Aldehyde-selective adsorbents

At present considerable research is being carried out to investigate reductants other than hydrogen the emphasis here is on possible structural sensitivity. For example, hydrazine and aldehydes (such as glucose) at low coverages are selectively adsorbed by particular sites of the basic metal, and can then be used to induce the deposition of additives on those sites [8]. 

The absence of a solvent when working under vapor phase conditions strongly increases the intraporous alpha-pinene oxide concentration. This leads to a decreased campholenic aldehyde selectivity. A competitive inert co-adsorbate may be added to the reactor feed to control the alpha-pinene oxide concentration. When dichloroethane is chosen as a co-adsorbate, a very high selectivity to campholenic... 

Cyclooctadiene (1,5-COD) is converted selectively to cyclooctene (97.8% on the addition of 1 mol of H2 per mol of diene) over a colloidal Pd supported on poly(N-vinyl-2-pyrrolidone) in methanol. The reaction is in part direct and in part proceeds through isomerization to the 1,4-COD, which is observed, and, presumably, through 1,3-COD, which is not observed. 1,3-COD is the most reactive and selective of the three dienes at its complete conversion it yields 99.9% cyclooctene. Nishimura discovered that the reduction of 1,5-COD is highly selective over Pd/CaCOa or Pd black in the presence of phenylacetalde-hyde with almost complete suppression of the reduction of cyclooctene. The inhibition is probably due to the decarbonylation of the aldehyde the adsorbed CO competes with the cyclooctene for surface sites. ... 

Palmer 10 found that although ethanol decomposed to acetaldehyde in the presence of copper at 300° C. without the formation of secondary decomposition, this was not true when aldehyde alone was used. If hydrogen and acetaldehyde are passed over copper at 250° to 300° C. much of the aldehyde decomposes into secondary products. This anomaly, Palmer explains on the basis of the alcohol being selectively adsorbed by the catalyst surface so that the copper surface is covered with a layer of alcohol molecules which prevent the adsorption and consequent destruction of the aldehyde. The three steps in the dehydrogenation reaction were postulated to be (1) adsorption of alcohol, (2) activation of certain alcohol molecules, (3) evaporation of hydrogen and acetaldehyde from the catalyst surface. 

A mixture of hydrocarbons can be separated by three selective adsorbents molecular sieves which retain the n-paraffins, mercuric perchlorate which adsorbs the unsaturated compounds, and a stationary liquid phase which retains the aromatic hydrocarbons. Boric acid on a stationary phase such as Chromosorb P is used for the removal of alcohols from a mixture of organic compounds, the products are nonvolatile esters. A similar technique was used for removal of terpene alcohols from a mixture of terpenoids. Acids can be adsorbed on potassium hydroxide deposited on quartz powder. This technique was found to be suitable for the analysis of compounds having active hydrogens such as fluorene, indene, carbazole, indole, and pyrrole as well as steroids such as estrogens and ketosteroids. o-Dianisidine quantitatively subtracts aldehydes, ketones, and epoxides, and phosphoric acid subtracts epoxides. 

It is noteworthy that even a separate treatment of the initial data on branched reactions (1) and (2) (hydrogenation of crotonaldehyde to butyr-aldehyde and to crotyl alcohol) results in practically the same values of the adsorption coefficient of crotonaldehyde (17 and 19 atm-1)- This indicates that the adsorbed form of crotonaldehyde is the same in both reactions. From the kinetic viewpoint it means that the ratio of the initial rates of both branched reactions of crotonaldehyde is constant, as follows from Eq. (31) simplified for the initial rate, and that the selectivity of the formation of butyraldehyde and crotyl alcohol is therefore independent of the initial partial pressure of crotonaldehyde. This may be the consequence of a very similar chemical nature of both reaction branches. 

Recently, it has been demonstrated that coordination vacancies on the surface metal cations are relevant to the unique redox reactivity of oxide surfaces]2]. Oxidation of fonnaldehyde and methyl formate to adsorbed formate intermediates on ZnO(OOOl) and reductive C-C coupling of aliphatic and aromatic aldehydes and cyclic ketones on 1102(001) surfaces reduced by Ar bombardment are observed in temperature-prognunmed desorption(TPD). The thermally reduced 1102(110) surface which is a less heavily damaged surface than that obtained by bombardment and contains Ti cations in the -t-3 and +4 states, still shows activity for the reductive coupling of formaldehyde to form ethene]13]. Interestingly, the catalytic cyclotrimerization of alkynes on TiO2(100) is also traced in UHV conditions, where cation coordination and oxidation states appear to be closely linked to activity and selectivity. The nonpolar Cu20( 111) surface shows a... 

Photocatalytic oxidation is a novel approach for the selective synthesis of aldehyde and acid from alcohol because the synthesis reaction can take place at mild conditions. These reactions are characterized by the transfer of light-induced charge carriers (i.e., photogenerated electron and hole pairs) to the electron donors and acceptors adsorbed on the semiconductor catalyst surface (1-4). Infrared (IR) spectroscopy is a useful technique for determining the dynamic behavior of adsorbed species and photogenerated electrons (5-7). 

In contrast to kinetic models reported previously in the literature (18,19) where MO was assumed to adsorb at a single site, our preliminary data based on DRIFT results suggest that MO exists as a diadsorbed species with both the carbonyl and olefin groups being coordinated to the catalyst. This diadsorption mode for a-p unsaturated ketones and aldehydes on palladium have been previously suggested based on quantum chemical predictions (20). A two parameter empirical model (equation 4) where - rA refers to the rate of hydrogenation of MO, CA and PH refer to the concentration of MO and the hydrogen partial pressure respectively was developed. This rate expression will be incorporated in our rate-based three-phase non-equilibrium model to predict the yield and selectivity for the production of MIBK from acetone via CD. 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

Largely, the same principles apply for water treatment. Consequently, activated carbon is suitable for organic molecules that are nonpolar and of high molecular weight. Trichloroethylene, benzene, ethylbenzene, toluene, and xylene are easily adsorbed in the gas phase when activated carbon, for instance, is used. On the other hand, adsorption is not preferably selected in applications in relation to aldehydes, ketones, and alcohols. In a successful application, reduction in emissions from 400-2000 ppm to under 50 ppm can be achieved (EPA, 1999), especially for VOCs with boiling points between 20 -and 175 °C. 

The flexibility of the gas-solid extraction method stems from the choice of many adsorbing phases, each offering a particular selectivity. It is possible to stabilise certain molecules that would normally decompose upon contact with the sorbent by incorporating into the sorbent a reagent that carries out a specific derivatisation. For example, aldehydes can be converted into oxazolidines, which are stable and desorbable compounds, by reaction with 2-hydroxymethylpiperidine. 

An important distinction between dimerization and acrolein formation is that the selectivity of the former is evidently connected with a partially reduced state of the catalyst. It is commonly accepted, therefore, that cations like Bi3+, Sn4+, etc. play a role, presumably by adsorbing the allyl radical intermediate. Several authors assume that this is the case for allylic oxidation in general and that the role of a second oxide component is to promote dimerization byi stabilization of the allyl radical, or to direct the oxidation to aldehyde formation via a cationic allyl complex. Seiyama et al. [285] further suggest that the acidity of the promoting oxides is an important factor in this connection, and may, in part, explain why acidic oxides like Mo03 direct the oxidation to aldehydes, while basic compounds favour dimerization. 

Thus oxidation of the nucleus may be activated and may form a possible explanation of the relative low initial selectivity generally observed [345, 347]. As to side chain oxidation, introduction of oxygen and dissociation of a second hydrogen atom yields the aldehyde as the first desorb-able aromatic product. On V2Os -based catalysts the aldehyde is easily converted into the acid. The transition of an adsorbed aldehyde into a sym-... 

An interesting means of improving the selectivity of Pd for the conversion of unconjugated dienes, such as 1,4-cyclooctadiene to the monoene is to add phenylacetaldehyde to the mixture undergoing reaction (ref. 36). The mechanism of action is not established but it may involve aldehyde decarbonylation to form adsorbed CO but the addition of small amounts of CO to the reactants does not reproduce the effect of the aldehyde (ref. 37). Means to modify the metal suface in other ways can prove effective, the studies of Ni catalysts by Okamoto et al. afford an interesting example of an attempt to reach a more fundamental understanding of catalyst selectivity. 

Introducing adsorbents into a supercritical fluid extraction system is an alternative attractive method to improve the selectivity for the citrus oil processing. Several applications using silica gel as adsorbent have been reported in the last decade [1-3]. Yamauchi and Saito [3] fractionated lemon peel oil into 3 fractions with gradual increase in pressure by supercritical fluid chromatography, where terpene rich fraction, ester rich, and alcohol and aldehyde rich fraction were obtained at 10 MPa, 20 MPa, and 20 MPa with ethanol as a cosolvent,... 

---