Patent Application
20180050906
Aspects of the invention relate generally to improved processes and catalysts to split water (i.e., to separate water into hydrogen and oxygen). The invention is particularly useful for the in situ generation of hydrogen via microwave irradiation of a ruthenium chitosan composite catalyst and the convenient reduction of nitro compounds in an aqueous medium.
Increasing energy demands and the impending global warming crisis has forced a paradigm shift in thinking towards the development of carbon-neutral, sustainable and inexpensive energy-consuming strategies. This has inspired scientists to design and develop methods for splitting water, primarily as a means for producing hydrogen (Scheme 1). The total water splitting process consists of two steps: (Step 1) proton reduction and (Step 2) water oxidation. Step 1 requires less energy compared to the key step in water splitting, i.e., water oxidation. Due to the complexity of the Step 2 reaction, the challenges in the water oxidation process involve multiple proton-coupled electron transfer processes and O—O bond formation.
2H2O+hν→2H2+O2
Step 1 2H++4e−→H2
Step 2 2H2O→O2+4H++4e−
Scheme 1. Splitting of Water
There has been great effort over the years to develop a method to split water under a variety of conditions. This includes developing methods such as 1) electrolysis, 2) photoelectrochemical water splitting, 3) photocatalytic water splitting, 4) photobiological water splitting, and 5) thermal decomposition of water.
Hydrogen production from water using electrical means is not a viable option, as the energy consumed in this process is more than what is produced in the form of hydrogen. Photoelectrochemical, photocatalytic and photobiological methods are no doubt of prime importance in water splitting research. However, to date there has been very limited success due to the high complexity and cost of such methods as well as their low efficiency in producing hydrogen.
Comprehensive efforts have been made to understand the reaction mechanisms of water splitting using a catalyst. The catalytic concepts center around the use of transition metals or biological systems coupled with sunlight. However, most of the reported literature is confined to the use of conjugated complex ligands or metal oxides, and they are often designed or selected in view of their efficiency in sunlight absorption, thus mimicking nature. There has been little to no progress in the use of low-energy microwaves and non-conjugated ligands in the generation of hydrogen via water splitting.
In view of the above, there is a need to develop catalysts and reactions capable of splitting water and generating hydrogen. Accordingly, an object of the invention is to provide such a process and catalyst. Other objects will also be apparent from the detailed description of the invention.
Broadly stated, the objects of the invention are realized, according to one aspect of the invention, by synthesizing a chitosan-ruthenium catalyst and using this catalyst in a reaction to split water and produce hydrogen. There is no history of using low-energy microwaves and non-conjugated ligands in the generation of hydrogen via water splitting. In continuation of the inventors ongoing efforts to discover the versatility of renewable biopolymers as catalyst supports in organic synthesis, they serendipitously discovered in situ generation of hydrogen via water splitting using a chitosan ruthenium composite under microwave (MW) irradiation conditions. The processes of the invention also relate to the aqueous reduction of nitro compounds.
In one embodiment, the invention embraces a process for generating hydrogen and oxygen from water. The process includes the following steps: preparing a ruthenium on chitosan catalyst, bringing the catalyst into contact with an aqueous mixture under basic conditions, and applying microwave energy to the aqueous mixture to produce hydrogen and oxygen.
In one embodiment, the catalyst is prepared by a process including the following steps: suspending chitosan in water; adding RuCl3.3H2O; adjusting the pH of the mixture to about 8.5 to about 9.5 while continuously stirring the mixture; separating the catalyst from the liquid in the mixture using a centrifuge; and drying the mixture under a vacuum. The pH of the mixture may be adjusted using ammonia, and the catalyst may be dried under a vacuum at about 30° C. to about 60° C.
In the process of separating water into hydrogen and oxygen, the aqueous mixture may be maintained at about 125° C. during the reaction. In one embodiment, the aqueous mixture is maintained at this temperature for about 30 minutes during the reaction. In another embodiment, K2CO3 is used to maintain basic conditions in the aqueous mixture.
In one embodiment of the invention, the concentration of base in the water is about 0.4 mmol/mL. In another embodiment, the microwave energy is applied at a power of about 100 watts for a time of about 60 to about 70 minutes.
In one embodiment of the invention, a nitrobenzene compound is added to the aqueous mixture prior to application of microwave energy. The nitrobenzene compound may be 4-bromo-nitrobenzene. In another embodiment of the invention, the reaction also produces a reduced organic compound. The reduced organic compound may comprise a 4-bromo-aniline.
In one embodiment of the invention, the concentration of hydrogen produced is at least 27.79%.
The invention is more fully described by reference to the following detailed description and the accompanying drawings wherein:
Referring more specifically to the drawings,
While demonstrating the application of ruthenium chitosan composite ChRu in C—O aryl bond formation in aqueous media (Scheme 2), the inventors observed the formation of three products: p-amino phenol A (
The study was undertaken with 4-bromo-nitro-benzene as a model substrate in order to find out the best possible conditions for the reduction reaction. The reduction of nitro group occurs under MW heating conditions exclusively in basic media (See Table 1,
[a]Reaction condition: 4-Nitro-bromobenzene (1 mmol), RuCh-catalyst (25 mg),
After optimizing the reaction conditions that provided an ideal environment for reduction, the inventors searched for the source of the nitro reduction. Their studies led them to hypothesize that hydrogen may be generated in-situ in the reaction mixture, which could be responsible for the reduction of the nitro group. The hydrogen evolution through splitting of water is possible through use of a specially designed, sophisticated and highly conjugated ruthenium metal complex under photochemical conditions. In order to detect the generation of hydrogen in the reaction mixture, the inventors designed an experimental set up to perform the MW reaction in a sealed tube (see
Accordingly, the inventors carried out experiments and studied the head space atmosphere. The head space analysis after the reaction was encouraging, as the inventors observed that substantial percentages of hydrogen were detected (˜27%) (Table 2). Under neutral conditions, no splitting of water was discerned even after prolonged exposure to MW, UV-visible light and conventional heating (Table 2, entries 1-4). However, when the catalyst, suspended in aqueous K2CO3, was exposed to MW heating for 30 min, 27.79% of hydrogen (Table 2, entry 8) was detected in head space analysis (
[a]Reaction conditions: Base (2 mmol), water (5 mL);
b1 mmol of K2CO3, water (5 mL), MW, 125° C.;
cReaction was performed at 100° C., using 2 mmol of K2CO3 in 5 mL of water.
The microwave irradiation of ChFe, ChPd, CelluFe, CelluPd, CelluRu, (see
aReaction condition: K2CO3 (2 mmol), Water (5 mL), MW, Metal catalyst (25 mg).
bPercentage after Head space analysis
[a]Reaction condition: 1 mmol of 4-nitro bromobenzene, 2 mmol of K2CO3,
The selective reduction of nitro compounds to amines is one of the very important transformations in organic synthesis, and consequently there are number of procedures for the reduction of nitro compounds (Scheme 3). Due to concern for the environment, the search for simple, chemo-selective, affordable and environmentally benign methods that avoid the use of hazardous and expensive reducing agents in stoichiometric amounts has gained prime importance. The use of a binary mixture of CO and H2O as a hydrogen source along with specially designed Rh, Au or Ru metal complex catalysts has been considered as one of the most useful methods for nitro reduction. The in-situ generation of hydrogen from water and its utilization in reduction of nitro compounds using simple biodegradable renewable chitosan has never been reported (see bottom, Scheme 3).
[a]Reaction condition: 1 mmol of nitro compound, 2 mmol of K2CO3, ChRu (25 mg), 5 mL water, MW,
The experimental procedure entailed placing aromatic nitro compound (1 mmol) into a microwave reaction tube, to which K2CO3 (2 mmol), ChRu (25 mg) and 5 mL of water were added and exposed to microwave irradiation for 60-70 min at 125° C. The reaction was monitored using GCMS. In almost all the cases, the nitro group is selectively reduced to the corresponding amine (Table 3).
The recovery of the catalyst is the most important goal in a sustainable organic synthesis. In industrial application of heterogeneous system, the lifetime of the catalyst and its level of reusability are significantly important factors. To demonstrate recyclability of ruthenium chitosan composite, a set of experiments was conducted for the hydrogenation of 4-nitro bromo benzene using the recycled ChRu catalyst. After the completion of the reaction, the catalyst was recovered, washed with acetone, and dried under vacuum. A fresh reaction was then set-up using fresh reactants and recycled catalyst and subjected to MW irradiation. The ChRu catalyst could be used at least five times without any change in its activity. Metal leaching was studied using ICP-AES analysis of the catalyst before and after the completion of the reaction. The Ru concentration was found to be 6.41% before the reaction and 6.35% after the reaction. The very small amount of Ru metal was detected in the aqueous mixture. The negligible amount of Ru leaching may be due to the well-defined structure of chitosan with its abundance of amine and hydroxyl functional groups. While it is only theory, chitosan's advantageous structural characteristics may result in Ru chelating via non-covalent interaction.
The Ruthenium on Chitosan catalyst was made as follows. Chitosan (3 g, medium molecular weight, Aldrich, CSA #9012-76-4) was suspended in 100 mL of water. To this suspension, 500 mg of RuCl3.3H2O was added. The pH was adjusted to 9 using 25% ammonia, and the suspension was continuously stirred overnight. The catalyst was separated using a centrifuge operated at 5000 rpm for 5 minutes. The catalyst was then dried under vacuum at 50° C. for about 2 hours to about 3 hours.
Nitro compound (e.g., the aromatic nitro compound, 4-bromonitrobenzene, 1.0 mmol), ChRu (25 mg, 0.015, mol %) and K2CO3 (2 mmol) were placed in a crimp-sealed thick-walled glass tube equipped with a pressure sensor and a magnetic stirrer. Water (5 mL) was added to the reaction mixture. The reaction tube was placed inside the cavity of a CEM Discover focused microwave synthesis system and operated at 125° C. (temperature monitored by a built-in infrared sensor) and 100 Watts for 60-70 minutes. After completion of the reaction, the catalyst was removed from the reaction mixture using a centrifuge. Reduction of the nitro group has been monitored using GCMS. The clear liquid was cooled slowly, and, in most cases, an analytically pure sample of corresponding amine was obtained. The samples can be isolated from the water medium by simple decantation.
In brief, the inventors have demonstrated the unprecedented use of pure water as a hydrogen source in the catalytic hydrogenation of nitro compounds wherein ruthenium chitosan composite can split water under microwave irradiation conditions. See
As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it will be understood that the invention is not limited by the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the metes and bounds of the claims. Accordingly, the invention is defined by the appended claims.
