Dehydration of alcohols in the presence of carbonyl compounds and carboxylic acids in a Fischer–Tropsch hydrocarbons matrix

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Abstract

The dehydration of alcohols in the presence of carboxylic acids, ketones and aldehydes in Fischer–Tropsch hydrocarbon fractions was investigated over η-alumina at different temperatures and mass hourly space velocities. Quantitative dehydration of the alcohols was achieved while reactions of carbonyl compounds and acids were incomplete and short-lived. At least 70% of the α-olefins in the feed were initially isomerised to internal olefins, while primary alcohols were dehydrated to mostly internal olefins. Over a period of 8 days, at 350 °C and a liquid hourly space velocity of 6 L feed/(L cat h−1), isomerisation of olefins gradually ceased until the fraction of α-olefins equalled the original concentration in the feed (16 mass%) plus the fraction formed from the dehydration of alcohols (8 mass%). The conversion of carbonyl compounds and acids also diminished to a negligible level. The isomerisation activity was linked to catalyst sites which were selectively poisoned by carbonyl compounds and acids. Regeneration of the catalyst with oxygen at a temperature of 480 °C resulted again in removal of carbonyl compounds and acids as well as olefin isomerisation. The alcohol dehydration reaction appeared not to be affected by the poisoning of the catalyst.

Introduction

The Fischer–Tropsch reaction between CO and H2 produces a mixture of hydrocarbons and oxygenates, comprising mainly alcohols, ketones, aldehydes, carboxylic acids and esters [1], [2]. The hydrocarbons produced in the Low Temperature Fischer–Tropsch (LTFT) reaction consist mostly of paraffins and olefins with a negligible percentage of aromatics. The product from the High Temperature Fischer–Tropsch (HTFT) reaction is more olefinic and contains about 5% aromatics [3].

In both product slates the olefin fractions are predominantly linear α-olefins which are useful as starting material in many applications, e.g. polymers, alkylbenzenes for detergents, production of alcohols via hydroformylation, etc. [4]. Oxygenates are undesirable in these olefinic feedstocks because they often cause loss of catalyst activity. Catalytic dehydration of alcohols to olefins is a well-known reaction which can be successfully carried out over a wide range of solid acid catalysts [5], [6]. The alcohols produced via FT synthesis are mostly primary and in principle they can react to form α-olefins which increase the pool of useful olefins. However, in the presence of acid catalysts a considerable portion of the olefins isomerises to internal olefins which are thermodynamically more stable. Both double bond shift and skeletal isomerisation are known to occur over alumina at 325 °C [7], [8], [9]. These reactions are undesirable as internal olefins have little value as chemical feedstocks.

The percentage of internal olefins can be lowered by decreasing the operating temperature of a dehydration process [9], but on a commercial scale that will negatively impact on production. Another way of reducing isomerisation is by impregnating the alumina with alkali hydroxide to neutralise the Brönsted acidic sites on the surface [10], [11]. Improvements in the α-olefin selectivity were also reported for alumina catalysts treated with a variety of primary amines with basic properties [12], [13].

γ-Alumina catalyses the bimolecular reaction of carboxylic acids to form ketones or aldehydes depending on the number of α-hydrogens of the carboxylic acid. Since FT product contains predominantly aliphatic linear carboxylic acids with 2 α-hydrogens, the reaction products should be mainly ketones. This reaction takes place at temperatures similar to alcohol dehydration. For example, acetic acid in a hydrogen gas stream forms acetone above 300 °C although decomposition to CO2 and water occurs as well [14].

Dehydration of aldehydes and ketones (carbonyl compounds) to dienes is known to occur over a variety of oxide catalysts, e.g. aluminium silicate, TiO2 and SiO2 although phosphorus containing catalysts proved to be more active [15], [16]. The dehydration reaction of carbonyl compounds was however characterised by rapid catalyst deactivation, one of the factors being coke deposition [17]. Dehydration of carbonyl compounds over alumina has not been reported according to the writer's knowledge.

The purpose of this investigation was two-fold: firstly, to dehydrate alcohols over alumina without sacrificing α-olefins already present in the hydrocarbon fraction and, secondly, to determine whether the remaining oxygenates could be reacted to hydrocarbons under the same operating conditions.

Section snippets

Experimental

The dehydration reactions were carried out in a 1 m long tube with a 27 mm inside diameter in the downflow mode at near atmospheric pressure. Temperatures inside the reactor were measured by means of six movable thermocouples inside a 6 mm sheath fitted axially in the reactor tube. Heating was provided by four independently controlled heating blocks clamped around the tube. Feed was pumped into the reactor at the top without a carrier gas. The feed rate was determined from level readings of a tall

Dehydration of alcohols

First the results with the C8–C18 feed are presented. The conversion of alcohols at 350 °C and 2 h−1 WHSV was above 95% and the dehydration product consisted only of olefins. As mentioned before, isomerisation of olefins occurs over pure γ-alumina. η-Alumina strongly resembles γ-alumina and olefin isomerisation is therefore to be expected.

Initially, only about 5 mass% α-olefins was measured in the product, against a possible maximum of 22–23 mass% at conditions of complete dehydration of the

Discussion

In all the experiments, the reactions of the carbonyl compounds and acids over alumina ceased after a period of 2–5 days on stream, whereas the dehydration of alcohols was not measurably affected. Ketones are quite stable compounds and at the temperature of 250 °C, at which the dehydration/regeneration experiment was carried out, it is possible that the carbonyl compounds were adsorbed without undergoing a decomposition reaction. Nevertheless, at all the temperatures at which the dehydration

Conclusions

Alcohols in FT product streams containing olefins, carbonyl compounds and/or acids could be successfully dehydrated at 350 °C to olefins over pure alumina. Initially, considerable isomerisation of the α-olefins in the FT product occurred and the dehydration product of the alcohols consisted mostly of internal olefins. Other oxygenates were dehydrated or reacted to other products for a limited period only, due to poisoning of catalytic sites by carbonyl compounds and acids or their reaction

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