Chemical Engineering and Processing: Process Intensification
Nanosized catalysts as a basis for intensifications of technologies
Introduction
The main goal of constructing and maintaining an innovative economic model requires the use and development of nanotechnologies, as they can be used to create new economically competitive goods. The general discussion on the prospective implementation of nanosystems seems to be groundless and unreasonable; however, it is necessary to indicate the industrial branches and directions where the application of nanosystems is of certain value. For chemistry and chemical technology, such a branch is certainly represented by metal complex catalysis. The general applications of this method have been evolving in an explosive manner: while in the period of 1996–2005, papers on this subject made up approximately 1.7% of the total number of investigations, currently the number of works on catalysis has increased up to 5.4% from 2.9% of all scientific publications.
The possibilities of the development and application of nanosized catalysts were proven in numerous papers [1], [2], [3], [4], [5], [6], [7]. Special interest appears to be related to their use in such a field of fine organic synthesis, as preparation of materials for pharmaceutical and food manufacture (active pharmaceutical ingredients (API), vitamins, food additives, etc.).
Meanwhile, the actual situation in pharmaceutical market in Russia is as follows:
- (i)
More than 75% of medications and bioactive additives (BAA) in drugstores is imported.
- (ii)
90% of medications and BAA is produced using imported API and raw materials.
- (iii)
In 1992–2008 the capacity for the API domestic manufacture in the Russian Federation was reduced by more than six times.
- (iv)
The highest specific expenses in the production cost of vitamins and medications are related to the cost of raw materials (up to 40%), which affects retail prices very much with respect to the ruble devaluation at the end of 2008 and beginning of 2009.
- (v)
The most abundant delivery of API into the Russian Federation is being performed by Indian and Chinese suppliers, whose quality standards are sometimes questionable.
Taking the above mentioned into consideration the discussion of nanotechnical innovations for vitamin synthesis (including food additives) and medications seems very timely and useful. Moreover, the recently obtained results in this field are very informative for both the technological and environmental aspects of chemical industry, because the implementation of new efficient techniques should diminish the overall time it takes to obtain target materials and reduce the total number of synthesis steps and/or replace them with more efficient ones. The use of safe and environmentally friendly solvents instead of toxic and dangerous media should favor the goals of environmental protection.
Nowadays, at least one catalytic step is included into the modern industrial synthesis sequence to produce such compounds as vitamins A, E, K, C, B6, β-carotene, calcium gluconate, and a number of other next-generation API and BAA that often contain pure optically active isomers. One of the goals of presented investigation is to improve the existing technologies using an integrative innovative approach which includes the replacement of several synthesis steps with a catalytic one; optimization of the efficiency of the existing catalytic steps via utilization of new, more active, selective and operationally stable nanosized catalysts instead of traditional ones; application of new chiral modifiers immobilized on a polymeric matrix; minimization of side products; and using environmentally friendly solvents.
Heterogeneous catalysts are known to provide a more convenient choice for the needs of industry. Most of inorganic matrices are presented by the classical supports for heterogeneous catalysts (SiO2, Al2O3 and TiO2) [8]; however, they usually contain pores with a rather broad and poorly reproducible size distribution [9], [10]. The most important part of the nanostructured inorganic matrix is made up of nanocarbon materials (nanotubes, nanofibers, mesoporous soot, etc.) and zeolites which possess various regular structures and pores 0.3–1.0 nm in size that are characterized by quite narrow distributions (more than 150 types of zeolites). Another option is provided by the use of specially developed nanostructured polymeric matrices [11]. Micelles of amphiphilic block co-polymers immobilized on inorganic supports, polyelectrolytes of both cationic and anionic types, ultrathin nanoporous films (membranes) [11], [12], and dendrimers [7] have been investigated as nanostructured polymers which allow controlling the formation, morphology, sizes, and resulting properties of nanopolymeric composites. The nanoparticles formed in polymer matrix directly control the nanopore surface. Such hybrid materials prevent the agglomeration of nanoparticles and do not require stabilizing ligands, which makes nanoparticles more active in catalytic processes [9], [10]. The selection of hypercrosslinked polymeric matrices is confined by porous membranes on the basis of polyacrylic acid cross-linked by a bifunctional epoxide [13], [14], [15] and polystyrene-based materials (HPS) [16], [17].
In this review the data on the influence of HPS structure and conditions of catalyst formation on the properties of noble metal nanoparticles, their sizes and morphology, as well as catalytic properties in the syntheses of intermediates for the production of vitamins and medications were gathered and summarized. The catalysts developed were described by the physicochemical methods including X-ray fluorescence analysis (XFA), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), low-temperature nitrogen physisorption (BET), EXAFS and DRIFT spectroscopy of CO adsorption.
Section snippets
Synthesis and study of noble metal nanoparticles in pores of HPS
The representative of a new type of polymeric HPS-based networks, which was synthesized at the Laboratory of Sorption Processes Stereochemistry of INEOS RAS for the first time, has shown a unique topology and, therefore, a number of unusual properties [16], [17]. In order to prepare a hypercrosslinked net, two conditions needed to be fulfilled: (1) the polymer should be prepared in the presence of a proper solvent to prevent phase separation and the formation of an open network microstructure
Platinum-containing nanosized catalysts
The platinum-containing nanosized catalysts (Table 1, #1–5) were synthesized by the sorption of H2PtCl6·6H2O from the complex solvent THF–MeOH–H2O in the HPS matrix, followed by sodium bicarbonate treatment for the precipitation of platinum(IV) oxide [32], [33], [34]. The TEM investigation (Fig. 2a) revealed no influence of platinum quantity (Table 1, #1–5) on the resulting nanoparticle size; moreover, the average particle size was found (2.1–2.3 nm) not to differ significantly from those of the
Palladium-containing nanosized catalysts
The palladium-containing catalysts based on HPS (Table 1, #10–14) were prepared in a way similar to that described above for platinum materials: by the sorption of Na2PdCl4 from the complex solvent, followed by treatment with sodium bicarbonate. In contrast to platinum nanoparticles, palladium ones quite differ and have an alternative mechanism of formation. The TEM investigation revealed that the size distribution for the palladium nanoparticles is rather complex (Table 1, #10–14). For
Ruthenium-containing nanosized catalysts
For the synthesis of ruthenium nanoparticles stabilized into the HPS matrix, several specific features of ruthenium inorganic compounds and their behavior in solutions should be taken into account [32], [34]. Unlike other noble metals, ruthenium has the unique ability to take various valence states (from 0 to 8). The versatility of the transitions from one valence state to another results in the enormous complexity and originality of the chemistry of ruthenium compounds [32], [34]. As a
Polymetallic nanosized catalysts
The samples containing bi- and trimetallic nanoparticles were synthesized by the consecutive impregnation of the respective precursors into the HPS matrix from their solutions in the complex solvent, with the following treatment with an alkaline reagent (Table 1, #20–23). For the palladium-containing samples HPS-Pt (0.1%) -Pd(0.1%) and HPS-Pd (0.1%) -Ru(0.1%), the TEM data showed the formation of rather bulky nanoparticles with average sizes of 25.3 and 17.1 nm in a similar fashion to the
The catalytic properties of noble-metal-based nanoparticles for monosaccharides oxidation (the synthesis of intermediates of vitamin C and calcium gluconate)
Nowadays, many techniques for the production of vitamins and medications involve the reactions of monosaccharides oxidation [23], [40], [41], [42].
The oxidation of l-sorbose and its derivatives to 2-keto-l-gulonic acid or its derivatives is a key step for the synthesis of ascorbic acid (Vitamin C) [40], [41], [43], which attracts the persistent attention of numerous researchers.
There are several methods for the oxidation of l-sorbose to 2-keto-l-gulonic acid: chemical, electrochemical,
Conclusions
The developed preparation methods of mono- (Pt, Ru, Pd), bi- (Pt–Pd, Pt–Ru, Pd–Ru) and trimetallic (Pt–Pd–Ru) nanoparticles in the pores of HPS allowed synthesizing the active, stable, and selective nanocatalysts for the important processes widely used in the synthesis of API. According to the physicochemical investigation using XPA, XPS, EXAFS, TEM, DRIFT spectroscopy of CO adsorption and low-temperature nitrogen physisorption (BET), the size of metal-containing nanoparticles formed was proven
Acknowledgments
We sincerely thank Ministry of Education and Science of Russian Federation for financial support. This research was performed with the support of the following programs:
Program for Basic Researches of the Chemistry and Material Sciences Section, Russian Academy of Sciences (CMSS RAN) “Synthesis of target materials with pre-determined properties and the development of useful composites on their base: Enantioselective catalysis by nanocomposites of platinum group”; the Program for Basic
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