Formaldehyde catalyst deactivation

The bottom product of the pre-evaporation stage (Figure 6.7) can eventually be sub] ected to hydrogenation in a trickle bed reactor, to purify the solvent recycle stream by eliminating impurities in the form of formaldehyde and acetaldehyde, reducing them to methanol and ethanol, and also to eliminate traces of unconverted HP. Moreover, traces ofhydroperoxypropanol and hydroxyacetone are converted into 1,2-propanediol. This allows a considerable decrease in catalyst deactivation in the epoxidation reactor and the improvement of product quality [20k]. 

All samples show a low m-cresol conversion, mainly because of the reaction conditions (low temperature and poor solubility of the methanol in the liquid phase). The conversion is particularly low for both samples with a low surface area and samples havii a low number of medium basic sites. In the latter case catalyst deactivation occurs because formaldehyde (precursor of heavy compounds) is formed by methanol dehydrogenation on strong basic sites the latter then soon deactivate. The most active catalyst is the Mg/Al/O 2/1 sample, characterized by the highest number of medium-strength sites. 

As Shown in Figure 4, catalyst deactivation is an important factor in the hydroxy-alkylation reactions. This makes quantitative comparison difficult, as each material has a balance between activity and deactivation. The main conclusion from the results is that materials having a low acidity, ALPO-5 (vide supra) and 7-alumina, give best results, because of their low rate of deactivation. Already when SAPO-5, having an acid strength between ALPO-5 and aluminosilicates, is used, a rapid deactivation was observed in the phenol/formaldehyde reaction. 

Silico)aluminophosphate-molecular sieves are interesting materials as catalysts for the condensation of phenoi with carbonyl compounds, presumably because of the low acidity and the spacious constraints, decreasing catalyst deactivation. Especially ALPO-5, having a very low acid strength, gives good yields in the reaction of phenol with formaldehyde towards dihydroxydiphenylmethanes. In case of the phenol/isobutanai reaction, mainly monosubstituted products are obtained shape selectivity seems to retard a second condensation step. Ortho-substitution prevails, possibly caused by a contribution of Lewis-acid catalysis. 

The polymerization of formaldehyde has been studied using various catalysts and complexing co-catalysts. When a bed of NaX-zeolite spheres was used, aqueous formaldehyde at 95 °C gave an initial conversion to formose sugars of 50% at pH 5—7. Rapid irreversible catalyst deactivation then occurred due to the presence of formic acid produced in the undesired Cannizaro reaction. The problem was overcome by incorporation of 0.86 cm sodium hydroxide per cm formaldehyde into the combined feed to the reactor... 

The catalytic performance of both Co- and Ce-POM supported on NH2-fimctionalized mesoporous silica was assessed in the aerobic oxidation of formaldehyde in H2O under mild conditions (20-40 °C, 1 atm of air) [97,118]. While the Co-POM-based catalyst underwent rapid deactivation, the Ce-POM catalyst could be used repeatedly without significant loss of the catalytic activity as one ean judge from Figure 4 [118]. 

According to Platonov et al. (309), nickel can be poisoned by H2S, S02, H2SO4, As203, and P206, after which it will direct the decomposition of formic acid preferentially in one of three possible ways above a critical amount of the poison the latter ceased to act selectively and the catalyst was deactivated. In another study (310) Platonov and co-workers demonstrated that over rhenium catalysts, partly poisoned with H2S or AS2O3, methanol was converted to formaldehyde in preference to the complete decomposition which occurred over the unpoisoned catalyst. However, rhenium sulfide itself was later shown to be a good catalyst for the reaction reported and consequently the poisoning phenomenon may not be applicable to this case (308). 

The highest propene oxide yields were obtained with both the Ti-SBA-15- and the Ti-silica-supported catalysts, although a higher reaction temperature was needed in comparison to the titania-supported catalyst. The deactivation for these catalysts was also considerably less. At lower temperatures (up to 423 K), all catalysts had an inhibition period for both propene oxide and water formation, which is explained by product adsorption on the support. The side products produced by all catalysts were similar. Primarily, carbon dioxide and acetaldehyde were produced as side products and, in smaller quantities, also propanal, acrolein, acetic acid, and formaldehyde. Propanol (both 1- and 2- as well as propanediol), acetone, carbon monoxide, and methanol were only observed in trace amounts. 

The most selective catalysts for the oxidation of methanol to formaldehyde are molybdates. In many commercial processes, a mixture of ferric molybdate and molybdenum trioxide is used. Ferric molybdate has often been reported to be the major catalytically active phase with the excess molybdenum trioxide added to improve the physical properties of the catalyst and to maintain an adequate molybdenum concentration under reactor conditions(l,2). In some cases, a synergistic effect is claimed, with maximum catalytic activity for a mixture with an Fe/Mo ratio of l.T( 3j. A defect solid solution was also proposed( ). Aging of a commercial catalyst has been studied using a variety of analytical techniques(4) and it was concluded that deactivation can largely be account for by loss of molybdenum from the catalyst surface. 

S3-85) catalyst [39] and over a Raney Cu catalyst [26], The deactivation was explained as fouling of the active Cu sites by polymerized formaldehyde, an intermediate in the reaction [26], Steam was found to be a very good agent for regeneration of the deactivated catalysts [39],... 

In some exploratory experiments test conditions were selected for the phenol/ formaldehyde condensation. It was found that catalytic test experiments could best be carried out at a relatively high temperature, 180 C, to increase conversions generally a reaction time of 4Vi h was found suitable to compare materials with higher and lower deactivation rates. Application of 1,4-dioxane as a solvent did not improve the selectivity much, and therefore the experiments were carried out solvent free. Selectivities could be improved considerably when a higher phenol/formaldehyde-ratio was applied (see for example Figure 3) but, again to compare different catalysts, a molar ratio of 2/1 was considered most suitable in the catalytic test experiments. 

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