Selective catalytic reduction synthesis methods

In-Sik Nam and Moon Hyeon Kim (Pohang University of Science and Technology, Korea) review new materials for the selective catalytic reduction of NOx from combustion processes. Despite significant research efforts over the last 20 years, there are still unresolved issues, such as inhibition and deactivation by steam. The authors show how new synthesis methods, especially for zeolites, can be used to improve these catalysts. 

Zirconium dioxide and zeolites of pentasil structure are widely used as catalysts and efficient carriers in many heterogeneous reactions, and particularly in the process of selective catalytic reduction of nitrogen oxides by hydrocarbons (SCR-process) [1,2]. Synthesis of new catalytic systems for NOx SCR-process by CnHm is therefore related with searching for their optimum composition and preparation methods to attain maximum activity in this reaction. 

Titania-supported vanadia catalysts have been widely used in the selective catalytic reduction (SCR) of nitric oxide by ammonia (1, 2). In an attempt to improve the catalytic performance, many researchers in recent years have used different preparation methods to examine the structure-activity relationship in this system. For example, Ozkan et al (3) used different temperature-programmed methods to obtain vanadia particles exposing different crystal planes to study the effect of crystal morphology. Nickl et al (4) deposited vanadia on titania by the vapor deposition of vanadyl alkoxide instead of the conventional impregnation technique. Other workers have focused on the synthesis of titania by alternative methods in attempts to increase the surface area or improve its porosity. Ciambelli et al (5) used laser-activated pyrolysis to produce non-porous titania powders in the anatase phase with high specific surface area and uniform particle size. Solar et al have stabilized titania by depositing it onto silica (6). In fact, the new SCR catalyst developed by W. R. Grace Co.-Conn., SYNOX , is based on a titania/silica support (7). 

Among various methods to synthesize nanometer-sized particles [1-3], the liquid-phase reduction method as the novel synthesis method of metallic nanoparticles is one of the easiest procedures, since nanoparticles can be directly obtained from various precursor compounds soluble in a solvent [4], It has been reported that the synthesis of Ni nanoparticles with a diameter from 5 to lOnm and an amorphous-like structure by using this method and the promotion effect of Zn addition to Ni nanoparticles on the catalytic activity for 1-octene hydrogenation [4]. However, unsupported particles were found rather unstable because of its high surface activity to cause tremendous aggregation [5]. In order to solve this problem, their selective deposition onto support particles, such as metal oxides, has been investigated, and also their catalytic activities have been studied. 

Because of the pure performance of traditional Cu catalysts in the hydrogenation of C02, efforts have been made to find new, more effective catalysts for direct C02 hydrogenation. The problem is to improve selectivity, specifically, to find catalysts that display high selectivity toward methanol formation and, at the same time, show low selectivity in the reverse water-gas shift reaction, that is, in the formation of CO. It appears that copper is the metal of choice for methanol synthesis from C02 provided suitable promoters may be added. Special synthesis methods have also been described for the preparation of traditional three-component Cu catalysts (Cu-ZnO-A1203 and Cu-Zn0-Cr203) to improve catalytic performance for C02 reduction. 

When simulating the alcohol dehydrogenase, the investigators met with economic problems because the NAD+ co-factor is very expensive. Therefore, several methods of its regeneration were developed, among which the most effective method is non-enzymatic continuous regeneration of catalytic amounts of NADHL and NAD+ with sodium dithionite [127], This method can be used for HLADH synthesis, used in the catalytic reduction of a wide selection of aldehydes and ketones. 

In the triene series for the synthesis of the stereoisomers of (15 3)-anacardic acid, namely the 8(Z),11(Z),14 8(E),11(E),14 8(Z),11(E),14 and 8(Z),11(E),14 compounds by the alkylation of the ArC., intermediate (ethyl 2-methoxy-6-methybenzoate) with a 0 4 component these boration methods have been of value as an addition to selective catalytic hydrogenation and the use of terminal trimethylsilylation (ref. 167). At this stage for synthetic purposes the selective reductive use of boration methods has been mainly exploited. The chief use of combined addition/alkylation procedures is for obtaining 8(E), and 11 (E) isomers. For this, the sequence of synthons, for the side chain has to follow the different series, Ar9->ArCi2->ArCi5. 

A catalytic reductive cycUzation of olefinic iodoethers 25 is performed by use of cat. Cp2TiCl2 in the presence of Mn and MosSiCl (Schane 2.16). This protocol provides a selective method for the synthesis of multi-substituted tetrahydrofurans... 

Buchwald reported an important advance in enantioselective C=N reductions with the chiral titanocene catalyst 186 (X,X = l,l -binaphth-2,2 -diolate) [137]. The reduction of cyclic imines with 186 and silanes afforded products with high selectivity however, reductions of acyclic imines were considerably less selective. It was suggested that this arose from the fact that, unlike cyclic imines, acyclic imines are found as mixtures of equilibrating cis and trans isomers. An important breakthrough was achieved with the observation that in situ activation of the difluoride catalyst 187 (X = F) gave a catalytically active titanium hydride species that promotes the hydrosilylation of both cyclic and acyclic amines with excellent enantiomeric excess [138]. Subsequent investigations revealed that the addition of a primary amine had a beneficial effect on the scope of the reaction [138, 139]. A demonstration of the utility of this method was reported by Buchwald in the enantioselective synthesis of the alkaloid frans-solenopsin A (190), a constituent of fire-ant venom (Scheme 11.29) [140]. 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

The third way to prepare CNT-ceramic composite powders is via the synthesis of CNT by a CCVD process, in situ in the ceramic powder. A ceramic powder which contains catalytic metal particles at a nanometric size, appropriate to the formation of CNTs, is treated at a high temperature (600-1100°C), in an atmosphere containing a hydrocarbon or CO. In the method reported in 1997 by the present authors,27 iron nanoparticles are generated in the reactor itself, at a high temperature (>800°C), by the selective reduction in H2/CH4 (18% CH4) of an a-Al203 based oxide solid solution ... 

---