Elsevier

Catalysis Today

Volume 246, 15 May 2015, Pages 35-45
Catalysis Today

Effects of amine structure and base strength on acid–base cooperative aldol condensation

https://doi.org/10.1016/j.cattod.2014.08.007Get rights and content

Highlights

  • The effect of amine structure and base strength is analysed by kinetic modelling.

  • The arrangement of the amine active sites depends on the amine type.

  • Intermediate and inhibiting species formation was determining the TOFs.

  • The base strength affects the activation entropies and energies of the reactions.

  • A cyclohexyl substituted on the amine results in steric hindrance.

Abstract

Aminated silica materials are known to efficiently catalyse aldol condensations, especially when silanol groups are neighbouring the amine function. The effect of the amine structure and base strength has been analysed experimentally and by kinetic modelling using commercially available precursors to graft primary, secondary and tertiary amines on the silica surface. While primary amines are arranged in a clustered manner on the catalyst surface, secondary amines are arranged randomly which results in a higher percentage of promoted amines in the low silanol-to-amine ratio range. An enamine compound formed by the reaction between the amine active site and acetone has been identified as the key intermediate to explain the experimental observations. In the case of a primary amine this enamine intermediate can form an inhibiting imine with which it is in equilibrium. As a secondary amine has only one hydrogen atom bonded to the nitrogen atom, the inhibiting imine cannot be formed, resulting in a comparatively higher concentration of reactive enamines on the catalyst surface. In case of a tertiary amine the formation of the reactive intermediate is impossible due to the absence of any hydrogen atom bonded to the nitrogen atom. The activation entropies of all reaction steps occurring on the amine sites, as obtained by regression, could be correlated to the deprotonation entropies of the amine sites. As the deprotonation enthalpy does not account for steric effects, no such correlation could be found between the activation energies of these reaction steps and the deprotonation enthalpies of the amine sites.

Introduction

There are some inherent disadvantages to homogeneous catalysis such as (i) the necessity of energy intensive separation steps, (ii) a short catalyst life time and (iii) the low reusability, (iv) important waste streams and (v) intensive equipment corrosion [1], [2]. Heterogeneous catalysis may help to overcome many of these disadvantages. Therefore, it is important to develop new and optimize existing heterogeneous catalysts that can, potentially, replace homogeneous ones. Since about a decade, so-called high-throughput technologies have emerged in order to speed up catalyst development and optimization. However, the discovery rate may be further enhanced by exploiting more rational feedback obtained by means of kinetic modelling in the catalyst design cycle [3], [4], [5].

Aldol condensations are important reactions, typically employed in the pharmaceutical industry and fine chemicals production, to create new Csingle bondC bonds and, hence, heavier and more complex molecules [6], [7], [8], [9]. A bright future has also already been forecasted for aldol condensation reactions in the transition from a fossil resources based towards a more sustainable society, e.g., in the valorization of glycerol, which is a byproduct in the production of biodiesel, or the conversion of furan compounds such as 2-furaldehyde (furfural) and 5-(hydroxymethyl)furfural (HMF), which are obtained by dehydration of sugars, into hydrocarbon fuels [10], [11], [12], [13], [14], [15]. At present aldol condensations are industrially mostly catalysed by strong, homogeneous base catalysts such as KOH, Ca(OH)2, NaOH or Na2CO3. Recent research focuses on hydrotalcite catalysts and other layered double hydroxides [16], [17], [18], [19], [20], [21] or functionalized silica materials [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35] as possible alternatives. Silica materials on which amine containing silanes are grafted by means of a stirring or a reflux procedure have been found to perform well [24], [25], [26], [27], [28], [29], [30], [31], [32]. Moreover, many authors agree that the incorporation of weak acid sites, such as silanol groups, next to the base amine sites has a enhancing effect on the catalytic activity of these amines in aldol condensations and other important Csingle bondC coupling reactions [22], [23], [24], [25], [26], [27], [28], [29], [30], [33], [34], [35], [36], [37], [38]. Recently it has been demonstrated that an increasing acid strength of the promoting site leads to a decreasing activity which could be explained by a pronounced shift in the equilibrium from the free acid and free base towards the resulting neutralized ion pair [25], [26], [32].

In this work, a kinetic model is developed for the acid–base cooperative catalysed aldol condensation reaction. The focus is on the effect of the structure and base strength of the amine functional groups on the aldol condensation kinetics. A catalyst library comprising both base and acid–base catalysts was synthesized by functionalization of a commercial, mesoporous silica, Silicagel 60, with different commercially available primary, secondary and tertiary amine containing silanes. The effect of the reaction conditions as well as the structure and base strength of the active site on the catalyst activity is investigated by means of both an experimental kinetic study and kinetic modelling.

Section snippets

Grafting of amines on Silicagel 60

Different catalysts were prepared using five commercially available amine containing silanes, (3-aminopropyl)triethoxysilane (98%, APTES, ABCR), N-methylaminopropyltrimethoxysilane (MAPTMS, ABCR), N-cyclohexylaminopropyltrimethoxysilane (CAPTMS, ABCR), N-phenylaminopropyltrimethoxysilane (95% PAPTMS, ABCR) and (N,N-diethyl-3-aminopropyl)trimethoxysilane (DEAPTMS, ABCR). These aminosilanes were grafted on a mesoporous silica, Silicagel 60 (Grade 7734, Sigma–Aldrich). First, the silica is

Catalyst characterization

A type IV nitrogen adsorption–desorption isotherm is obtained for the calcined Silicagel 60 material. It has a specific BET surface of 497 m2/g, a total pore volume of 0.69 cm3/g and an average pore diameter of 5.6 nm.

After grafting the precursor on the silica material the presence of amine groups is demonstrated via Diffuse Reflectance Infrared Fourier Transform (DRIFT) measurements, see Fig. 2. The unfunctionalized pretreated silica exhibits a narrow vibration band at 3745 cm−1 which is

Interaction parameter estimation and activity coefficient calculation

The activity coefficients which were determined using COSMO-RS are used to estimate the binary interaction parameters of the UNIQUAC model. The obtained F value for the global significance of the regression amounts to 7476, which exceeds the tabulated value by 4 orders of magnitude. Hence, the model behaviour can be considered to be acceptable. The binary interaction parameters and corresponding 95% confidence intervals are shown in Table 5. A figure showing the effects of the liquid

Conclusions

Distinct differences in turnover frequency (TOF) were obtained when different types of amines were used to catalyse the aldol condensation between acetone and 4-nitrobenzaldehyde. The activity of all amine active sites could be enhanced by incorporating silanol groups in the catalyst. In contrast with primary amine active sites which were found to be positioned in a clustered manner, secondary and tertiary amine active sites were randomly distributed over the silica surface.

While primary amines

Acknowledgments

This work was supported by the Long Term Structural Methusalem Funding by the Flemish Goverment. DE is a postdoctoral researcher of the FWO-Vlaanderen (Fund Scientific Research – Flanders) grant number 3E10813W. The authors would like to thank Tom Planckaert of the department of Inorganic and Physical Chemistry for nitrogen sorption measurements and CHNS elemental analysis.

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