Acetaldehyde oxidation improvements

Another study on the use of Fe showed that the oxidation rate of acetaldehyde was improved with Ti02 catalysts doped with Fe and Si s)mthesized by thermal plasma (Oh et al., 2003). A Fe content lower than 15% rendered higher activities than the untreated catalyst. The catalyst preparation technique involved a complex procedure using a plasma torch, with all this likely leading to an expensive photocatalyst of mild prospects for large-scale applications. 

Acetaldehyde oxidation was also marginally improved, especially for the manufacture of acetic anhydride. In 1935, workers at Shawingan Chemicals discovered that the oxidation of acetaldehyde, if conducted in the presence of cobalt, copper, or better yet, a mixture of the two catalysts, yielded a mixture of acetic anhydride and acetic acid providing the water co-product was rapidly separated by azeotropic distillation, normally with a compatible material such as ethyl acetate. It would not be until the 1940 s that this became widely practiced, but the process was eventually widely adopted. While experimental units produced ratios of acetic anhydride acetic acid as high as 4 1, it appears that the commercial process normally gave a 5 4 mixture of acetic anhydride acetic acid... 

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equihbrium Ethane to ethylene, propane to propylene, water-gas shirt reaction CO -I- H9O CO9 + H9, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

Examples for necessary process improvements through catalyst research are the development of one-step processes for a number of bulk products like acetaldehyde and acetic acid (from ethane), phenol (from benzene), acrolein (from propane), or allyl alcohol (from acrolein). For example, allyl alcohol, a chemical which is used in the production of plasticizers, flame resistors and fungicides, can be manufactured via gas-phase acetoxylation of propene in the Hoechst [1] or Bayer process [2], isomerization of propene oxide (BASF-Wyandotte), or by technologies involving the alkaline hydrolysis of allyl chloride (Dow and Shell) thereby producing stoichiometric amounts of unavoidable by-products. However, if there is a catalyst... 

As oxidation also converts the original chiral terpene-derived group to an alcohol, it is not directly reusable as a chiral auxiliary. Although this is not a problem with inexpensive materials, the overall efficiency of generation of enantiomerically pure product is improved by procedures that can regenerate the original terpene. This can be done by heating the dialkylborane intermediate with acetaldehyde. The a-pinene is released and a diethoxyborane is produced.204... 

Together with the fast oxidation (at low temperatures) of NO to N02, the plasma causes the partial HC oxidation (using propylene, the formation of CO, C02, acetaldehyde and formaldehyde was observed). Both the effects cause a large promotion in activity of the downstream catalyst [86]. For example, a "/-alumina catalyst which is essentially inactive in the SCR of NO with propene at temperatures 200°C allows the conversion of NO of about 80% (in the presence of NTP). Formation of aldehydes follows the trend of NO concentration suggesting their role in the reaction mechanism. Metal oxides such as alumina, zirconia or metal-containing zeolites (Ba/Y, for example) have been used [84-87], but a systematic screening of the catalysts to be used together with NTP was not carried out. Therefore, considerable improvements may still be expected. 

Recently, researchers at Catalytica proposed a new technology for ethylene oxidation (368). Typical compositions are aqueous ca. O.l mM Pd2+, 5-25 mM Cl , and ca. 0.30 M NavH(3+ -v)PVxMoi2 04o (preferably x = 2-3). The Pd2+ and chloride concentrations are only 1/100 those in the oridinary Wacker system. The solutions at pHO-l result in high reaction rates and stability of Pd2+, as shown in Fig. 65. The stability of Pd2+ is further improved by the presence of chloride ion in a concentration of about 0.01 M. In this system, the phosphomolybdate serves two functions in the Pd° reoxidation (l) It solubilizes high concentrations of Vs + in aqueous solution and (2) it accelerates the reoxidation of V4+ by dioxygen. Kinetics (the reaction is first-order in Pd2+ and in ethylene concentrations and zero-order in Vs + concentration) shows that the oxidation of ethylene to produce acetaldehyde is rate-determining. 

To make the DERA-catalyzed process commercially attractive, improvements were required in catalyst load, reaction time, and volumetric productivity. We undertook an enzyme discovery program, using a combination of activity- and sequence-based screening, and discovered 15 DERAs that are active in the previously mentioned process. Several of these enzymes had improved catalyst load relative to the benchmark DERA from E. coli. In the first step of our process, our new DERA enzymes catalyze the enantioselective tandem aldol reaction of two equivalents of acetaldehyde with one equivalent of chloroacetaldehyde (Scheme 20.6). Thus, in 1 step a 6-carbon lactol with two stereogenic centers is formed from achiral 2-carbon starting materials. In the second step, the lactol is oxidized to the corresponding lactone 7 with sodium hypochlorite in acetic acid, which is crystallized to an exceptionally high level of purity (99.9% ee, 99.8% de). 

The sluggish oxidation of p-toluic acid or o-nitro-substituted alkylaromatic compounds is dramatically improved by the simultaneous use of auxiliary organic or inorganic compounds such as acetaldehyde, methyl ethyl ketone, butane, xylene, or nitric acid. The principle of co-oxidation is based on the formation of additional Co ions, mediated by the co-oxidizing reagents. More important, the tatter act as a perfect source for radicals. 

As soon as the fermentation is stopped sulfur dioxide is added to bring the free SO2 concentration to 25 ppm for the subsequent clarification steps. This prevents oxidation, assists in biological stabilization, and binds aldehydes in the new wine. The binding of aldehydes (mostly acetaldehyde) improves the taste of the wine. 

Combination of T1P2O5 with V-P oxide, which improves the performance in the reaction of HCHO with acetic acid, increases the activity markedly, but the maximum yield of acrolein decreases from 79 to 63 mol% based on the charged acetaldehyde. 

There were also improvements in acetaldehyde and acetic anhydride manufacture. Ag based catalysts for the partial oxidation of ethanol became available around 1940. When used to oxidatively dehydrogenate ethanol [14], the conversion of ethanol to acetaldehyde was no longer equilibrium limited since the reaction was now very exothermic. Fortunately, the process still displayed excellent selectivity (ca. 93-97%) for acetaldehyde. This technology replaced the older Cu-Cr processes over the period of the 1940-1950 and made ethanol a much more attractive resource for acetaldehyde. When ethylene became available as a feedstock in the 1940 s through 1950 s, ethanol became cheaply available via ethylene hydration (as opposed to traditional fermentation). With ethanol now cheaply available from ethylene, the advent of the Ag catalyzed oxidative dehydration to acetaldehyde rapidly accelerated the shutdown of the last remaining wood distillation units. 

To the best of our knowledge, no Pt-based catalyst is able to promote the selective oxidation of ethanol to either CO2, acetic acid or acetaldehyde. However, the product composition can be modified by alloying Pt with other metals. For example, the addition of Sn to Pt increases the catalytic activity and also the production of acetic acid, while the addition of Ru improves the electrical performance without changing the product selectivity. Several authors have demonstrated that Ru and Sn are able to activate water al lower potential than pnre Pt, thus boosting the formation of CO2 and acetic acid, while Sn is particularly suited to reduce C-C bond breaking through a sort of dilution of adjacent Pt atoms. ... 

The chemical reactivity of the catalyst support may make important contributions to the catalytic chemistry of the material. We noted earlier that the catalyst support contains acidic and basic hydroxyls. The chemical nature of these hydroxyls will be described in detail in Chapter 5. Whereas the number of basic hydroxyls dominates in alumina, the few highly acidic hydroxyl groups also present on the alumina surface can also dramatically affect catalytic reactions. An example is the selective oxidation of ethylene catalyzed by silver supported by alumina. The epoxide, which is produced by the catalytic reaction of oxygen and ethylene over Ag, can be isomerized to acetaldehyde via the acidic protons present on the surface of the alumina support. The acetaldehyde can then be rapidly oxidized over Ag to COg and H2O. This total combustion reaction system is an example of bifunctional catalysis. This example provides an opportunity to describe the role of promoting compounds added in small amounts to a catalyst to enhance its selectivity or activity by altering the properties of the catalyst support. To suppress the total combustion reaction of ethylene, alkali metal ions such as Cs+ or K+ are typically added to the catalyst support. The alkali metal ions can exchange with the acidic support protons, thus suppressing the isomerization reaction of epoxide to acetaldehyde. This decreases the total combustion and improves the overall catalytic selectivity. 

Steam reforming of ethanol has been demonstrated in the MCFC and proceeds rather differently to the reforming of hydrocarbons [125, 126]. Rinaldi et al. [121] studied ethanol reforming over supported metal catalyst (nickel on doped magnesium oxide). They concluded that acetaldehyde is the main unwanted product. Further catalyst optimization may improve the selectivity in the MCFC. 

It was proposed by Roh et al. [37] that the presence of partially oxidized Ce sites in Ce,cZri 02 suppresses CH4 formation by acetaldehyde decomposition, thus optimizing the hydrogen yield. In addition, Ce Zri c02 promotes noble metal and transition metals for the water-gas shift reaction (Eq. (24.3)) [38]. Moreover, in the reduced state, CexZii x02 niay reduce water to directly yield hydrogen [39]. Finally, Ce Zri, (02 improves the catalyst stability by (i) limiting the formation of ethylene and (ii) promoting carbon gasification [40]. 

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