Hydroformylation catalyst poisons

In the normal oxo reaction a certain amount of hydrogenation occurs, a minor amount of olefins being converted to paraffins in the case of certain olefinic compounds hydrogenation indeed occurs to the exclusion of hydroformylation. It is a remarkable fact that this catalytic reaction occurs in the presence of carbon monoxide and also of sulfur compounds, although cobalt metal is notoriously poisoned by traces of these compounds. The significance of this was pointed out by Adkins and Krsek (23) and Wender, Orchin, and Storch (25) in terms of the concept that the hydroformylation catalyst is a homogeneous one, not sensitive to carbon monoxide or sulfur compounds and in this respect different from usual solid cobalt catalysts. 

The TPPTS catalyst system is not sensitive toward sulfur and most of the other common poisons for hydroformylation catalysts. One reason is the continuous withdrawal of organic and other by-products with the product phase and the vent stream from the decanter (see Figure 2), avoiding the accumulation of poisons in the catalyst solution. 

The screening of heterobimetallic hydroformylation catalysts with iron as one constituent received more attention than the use of monometallic Fe catalysts [9]. Earlier attempts were encouraged by the assumption that iron carbonyls, which can be formed in steel autoclaves under carbon monoxide, act as poisons for cobalt or rhodium catalysts [10]. Especially, the property of Fe(CO)5 to catalyze the aldol condensation of product aldehydes was considered to be detrimental to hydroformylation. Usually, this problem is solved either by technological means (fast separation of the product) or by the addition of chelating agents for iron [11]. 

A special concern is the purity of the gas feed. Poisons may have a remarkable influence on the long-time stability of the hydroformylation catalysts and, consequently, the degree of conversion and selectivity is influenced. In smaller companies, the production of aldehydes via hydroformylation may become a serious problem due to the significantly higher price of CO in comparison to This... 

Poisoning phosphites are particularly undesirable because their smaller steric bulk allows them to bind to the rhodium catalyst and inhibit hydroformylation. 

Addition of carbon monoxide and hydrogen to an alkene linkage in the presence of cobalt catalysts gives aldehydes in an average yield of 50%. The reactions may be carried out in the usual hydrogenation apparatus. The poisonous properties of carbon monoxide and cobalt carbonyls call for considerable care. Compounds made by hydroformylation include cyclopentanealdehyde from cyclopentene (65%), /3-carbethoxy-propionaldehyde from ethyl acrylate (74%), and ethyl /3-formylbutyrate from ethyl crotonate (71%). 

Bell and coworkers showed that both reactions can be mediated by the same Ru catalyst in a one-pot manner. It may be advantageous to adapt the conditions to each reaction. In order to draw benefit from the whole hydrogenation activity of a RuCl2(PPhg)3 catalyst, removal of traces of CO was mandatory [42]. Thus it was found that in the batch hydroformylation of 1-hexene with stoichiometric amount of carbon monoxide, the residual CO poisoned the Ru catalyst. Only utilization of sub-stoichiometric quantities of CO and a conversion of nearly 100% or venting the hydroformylation gases allowed the subsequent efficient hydrogenation. 

In general, rhodium-catalyzed hydroformylation of alkynes proceeds much slower than the reaction with olefins. It should be remembered that homogeneously catalyzed hydroformylation of olefins with unmodified rhodium catalysts can be irreversibly poisoned by the presence of even trace quantities of alkynes. As Liu and Garland [94, 95] found by means of in situ IR spectroscopy, the reason is likely the formation of dinuclear rhodium-carbonyl complexes I, which are stable even in the presence of hydrogen (Scheme 4.17). Therefore, alternative pathways for the production of a,P-unsaturated aldehydes have been suggested, consisting of Ni-catalyzed hydrocyanation followed by chemoselective hydrogenation [96]. 

When a rhodium monophosphite catalyst was subjected to hydroformylation of a technical feedstock poisoned with 14% 9,12-diene, decreased conversion and yield were found in comparison to the use of the pure substrate [23]. This effect was explained by the formation of stable rhodium-jt-allyl intermediates [32], which require more severe conditions in order to obtain the same magnitude of TOP (turnover frequency). 

A number of compounds are known to poison the hydroformylation reaction. The poisons may act in different ways. On the one hand they may prevent hydrocarbonyl formation from the cobalt compounds fed to the 0X0 reactor (which would lead to troubles in discontinuous operations or in start-up in a continuous operation), on the other hand they may react with finished hydrocarbonyl to form compounds which are insoluble in the reaction mixture or inactive as oxo-catalyst. 

Catalytic hydrogenation by H2, like hydroformylation, is almost ideally suited for the application of phase-separation techniques, as it involves only two reagents (the substrate and hydrogen), requires no additional ionic or non-ionic components and, most importantly, produces only the single target product without any by-products that may accumulate in the reaction system and poison the catalyst. 

The water-soluble rhodium complex [Rh(p.-pz)(CO)(TPPTS)]2 (pz = pyrazolate) was used as catalyst precursor during the two-phase catalytic hydroformylation of different olefins at 100 °C, 50 bar (CO H2 = 1 1), 600 rpm, and substrate catalyst ratio of 100 1. A reaction order- 1-hexene > styrene > allylbenzene > 2,3-dimethyl-1-butene > cyclohexene-was found. The experiments also showed that the binuclear catalyst precursor was resistant to possible sulfur poisons [108]. 

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