise the invention. Other embodiments may be utilized and structural and material changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[028] Present disclosure relates generally to supported nanowire catalysts. In present context, a supported nanowire catalyst refers to a catalyst having a plurality of nanowires supported thereon. The plurality of nanowires may be supported by attachment or binding to surfaces of the catalyst. For example, a plurality of nanoparticles are first deposited onto a substrate. The plurality of nanoparticles act as seeds and facilitate the growth of nanowires therefrom.

[029] The catalyst of present disclosure can be used in a variety of catalytic reactions, e.g. in fixed bed catalysis for chemical reactions, such as a photocatalysis, electrocatalysis.

[030] In one disclosed example to be described later, for fixed bed catalysis, the nanowires, for example, gold nanowires having of=5nm and /=1 m, are vertically grown on a fiber substrate, such as a glass fiber substrate. Glass fibers are used as support in this example as it has a large surface area due to its small width (about 10μητι), and the length of 10 cm or greater makes their handling convenient. This greatly improves the catalyst loading per unit support area. The glass fiber substrate has very large pores in the support structure, and this greatly improves the flow rate in fixed bed catalysis systems for chemical reactions. The glass fiber substrate used also forms a porous network with reasonable surface area for growth of the gold nanowires, and the catalytic fibers (i.e. the present catalysts) can be loosely packed into a simple column to demonstrate its use in fixed bed catalysis. Furthermore, the gold nanowires supported glass fibers are flexible and can also be packed into a column directly to form a network catalyst bed. Compared to conventional fixed-bed catalysis, present system offers lower transport resistance of the reaction mixture through the glass-fiber bed, resulting in high flow velocities with similar catalytic activity.

[031] In one disclosed demonstration whereby the catalytic performance of present catalysts in fixed bed catalysis is shown, the glass fiber supported gold nanowire catalyst (packed in a column) is used in a reaction to reduce 4-nitrophenol to 4-aminophenol using sodium borohydride (NaBH₄) as the reductant. The reaction took just 16min (Turnover frequency = 0.09/s), as compared to 60min (Turnover frequency = 0.03/s) when using literature reported gold nanoparticles anchored on silica microspheres as the catalyst for the same chemical reaction. The supported gold nanowire catalyst may be further coated with palladium, for example, for use in other chemical reactions (e.g. debenzylation reactions).

[032] The processing rate of a fixed bed system is determined by the lower rate of (a) the physical flow rate, and (b) the overall rate of chemical reactions. More catalysts can lead to faster reactions; but often slower flow rates. There is a limit in increasing the surface density of NPs on a support. Thus, traditional approaches are to use more support materials and/or smaller support particles, both of which reduce the flow rate. Even when high pressure is used, the flow rate is still a major limitation for improving the processing rate.

[033] To this end, present inventors have surprisingly found that by growing vertical nanowires on a fiber substrate, the catalyst loading per unit support area is greatly improved. With this new loading mode, it can now be afforded to have very large pores in the support structure, thus greatly improving the flow rate.

[034] Thus, in accordance with a first aspect of the disclosure, there is disclosed a catalyst comprising a fiber substrate and a plurality of nanowires attached to the fiber substrate. The plurality of nanowires are comprised of at least one noble metal selected from the group consisting of gold (Au), silver (Ag), palladium (Pd), platinum (Pt), iridium (Ir), osmium (Os), rhodium (Rh), and ruthenium (Ru).

[035] In various embodiments, the fiber substrate is comprised of a material selected from the group consisting of glass, silica, alumina, titania, strontium titanium oxide, lanthanum aluminum oxide, calcium carbonate, silicon, paper, and polymer. Preferably, the fiber substrate comprises a glass fiber substrate.

[036] As mentioned above, in certain embodiments, the plurality of nanowires may be coated with a further noble metal different from the noble metal of the nanowires. The further different noble metal may be selected from the group consisting of Au, Ag, Pd, Pt, Ir, Os, Rh, and Ru. For example, the plurality of nanowires may be comprised of Au while the Au nanowires may be coated with Pd. Such catalyst may be useful in catalyzing debenzylation reactions.

[037] In embodiments where the plurality of nanowires are comprised of gold, the catalyst may be formed by (a) contacting a glass fiber substrate with a functionalizing agent for attaching a plurality of gold nanoparticles thereto; (b) contacting the glass fiber substrate with a pluralit of gold nanoparticles for attachment; and (c) contacting the glass fiber substrate having the plurality of gold nanoparticles attached thereto with an aqueous solution comprising a ligand, gold ions, and a reducing agent, wherein the ligand is an organic compound having a thiol group (i.e. a second aspect of the disclosure).

[038] A nanowire refers generally to an elongated structure having a cross-sectional dimension that is in the nanometers range. For example, the nanowire may have a cross- sectional dimension that is less than 100 nm. The term "nanowire" as used herein may also be used to refer to other elongated nanostructures, such as nanorods, nanofibers, nanotubes, and nanoribbons. The cross-section of the nanowire may assume any shape, and may be uniform or non-uniform throughout the length of the nanowire. [039] A "nanoparticle" refers to a particle having a characteristic length, such as diameter, in the range of up to 100 nm. The term "diameter" as used herein refers to the maximal length of a straight line segment passing through the center of a figure and terminating at the periphery. The term "mean diameter" refers to an average diameter of the nanoparticles, and may be calculated by dividing the sum of the diameter of each nanoparticle by the total number of nanoparticles. Although the term "diameter" ts used normally to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of a nanosphere, it is also used herein to refer to the maximal length of a line segment passing through the centre and connecting two points on the periphery of nanoparticles having other shapes, such as a nanocube or a nanotetrahedra, or an irregular shape.

[040] Gold nanoparticles with a negative surface charge may be nanoparticles in which the negative charge of the gold nanoparticles is conferred by a carboxylic acid, sulfonic acid, carbolic acid or a mixture of the aforementioned acids which is immobilized at the surface of the gold nanoparticles. For example, the carboxylic acid may be, but is not limited to citric acid, lactic acid, acetic acid, formic acid, oxalic acid, uric acid, pyrenedodecanoic acid,

mercaptosuccinic acid, aspartic acid, to name only a few.

[041] In one specific embodiment in which gold nanoparticles are used, citric acid is used to form negatively charged gold nanoparticles comprising a surface layer of citrate ions. For example, the gold nanoparticles may be citrate-stabilized gold nanoparticles.

[042] To attach the gold nanoparticles to the substrate, a surface of the fiber glass substrate may be coated with a functionalizing agent for attaching the gold nanoparticles to the fiber glass substrate. Suitable functionalizing agents in this case include, for example, organofunctional alkoxysilane molecules such as, but are not limited to, (3-aminopropyl)-triethoxysilane, (3- aminopropyl)-diethoxy-methyls_.ilane, (3-aminopropyl)-dimethyl-ethoxysilane, (3-glycidoxypropyl)- dimethyl-ethoxysilane, (3-mercaptopropyl)-trimethoxysilane, (3-mercaptopropyl)-methyl- dimethoxysilane, or a mixture thereof. In various embodiments, the functionalizing agent comprises 3-aminopropyltrimethoxysilane (APTMS).

[043] In other embodiments, the gold nanoparticles may be attached to the substrate without the use of a functionalizing agent. For example, in case the substrate is positively charged and the nanoparticles are negatively charged, the nanoparticles may be attached to the substrate by electrostatic interaction, whereby the term "electrostatic interaction" refers to attraction between electrically charged molecules, such as between a negatively charged molecule and a positively charged molecule.

[044] The glass fiber substrate may be incubated in a suspension comprising gold nanoparticles to allow adsorption of the gold nanoparticles on the substrate, hence attaching the gold nanoparticles onto the substrate. The time for incubation may be any suitable time necessary to allow adsorption of the gold nanoparticles. For example, the incubating time may range from about 1 min to about 5 hours, such as about 5 minutes to about 2 hours, about 10 minutes to about 1 hour, or about 10 minutes.

[045] The gold nanoparticles attached to the glass fiber substrate may have an inter-particle distance of less than 5 nm, such as less than 4 nm, less than 3 nm, less than 2 nm or less than 1 nm. Generally, the larger the surface area of the substrate, the larger the number of gold nanoparticles that may be attached to the substrate.

[046] As mentioned above, in various embodiments, the fiber substrate comprises glass fiber substrate.

[047] In various embodiments, the noble metal comprised in the plurality of nanowires is gold. [048] In the method for forming the present catalyst wherein the ligand is an organic compound having a thiol group, the ligand may be selected from the group consisting of 4- mercapto-phenylacetic acid (4-MPAA), 4-mercaptobenzoic acid (4-MBA), 3-mercaptobenzoic acid (3-MBA), 4-mercaptophenol (4-MPN), and a mixture thereof. Preferably, the ligand comprises 4-mercaptobenzoic acid (4-MBA).

[049] Other ligands are also suitable. For example, the ligands mentioned in PCT Publication No. WO 2013/043133 may be used in forming the present catalysts, the content of which is incorporated herein in its entirety.

[050] The aqueous solution also includes gold ions. For example, the aqueous solution containing gold ions may comprise chloroauric acid, tetrachloroauric acid, a lithium salt of tetrachloroauric acid, a sodium salt of tetrachloroauric acid, a potassium salt of tetrachloroauric acid, tetrabromoauric acid, a lithium salt of tetrabromoauric acid, a sodium salt of

tetrabromoauric acid, a potassium salt of tetrabromoauric acid, tetracyanoaurio acid, a sodium salt of tetracyanoauric acid and a potassium salt of tetracyanoauric acid. In various

embodiments, the gold ions may be provided by a gold source such as chloroauric acid, gold trichloride, gold potassium chloride, and combinations thereof. In various embodiments, chloroauric acid is used as the source of gold ions.

[051] The aqueous solution further comprises a reducing agent. The term "reducing agent" as used herein, refers to an agent that donates electrons in an oxidation-reduction reaction.

Examples of a reducing agent include, but are not limited to, a hydrazine compound, sodium . citrate, hydroquinone, ethylene glycol, oxalic acid, sodium borohydride, hydrogen, formaldehyde, ascorbic acid, and hydroxylamine. Examples of a hydrazine compound that may be used include hydrazine, hydrazine hydrochloride, hydrazine sulfate, hydrazine hydrate, hydrazine monohydrate, phenyl hydrazine, benzyl hydrazine, and ethyl hydrazine. [052] In various embodiments, the reducing agent is selected from the group consisting of hydroquinone, sodium citrate, hydrazine, ethylene glycol, oxalic acid, sodium borohydride, formaldehyde, ascorbic acid, and combinations thereof. In one embodiment, the reducing agent comprises ascorbic acid.

[053] In order that the invention may be readily understood and put into practical effect, particular embodiments will now be described by way of the following non-limiting examples.

EXAMPLES

[054] Materials. All solutions were prepared using deionized water (resistivity > 18 ΜΩ-cm^"1). 4-mercaptobenzoic acid (MBA, 90%, Sigma Aldrich), hydrogen tetrachloroaurate (III) (HAuCI₄, 99.9%, Au 49% on metals basis, Alfa Aesar), 3-aminopropyltrimethoxysilane (APTMS, Sigma Aldrich), sodium citrate tribasic dihydrate (99.0%, Sigma Aldrich), 4-nitrophenol (Sigma Aldrich), sodium borohydride (NaBH₄, Sigma Aldrich), L-ascorbic acid (Sigma Aldrich), glass fiber (Sigma Aldrich) and ethanol (analytical grade) were used as received.

[055] Characterization. Transmission electron microscopy (TEM) images were collected on a JEM-1400 (JEOL) operated at 100 kV. Field emission scanning electron microscopy (SEM) images were collected on a JEOL JSM-7600F. Ultraviolet-visible (UV-vis) spectra were collected on a Cary 100 spectrophotometer. The loading amount of the catalyst on supports was calculated based on the data collected on an Agilent 7700x inductively coupled plasma mass spectrometer (ICP-MS) equipped with a 3^rd generation He reaction/collision cell (ORS3) to minimize interferences. ¹ H nuclear magnetic resonance (NMR) spectrum was recorded at a Bruker 300 MHz spectrometer using tetramethylsilane (TMS) as an internalreference (6 = 0 ppm).

[056] Analytical reverse-phase high-performance liquid chromatography (HPLC) analysis was performed on a Shimadzu HPLC system using an Alltima C-18 (250 x 10 mm) column at a flow rate of 3.0 ml/min. An eluting system consisting of A (0.1 % 2,2,2-trifluoroacetic acid aqueous solution) and B (0.1 % 2,2,2-trifluoroacetic acid acetonitrile solution) was used under a linear gradient, monitored by UV absorbance at 254 nm. The linear gradient stretched over 20 minutes from t = 0 minutes at 20% solution B to t = 20 minutes at 80 % solution B.

[057] Preparation of TEM Samples. TEM grids were treated with oxygen plasma in a Harrick plasma cleaner/sterilizer for 45 s to improve the surface hydrophilicity. The hydrophilic face of the TEM grid was then placed in contact with the sample solution. A filter paper was used to wick off the excess solution on the TEM grid, which was then dried in air for 30 min.

[058] Preparation of citrate-stabilized AuNPs. A 50 mL flask was charged with 0.2 mL sodium citrate (1 wt%) and 14 mL aqueous solution of HAuCI₄ (0.3 mM). Then 0.6 mL of ice- cold NaBH₄ solution (0.1 M) was added with vigorous stirring. The solution turned from orange- yellow to brownish-red, indicating the formation of AuNPs. The size of the resulting AuNPs are about 3-5 nm, as shown in Figure 6.

[059] Preparation of AuNWs on glass fibers. To prepare vertical AuNWs on glass fibers, glass fibers (600 mg) were cleaned with piranha solution (H₂0₂: H₂S0₄ = 1 :3) to improve its surface hydrophilicity. The glass fibers were then functionalized with amino group by reacting with APTMS solution (5 mM) for 0.5 h for the consequent absorption of Au seeds.

Subsequently, the fibers were soaked in excess citrate-stabilized AuNPs (3-5 nm) solution for 0.5 h to ensure the adsorption of Au seeds and rinsed with water twice to remove the excess Au seeds. The seed-adsorbed fibers were then immersed in a water/ethanol (v/v = 2:1 ) solution (120 mL) containing the ligand MBA (0.6 mM), HAuCI₄ (1.1 mM), and L-ascorbic acid (2.4 mM) for 15 min. Finally, the resulting fibers were rinsed with ethanol and dried in air. The loading of Au on glass fiber was calculated to be 2.2 wt% on the basis of ICP measurements. The SEM image of the product was shown in Figures 1 b,c and 5. [060] For preparing catalytic fibers with lower loading ratio of Au (1.6 wt%), all reaction conditions were unchanged except that 1 g glass fibers were used as support (Figure 11).

[061] Preparation of AuNPs on silica spheres. 1 g of silica spheres (d = 60 μιτι) were incubated with APTMS solution (5 mM) for 0.5 h to be functionalized with amino group.

Subsequently, the silica spheres were soaked in excess citrate-stabilized AuNPs (d = 3-5 nm) solution for 0.5 h to ensure the adsorption of Au seeds and rinsed with water twice to remove the excess Au seeds and residue chemicals.

[062] Catalytic reactions using catalytic fibers monitored by successive UV-vis spectra.

For testing the catalytic performance of the catalytic fibers, a few pieces of the catalytic fibers (2.2 wt% Au) were weighed (1.0 mg) and then added to the aqueous solution of 4-nitrophenol (2.5 mM) and NaBH₄ (250 mM). The reaction process was recorded by UV-vis spectra at 2 min interval, as shown in Figure 2. The reaction solution was shaken before each run to ensure full contact of catalysts with the reactants.

[063] Reduction process of 4-nitrophenol (2.5 mM) and NaBH₄ (830 mM) had also been monitored at 1 min interval. As shown in Figure 7, it took only 5 min to fully catalyze the reaction.

[064] Catalytic reactions using citrate-stabilized AuNPs monitored by successive UV- vis spectra. For testing the catalytic performance of the clean AuNPs (d = 3-5 nm), 350 μL· of as-prepared AuNPs solutions were added to the aqueous solution of 4-nitrbphenol (2.5 mM) and NaBH₄ (250 mM). The reaction process was recorded by successive UV-vis spectra at 0.5 min interval (Figure 8).

[065] Catalytic performance of the catalytic fibers in a column. 600 mg of the catalytic fibers were packed in a column (ci = 1 cm) to reach about 3 cm in height (Figure 1e). Then, aqueous mixture of 4-nitrophenol (20 mM) and NaBH₄ (2 M) was added into the column. As shown in Figure 1e,f, the reaction mixture remained yellow until it was allowed to flow through the catalyst bed. The exiting solution was completely colorless, indicating the full reduction of 4- nitrophenol. It took 40 s for fully catalyze 6 ml_ of reaction mixtures without external pressure (flow rate 9 mL/min). HPLC analysis shows that the exiting solution is in the absence of the reactant 4-nitrophenol (Figure 9). For ¹H NMR characterization, the exiting solution was extracted with ethyl acetate and the product was collected by removing the solvent via rotary evaporator (Figure 10). ¹H NMR (300 MHz, DMSO-d₆, TMS): δ 4.35 (s, 2H, NH₂), 6.39-6.47 (m, 4H, Ar H), 8.31 (s, 1 H, OH).

[066] Vacuum suction can be used to improve the flow rate in present system. In this case, 1.2 g of catalytic fibres were loosely packed into a column of 10 cm high for the reduction of 4- nitrophenol. Only 80 s was needed to convert 43 ml_ of 20 mM 4-nitrophenol to 4-aminophenol {i.e. 32 mL/min).

[067] For studying the durability of the catalytic column, a column with 195 mg of catalytic fibres and lower catalyst loading (1.6 wt% Au) was used (the setup is shown in Figure 14). 4- nitrophenol (10 mM) was reduced at a flow rate of 3.6 mL/min. Over 140 h, a total turnover number (TON) of 16,229 was achieved (Figure 4). This in turn confirms that present catalysts offer a better catalytic lifetime. After one run of the reaction is completed, the reaction solutions can be continuously added into the column for the next run, bypassing the recycle step for the catalysts. This advantage avoids the inevitable consumption and wastage of catalysts during the recycle step and thus improves the lifetime of the catalyst.

[068] Control experiment using AuNP-adsorbed glass fibers. The glass fibers after absorbing the seeds and before the growth of AuNWs are used as control sample. 600 mg of the seed-adsorbed glass fibers were packed into a column with the same height (h = 3 cm). Then, aqueous mixture of 4-nitrophenol (1 mM) and NaBH₄ (0.1 M) were then added into the column. Under the same flow rate (9 mL/min), the exiting solution had almost the same yellow color as the starting solution (Figure 11).

[069] Comparison of the flow rate in glass fiber and silica spheres systems. To compare flow rate issues, several control experiments have been conducted. Catalytic fibers were packed in a column with a height of 3 cm. In contrast, silica sphere with AuNPs were packed in two columns with a height of 0.3 and 4.5 cm (optimal condition), respectively (Figure 12). Cotton wools were packed at the bottom of all the catalyst beds in the experiments to ensure comparable volume. The flow rates were recorded for reduction of 20 mM of 4-nitrophenol: 9 mL/min for catalytic fibers, 1.2 mL/min for silica spheres column in which reaction was not completed (h = 0.3 cm), and 0.04 mL/min for silica spheres column (h = 4.5 cm), as shown in Table 2.

Table 2. Comparison of the flow rate using catalytic fibers or silica spheres decorated with AuNPs.^a>

Catalytic Fibers Silica Spheres with AuNPs

Bed Height

3 0.3 4.5

(cm)

Flow Rate (mUmin) 9 1.2 0.04 a⁾The catalytic fibers or 60 nm silica spheres decorated with AuNPs (d = 3-5 nm) were packed into columns (d = 1 cm) reachmgdifferent bed heights.

[070] Calculations:

[071] Comparison of Au surface area in the AuNPs and AuNWs adsorbed on support surface. For comparison, AuNPs and AuNWs are assumed to have the same density on support surface and AuNWs are grown to be 1 //m in length (Figure 3c,d). Moreover, the calculations do not take into account the interface area of AuNPs and AuNWs in contact with the support surface, which are difficult to determine. Under these assumptions: The surface area of one AuNP = 4πτ² = 78.5 nm². The surface area of one AuNW = 2nr\ = 15700 nm². Thus, AuNWs would have 200 times the surface area of Au spheres.

[072] Height of column packed with silica spheres of ideal catalytic capability. When 1 g of 60 μπ\ silica spheres packed in a column (d = 1 cm), the height of the column is 2.2 cm . Thus, the total volume of the silica-sphere bed occupied = 1 .7 cm³.

[073] Volume of silica spheres (1 g) = 1 g / 2.65 g/cm³ = 0.38 cm³

[074] According to the above experimental data, the density of packed silica spheres in the column = 0.38 cm³ / 1 .7 cm³ = 22 %

[075] The volume of silica spheres (d = 60 μιτι) in column is based on the assumption: namely the highest known TOF (0.43/s) and the highest surface density (5 nm AuNPs on silica with 30% coverage). In order to process 20 mM of 4-nitrophenol at 9 mL/min, the weight of Au in this column = 1 .4 mg.

[076] Number of AuNPs on one silica sphere = [0.3* An χ (3 * 10⁴)² nm²]//r2.5² nm² =1 .7 x 10⁸

[077] Weight of the AuNPs on one silica sphere = (4/3) 7 x 2.5³ nm³ x 1 .7* 10⁸ x 19.28 * 10^" ¹⁸ mg/ nm³ = 2.1 χ 10^"7 mg.

[078] Number of silica spheres adsorbed with 1 .4 mg of Au = 1 .4 mg /2.1 χ 1 Q^"7 mg = 6.6 χ 10⁶

[079] Volume of silica spheres = (4/3) rr (3 10^"3)³ cm³ χ 6.6 χ 10⁶ = 0.76 cm³ [080] Total volume of silica spheres in column = 0.76 cm³ / 22 % = 3.5 cm³ [081] Height of the column = 3.5 cm³/ {π 0.5² cm²) = 4.5 cm [082] Comparison of surface area in glass fibers and monolith structures. 600 mg of glass fibers with 10 //m in width were packed in a column (d = 1 cm) to reach 3 cm of height. Thus, surface area of these glass fibers = π χ 0.001 cm χ [(0.6 g / 2.65 g/cm³) / (77 * 2.5 χ 10^~7 cm²)] = 942 cm²

[083] Here, the monolith with square channels (diameter of square cells = 0.05 cm and wall thickness = 0.01 cm) was assumed to packed in a column with a same height.

[084] The surface area of each channel = 0.05 cm χ 4 χ 3 cm = 0.6 cm²

[085] The number of channels can be estimated = (ττχ 0.5² cm²) / (0.05+0.005)² cm² = 262

[086] Thus, internal surface area of the monolith = 0.6 cm² x 262 =157 cm²

[087] Therefore, surface area of glass fibers is about 6 times that of monolith structures.

[088] In this example, vertical Au nanowires (AuNWs, d = 5 nm) were grown on glass fibers to greatly improve the catalyst loading per unit support area. With this new loading mode, it can be afforded to have very large pores in the support structure, thus greatly improving the flow rate. Using a simple column (Figure 1 ), the catalytic performance is greatly improved; 20 mM of 4- nitrophenol was reduced at a flow rate of 32 mL/min.

[089] Thin glass fibers (d = 10 μιτι, /> 10 cm) were used as the catalyst support because they are robust and inexpensive, and can easily form porous network with reasonable surface area. The method of growing ultrathin AuNWs on oxide substrates has been previously reported. In a typical synthesis, glass fibers (600 mg) were cleaned using piranha solution, washed, and then functionalized with 3-aminopropyltrimethoxysilane. With amino groups on the glass fiber surface, small Au seeds (d = 3-5 nm) were easily adsorbed thereon. Finally, the fibers were immersed in a solution of 4-mercaptobenzoic acid (MBA, 0.6 mM), HAuCI₄ (1 .1 mM), and L- ascorbic acid (2.4 mM) to grow AuNWs. Upon immersion, the glass fibers quickly turned grey (< 1 min) and the color further darkened over time. After 15 min, the resulting black fibers (catalytic fibers) were rinsed with ethanol to remove the ligands on the AuNWs and then dried (Figure 1a). The equivalent amount of Au used in the reaction was about 17 mg. On the basis of inductively coupled plasma (ICP) measurement, the amount of Au loaded on the glass fibers was 2.2wt% (or a total of 13 mg).

[090] Scanning electron microscopy (SEM) of the glass fibers confirmed that a dense layer of vertical AuNWs have grown on their surface. The AuNWs were 5 nm in width and about 1 μητι in length (Figure 1c). The glass fiber shown in Figure 1 b was completely covered with the AuNW layer, where the difference in color was due to the viewing angle in SEM. The layer was so dense that the inventors had to find the cracks in the layer in order to characterize the constituent AuNWs (Figure 1c). The layer was also uniform at the multiple locations the inventors have characterized, consistent with the uniform black color of the glass fibers. In this method, the presence of strong MBA ligand inhibited the homogenous nucleation of Au to give colloidal NPs and thus, most of the reduced Au participated in the growth of AuNWs.

[091] To evaluate the catalytic performance of present system, the inventors used a well- known model reaction, where 4-nitrophenol was reduced to 4-aminophenol using NaBH₄ as the reductant. This reaction cannot proceed in the absence of Au catalyst. It has been widely used because the conversion of yellow 4-nitrophenol to colorless product can be easily monitored. Briefly, a few pieces of the catalytic fibers were weighed (1.0 mg with 2.2 wt% Au) and then added to the aqueous solution of 4-nitrophenol (2.5 mM) and NaBH₄ (250 mM) (Figure 2a,b). As shown by the UV-vis spectra (Figure 2c), the absorption peak at 400 nm gradually decreased with time and two new peaks appeared at 226 and 305 nm, indicating the formation of 4-aminophenol. On the basis of the peak at 400 nm, the reaction was completed after 14 min, where the average turnover frequency (TOF) was about 0.09/s (k = 6.5*10^"3 s^"1). Higher TOF of 0.26/s was achieved when a higher concentration of NaBH₄ (830 mM) was used. For a fair comparison, the TOFs below were calculated based on the total amount of Au in the system (the same is done for literature values), not the amount of surface atoms.

[092] This value is compared to the best examples in the literature for the same model catalysis. When AuNPs (d = 15 nm) were embedded in the porous silica shells of magnetic NPs, the TOF was estimated to be 0.02/s on the basis of total Au. When d = 3-5 nm AuNPs were anchored on silica microspheres, it took 60 min to fully reduce 1.5 mM of 4-nitrophenol. The TOF is estimated to be 0.03/s. For colloidal catalyst, the best performance involved AuNPs (d = 2-3 nm) stabilized by an ionic polymer. It took about 16 min to fully reduce 0.6 mM of 4- nitrophenol (estimated TOF = 0.07/s). Large citrate-stabilized AuNPs (d = 20 nm) gave about 0.02/s TOF. While citrate is a weak ligand favoring the catalytic reaction, the large NPs have small S/V. Surprisingly, the inventors were not able to find literature rates on using small and "clean" colloidal AuNPs (not hampered by ligand, polymer, or shells) as catalyst.

[093] Table 1 briefly summarizes the findings/comparison.

Table 1. Comparison of the turnover frequency in the reduction of 4-nitrophenol using catalytic Fibers or other catalysts in the literatures.

[094] To understand the unusual catalytic performance in present system (TOF = 0.09/s), the inventors tried to prepare a best control catalyst. HAuCI₄ was reduced in the presence of sodium citrate using NaBH₄. The resulting AuNPs were 3-5 nm in diameter and capped only with weak ligands. When these colloidal NPs were directly used as catalyst, 2.5 mM of 4- nitrophenol was fully reduced in 3 m in (TOF = 0.43/s). Being small and with all surfaces available for the catalysis, this simple control sample has better performance than all of the above systems.

[095] In comparison, for the vertical AuNWs (d = 5 nm) grown on glass fibers, the catalytic performance was reasonably good but still not comparable to the small and clean AuNPs. This is likely because the AuNWs have a lower S/V than (about 67% of) the colloidal AuNPs and the molecular diffusion may not be efficient in the dense forest of AuNWs. Moreover, even after washing there is probably some residue MBA on the AuNW surface.

[096] Thus, batch-wise reactions may not be the best platform for exploiting the advantages of present new method. The exceptional catalyst loading per unit support area is of great importance for continuous flow catalysis. For industry applications, the top concerns are to achieve an "automated" mode of continuous flow, and to improve the processing rate and recyclability. One can always use more catalysts for faster reactions, so long as the catalysts are recyclable.

[097] The catalytic fibers can be loosely packed into a simple column as a proof-of-concept demonstration. Specifically, 600 mg of the fibers were packed in a column (d = 1 cm) to reach about 3 cm in height (Figure 1e). Then, aqueous mixture of 4-nitrophenol (20 mM) and NaBH₄ (2 M) was added into the column. As shown in Figure 1e,f, the reaction mixture remained yellow until it was allowed to flow through the catalyst bed. The exiting solution was completely colorless. Aliquots taken from the solution showed no absorption at 400 nm, indicating the completed reaction. Without using external pressure, it took only 40 s to process 6 mL of the concentrated reaction mixture. The estimated TOF was about 0.05/s, a respectable number considering their short contact time with the flowing reactants.

[098] In comparison to the batch-mode reaction, the.ability to carry out continuous flow reaction with recyclable catalyst avoids the recycling step, which is costly in both time and money. Considering the time of recycling, the batch-mode reactions would be much slower in overall processing rate than present continuous-flow method. Given the small size of the column used (d = 1 cm), the processing rate that have been achieved is exceptional.

[099] In a fixed bed system, the flow rate depends on the crevices among the support particles. Typically, silica or polystyrene microspheres with large surface area are used as the support material. For 60 μπη spheres, the crevices among them should be about only a few μιτι in width, similar in size to the pores among gel fibers. It is known that the solution inside a gel flows very slowly. Hence, the more support materials used, the slower the flow rate. In contrast, the crevices inside a loose column of glass fibers can be much larger (Figure 3a,b).

[0100] Without external pressure, present catalytic column (d = 1 cm, h = 3 cm) can achieve a 9 mL/min flow rate, whereas a thin h = 0.3 cm column of silica spheres gave only a 1.2 mL/min rate. It is impossible to load enough catalyst in such a short column. In order to process 20 mM of 4-nitrophenol at 9 mL/min, it is needed to have a lot more catalysts and a lot more support materials. Assuming optimal conditions, namely the highest known TOF (0.43/s, vide supra) and the highest surface density (5 nm AuNPs on silica with 30% coverage), the column of 60

silica spheres needs to be at least 4.5 cm in height. Without external pressure, the flow rate through such a column was only 0.04 mL/min, clearly not matching with the catalytic capability. If TOF = 0.04/s, then a column of 45 cm high would be needed. Even with high external pressure, the flow rate through such a column would be very slow. [0101] An alternative approach is to directly adsorb d = 3-5 nm AuNPs on a loose network of glass fibers. Indeed, the glass fibers after absorbing the seeds and before the growth of AuNWs are a perfect control sample. A similar column packed with these fibers (h = 3 cm) was tested. Under a same flow rate (9 mL/min), even when a low concentration of 4-nitrophenol (1 mM instead of 20 mM) was used, the exiting solution had almost the same yellow color as the starting solution. Thus, the catalytic capability of this control sample was much less than 1/20 of the column with AuNWs.

[0102] These simple estimates illustrate the difficulty in improving the catalytic performance while matching the physical flow rate with the catalytic capability. While the NPs have large SA7, their small size necessitates a large quantity in order to present sufficient catalytic surface. But more catalyst means more support materials and thus, slower flow rate. For the two fixed-bed systems in the literature, the slow flow rate would make it difficult to use more support for more catalyst.

[0103] Presently proposed approach is to grow vertical AuNWs on the support. Assuming a same surface density on the support, AuNWs of 5 nm in diameter and 1 μπι in length would have 200 times the surface area of oi = 5 nm Au spheres (Figure 3d). This allows large pores in the support structure and easily balances the flow rate with the catalytic capability.

[0104] In industry, monolith with large and straight pores is often used as catalyst support. The pore size (thus the flow rate) is easily tunable, but the structure has only a small surface for absorbing catalyst (in comparison to using particles or fibers as support). For example, a similar sized column (d = 1 cm, h = 3 cm) with 0.5 mm pores would have only 1/6 of the surface area as the glass fibers used in present system. Thus, to load the same amount of catalyst, a longer column is needed. This would be problematic if 3-5 nm AuNPs are used as the catalyst, because the column needs to be 1200 times longer than present system. If AuNWs can be directly grown inside the monolith pores, the size of the monolith can be reduced. This is indeed possible because of present facile growth using a solution method.

[0105] External pressure can be applied to any fixed bed system to improve the flow rate, but it cannot resolve the issues discussed above. It can also be applied to present system, although not necessary. With minor modifications, the processing rate can be greatly improved: 1.2 g of the catalytic fibers (total 26 mg of Au) were loosely packed into a column of 10 cm high and vacuum suction was applied. It took only 80 s to process 43 ml_ of 20 mM 4-nitrophenol {i.e. 32 mL/min).

[0106] The performance of the catalytic column was studied over an extended period. Due to the high activity of the column, the inventors had to load less catalyst to cut down the required time and chemicals. A column with 195 mg of catalytic fibers and lower catalyst loading (total 3.1 mg of Au) was used. 10 mM of 4-nitrophenol was completely reduced at a flow rate of 3.6 mL/min. Over 140 h, a total turnover number (TON) of 16,229 was achieved with only slight loss of activity (Figure 4).

[0107] Given the small size of the AuNWs, they are unstable at high temperature. The NWs are relatively stable after being heated in an aqueous solution for 2 h. However, heating at 100 °C for 2 h caused the NWs to partially fuse, which would reduce the active surface for catalysis. Hence, the NWs are not ideal for high temperature reactions, but are suitable for low- temperature solution catalysis, which is common in pharmaceutical industry.

[0108] NPs are problematic for industrial catalysis, because to load them with a sufficient quantity requires a huge support surface. A careful analysis of the problem showed that there is no conventional solution. By directly growing vertical AuNWs on an oxide substrate, the available catalytic surface jncreased by about 200 times. This allowed highly porous support structure to be used for improved flow rate. This strategy of using 3-dimensional catalyst structure is conceivably applicable to other systems. It would help bridge the laboratory tested nano-catalysts to industrial applications.

[0109] In summary, the new capability of growing dense Au nanowire forest is advantageous for fixed bed catalysis. The solution method of growing Au nanowires on glass fibers is simple, reliable, and scalable. In comparison to the best examples in the literature, present system can catalyze the same model reaction with reasonable turnover frequency but greatly improved processing rate. The high loading surface area of Au on the glass fiber confers high catalytic activity and the large crevice area allows the reaction solution to pass through with low resistance. Without the recycle step, such catalysts show high stability even after reaction in the long run. They are expected to be of highly beneficial use in industrial applications, where it can lead a revolutionary path in the field of nano-catalysis.

[0110J By "comprising" it is meant including, but not limited to, whatever follows the word "comprising". Thus, use of the term "comprising" indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0111] By "consisting of" is meant including, and limited to, whatever follows the phrase "consisting of". Thus, the phrase "consisting of" indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0112] The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms "comprising", "including", "containing", etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.

[0113] By "about" in relation to a given numerical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.

[0114] The invention has been described broadly and generically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.

[0115] Other embodiments are within the following claims and non- limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.

Claims

A method of performing a chemical reaction, comprising reacting a mixture of reactants in the presence of a catalyst, wherein the catalyst comprises: a fiber substrate; and a plurality of nanowires attached to the fiber substrate, wherein the plurality of nanowires are comprised of at least one noble metal selected from the group consisting of gold (Au), silver (Ag), palladium (Pd), platinum (Pt), iridium (Ir), osmium (Os), rhodium (Rh), and ruthenium (Ru).

The method of claim 1 , wherein the plurality of nanowires are coated with a different noble metal selected from the group consisting of Au, Ag, Pd, Pt, Ir, Os, Rh, and Ru.

The method of claim 1 or 2, wherein the fiber substrate is comprised of a material selected from the group consisting of glass, silica, alumina, titania, strontium titanium oxide, lanthanum aluminum oxide, calcium carbonate, silicon, paper, and polymer.

The method of any one of claims 1 to 3, wherein the plurality of nanowires are comprised of Au.

The method of any one of claims 1 to 4, wherein the fiber substrate is comprised of glass.

The method of any one of claims 1 to 5, wherein a plurality of the catalysts are placed in a fixed bed column.

The method of claim 6, wherein the mixture of reactants is passed through the fixed bed column partially or completely packed with the plurality of catalysts.

A catalyst comprising: a fiber substrate; and a plurality of nanowires attached to the fiber substrate, wherein the plurality of nanowires are comprised of at least one noble metal selected from the group consisting of gold (Au), silver (Ag), palladium (Pd), platinum (Pt), iridium' (Ir), osmium (Os), rhodium (Rh), and ruthenium (Ru).

9. The catalyst of claim 8, wherein the plurality of nanowires are coated with a different noble metal selected from the group consisting of Au, Ag, Pd, Pt, Ir, Os, Rh, and Ru.

10. The catalyst of claim 8 or 9, wherein the fiber substrate is comprised of a material

selected from the group consisting of glass, silica, alumina, titania, strontium titanium oxide, lanthanum aluminum oxide, calcium carbonate, silicon, paper, and polymer.

11. The catalyst of any one of claims 8 to 10, wherein the plurality of nanowires are

comprised of Au.

12. The catalyst of any one of claims 8 to 11 , wherein the fiber substrate is comprised of glass.

13. A method for forming a catalyst, wherein the catalyst comprises a glass fiber substrate and a plurality of gold nanowires attached to the glass fiber substrate, the method comprising: contacting the glass fiber substrate with a functionalizing agent for attaching a plurality of gold nanoparticles thereto; contacting the glass fiber substrate with a plurality of gold nanoparticles for attachment; and contacting the glass fiber substrate having the plurality of gold nanoparticles attached thereto with an aqueous solution comprising a ligand, gold ions, and a reducing agent, wherein the ligand is an organic compound having a thiol group.

14. The method of claim 13, wherein the ligand is selected from the group consisting of 4- mercapto-phenylacetic acid (4-MPAA), 4-mercaptobenzoic acid (4-MBA), 3- mercaptobenzoic acid (3-MBA), 4-mercaptophenol (4-MPN), and a mixture thereof.

15. The method of claim 13 or 14, wherein the gold ions are provided by a gold source

selected from the group consisting of chloroauric acid, gold trichloride, gold potassium chloride, and a combination thereof.

16. The method of any one of claims 13 to 15, wherein the reducing agent is selected from the group consisting of hydroquinone, sodium citrate, hydrazine, ethylene glycol, oxalic acid, sodium borohydride, formaldehyde, ascorbic acid, and a combination thereof.

17. The method of any one of claims 13 to 16, wherein the functionalizing agent comprises or consists essentially of 3-aminopropyltrimethoxysilane (APTMS).

18. The method of any one of claims 13 to 17, further comprising depositing a layer of noble metal nanoparticles onto the gold nanowires, wherein the noble metal is selected from the group consisting of Ag, Pd, Pt, Ir, Os, Rh, and Ru.