*5.2.4. Ethyl pyruvate enantioselectivity hydrogenation*

*5.2.3. 2,3-butanedione hydrogenation*

198 New Advances in Hydrogenation Processes - Fundamentals and Applications

with an alcohol group.

was higher than 98%.

4.0 MPa H2, 368 K, *W*Cat = 2 g.

concentration.

**Figure 17** shows the scheme of reaction for the selective hydrogenation of 2,3-butanedione to 3-hydroxybutan-2-one. In this reaction one of the carbonyl groups requires to be hydrogenated

**Figure 17.** Scheme of reaction for the selective hydrogenation of 2,3-butanedione to 3-hydroxybutan-2-one.

**Figure 18.** 2,3-butanedione hydrogenation. Reaction conditions: isopropyl alcohol solvent, *C*<sup>0</sup>

All catalysts of Pd supported over composite supports, independent of the kind of polymer used and of the metal content, were active and highly selective for the reaction of interest. Among the composite catalysts series, those based on the BTAl support were more active than those based on the UTAl. This was more evident in the case of the catalyst of low metal

2,3-butanodione = 0.057 M,

The reaction of hydrogenation of 2,3-butanedione to 3-hydroxybutane-2-one is a reaction of interest for the industry of scents and fragrances [41–43]. **Figure 18** shows the graphs of total conversion of 2,3-butanedione (X2,3BD) as a function of time for the catalysts 0.3PdUTAl, 0.3PdBTAl, 1.3PdUTAl, and 1.3PdBTAl. In all cases the selectivity to 3-hydroxybutane-2-one

> The enantioselective hydrogenation of ethyl pyruvate has been an intensively studied research subject. It has been found that the Pt catalysts are the most appropriate for performing this reaction [45, 46], while the Ir, Ru, and Pdcatalysts display lower yields [45, 47]. The Ptcinchonidine catalysts favor the formation of (R)-ethyl lactate.

> **Figure 20** shows the scheme reaction for the selective hydrogenation of ethyl pyruvate to (R) ethyl lactate (using cinchonidine as modifier).In this reaction the objective is to get an enantiomeric excess (ee) of the (R) form over the (S) form.

> **Figure 21** shows the results of catalytic activity obtained with the catalysts 1PtBTAl and 1PtUTAl employing cinchonidine as quiral modifier. The composite catalysts prepared were found to be active, showing conversion values higher than 90%. In both cases it can be seen that 1PtBTAl is more active than 1PtUTAl.This could be related to the higher availability of active sites. Also, when cinchonidine is used as chiral modifier the enantiomeric excess (ee) of (R)-ethyl lactate achieved was greater for the case of the 1PtBTAl catalyst. This could be related

to the size of the metal particles. A big size would permit a better interaction with the chiral modifier.

**Figure 20.** Scheme of reaction for the enantioselective hydrogenation of ethyl pyruvate to (R) or (S) ethyl lactate.

**Figure 21.** Ethyl pyruvate hydrogenation. Reaction conditions: isopropyl alcohol solvent, *C*<sup>0</sup> Ethyl pyruvate = 0.346 M, *C*<sup>0</sup> Cinchonidine = 0.002 M, 2.0 MPa H2, 298 K, *W*cat = 0.5g.

#### **6. Conclusions**

In the present work a method was presented for the preparation of new organic-inorganic hybrid materials for being used as catalyst support, generally referred to in this work as "composites."BTAl and UTAl composites are formed by a combination of inorganic material (alumina) and an organic one (polymer).The organic phase was obtained by polymerization of organic molecules of functionality 2: bisphenol Aglycerolatedimethacrylate, diurethanedimethacrylate, and triethylene glycol dimethacrylate. The inorganic phase in the form of a powder was mixed with the monomers and a heat starter of polymerization (benzoyl peroxide).The mixture was extruded and then polymerized by applying heat. Thus, the obtained rigid material was used as a support for the preparation of supported noble metal catalysts.

The composites display the combined action of a hydrophilic material (alumina) and a hydrophobic one (polymer). This is an ideal combination for the preparation of catalysts with small thicknesses of active metal surface layer. The combination of these dual properties makes the process of preparation of metal/composite catalysts easier, faster, cheaper, and more repetitive, in comparison to the preparation of catalysts of metals supported over common supports. Conventional methods using these common supports need a strict control over the viscosity, contact time, and pH of the impregnating solution, the temperature of impregnation, drying, and calcination. **Figure 22** shows a graphical description of composites catalysts synthesis procedure.

**Figure 22.** Graphical description of composites catalysts synthesis procedure.

to the size of the metal particles. A big size would permit a better interaction with the chiral

200 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Figure 20.** Scheme of reaction for the enantioselective hydrogenation of ethyl pyruvate to (R) or (S) ethyl lactate.

**Figure 21.** Ethyl pyruvate hydrogenation. Reaction conditions: isopropyl alcohol solvent, *C*<sup>0</sup>

In the present work a method was presented for the preparation of new organic-inorganic hybrid materials for being used as catalyst support, generally referred to in this work as "composites."BTAl and UTAl composites are formed by a combination of inorganic material

nidine = 0.002 M, 2.0 MPa H2, 298 K, *W*cat = 0.5g.

**6. Conclusions**

Ethyl pyruvate = 0.346 M, *C*<sup>0</sup>

Cincho-

modifier.

Egg-shell supported metal catalysts have advantages over the catalysts with other metal distributions. They have a lower intraparticle mass diffusion resistance and enable a better control of the temperature on the surface of the catalyst, thus enabling an overall better control of the reaction and the reactor.

The results of hydrogenation of styrene using pellet catalysts and powder catalysts containing more than 1 wt% Pd showed that the composites UTAl and BTAl had higher values of the effectiveness factor (η) in comparison to the rest of the catalysts. These results can be attributed to the small thickness of the metal layer formed over the surface of the composite support, lower than the thickness in the other catalysts prepared using conventional supports as alumina or an activated carbon.

The Pd/composite catalysts of low metal content were compared to other commercial samples, LD265 (Axens) and ENGELHARD, in the reaction test of styrene hydrogenation. The Pd/ composite catalysts had slightly better catalytic properties than the commercial Pd catalysts, a fact pointing to the possibility of industrial use of these new materials.

In the tests of selective hydrogenation of terminal alkynes to terminal alkenes, the activity of the catalysts 0.3PdAl, 0.3PdCNR, 0.3PdUTAl, and Lindlar were evaluated using 1-heptyne as reactant. It was seen that at 180 min, Lindlar and 0.3PdUTAl achieved total conversion of the alkyne, with selectivity to 1-heptene of 85% in the case of the Lindlar catalyst and 97% in the case of 0.3PdUTAl. The 0.3PdAl and 0.3PdCNR catalysts suffered in this strong reaction deactivation. This was more noticeable from 1-heptyne conversion values of 10–20%.The 0.3PdUTAl catalyst had a similar activity as the commercial Lindlar catalyst. Some advantages of the new materials were evident. The Lindlar catalyst was available in powder form and was very expensive due to the high metal content (5 wt% Pd).The Pd/composites were pelletized and only had 0.25 wt% Pd content.

The good results of conversion and selectivity obtained during the reaction of hydrogenation of 3-hexyne, a nonterminal alkyne, to 3-hexene, over the 0.3PdUTAl catalyst, indicates that these composite catalysts can also be used in reactions comprising consecutive steps.

The reactions of hydrogenation presented were also favored by the use of composite supports because of their lower acid strength. This is an advantage for suppressing undesirable reactions, such as the oligomerization of alkenes and alkynes, leading to the formation of carbon deposits or gums over the catalysts. These and other reactions affect the selectivity and lifetime of the catalysts.

In the case of the reaction of synthesis of 3-hydroxybutane-2-one from 2,3-butanedione, which is of commercial interest for the perfume industry, it was seen that the Pd/composite catalysts could be used with values of conversion and selectivity close to 100%.The use of composites in the slurry reactors enabled sparing the filtering of the liquid phase at the end of the reaction. The difference in activity found between the PdBTAl and PdUTAl catalysts could be due to the differences in metal dispersion or to differences in the electronic properties of the surface Pd species.

During the enantioselectivity hydrogenation reaction of ethyl pyruvate using cinchonidineas a quiral modifier, both PtUTAl and PtBTAl composite catalysts were active and selective to (R)-Ethyl Lactate. PtUTAl catalyst had higher total conversion than PtBTAl but had less enantiomeric excess (ee). This activity and enantioselectivity behavior could be related, at least partly due to the electronic effects of the presence of chloride species that prevents the adsorption of the quiral modifier causing a decrease in the enantiomeric excess (ee), although geometrical effects of Ptδ+ OxCly species could not be discarded.

The advantage of the egg-shell catalysts that makes them to be preferred over other catalyst types is that they have smaller intraparticle mass transfer limitations. Having most of the metal phase on the outer surface layer, makes also the heat transfer more efficient, enabling a better control of the reaction and the reactor temperature.

A last advantage is related to the mechanical properties. The composites displayed higher values of diametrical and longitudinal resistance than alumina and silica and also suffered less attrition. This translates into a higher crush resistance when used in packed beds and a lower tendency to the formation of fines when used in slurry stirred reactors.

In summary, it can be said that the composite catalysts synthesized in this work had an eggshell metal distribution with very small metal surface layer thickness and proved to be superior to common metal catalysts prepared with commercial supports. This was attributed to a combination of the following properties: good mechanical resistance, simplicity of preparation procedure for support and catalyst, small thickness of the active phase, uniform distribution of the metal over the support surface, ease of preparation procedure scale-up, and possibility of achieving any desired pellet shape.
