Role of water in cyclopentanone self-condensation reaction catalyzed by MCM-41 functionalized with sulfonic acid groups
Graphical abstract
Introduction
Aldol condensation is an effective CC bond forming reaction that has the potential to play an important role in the industrial upgrading of biomass-derived compounds [1], [2], [3], [4], [5]. Some of the short carbonyl compounds derived from biomass (e.g. ketones, aldehydes, etc.) can be enlarged to a range of more valuable chemicals and high-quality fuels. Water is typically an inhibitor of the aldol condensation rate since it usually competes for active sites (bases or acids) [6], [7], [8], [9], [10], [11]. However, water is often unavoidable, particularly in biomass upgrading processes. Therefore, it is important to be able to quantify and control its influence on activity [12], [13], [14], [15]. For example, Ngo et al. have recently found that the presence of added water results in severe activity loss over MgO [16], [17]. After functionalization with octadecyltrichlorosilane (OTS), the catalyst remained active for much longer time than the pristine MgO catalyst. It was proposed that the hydrophobic alkyl chains of the organosilane inhibits the formation of a liquid film of water on the MgO surface. Thus, the catalytic stability of MgO was significantly enhanced after functionalization. On the bare MgO catalyst, the reaction was limited by the formation of the enolate intermediate. However, upon functionalization the rate limiting step shifted to the bimolecular CC coupling due to the interference of the alkyl chains between the sites. Small amounts of water promoted the CC coupling on the MgO-OTS catalyst by bridging the CPO molecule and a remote Mg2+ site by chain polarization assisted by water molecules connected via H-bonding. Here, we attempt to explore whether this water-mediated remote polarization is present in other systems as well, such as the acid-catalyzed aldol condensation.
Solid acids, including zeolites [18], [19], acid groups functionalized mesoporous silicas [20], [21], metal organic frameworks [22], [23], [24] and polymers [25], [26], have been widely used as catalysts for aldol condensation reactions. Among these catalysts, MOFs, silicas, and polymers containing sulfonic acid groups have been extensively studied [20], [21], [22], [23], [24], [25], [26]. Moreover, sulfonate polystyrene resins and sulfonated polysiloxanes are employed in industrial condensation processes [27]. One of the major advantages of these sulfonic acid-functionalized catalysts is the potential for tailoring the so-called cooperative catalysis, which strongly depends on the distances and topological distribution of the acid/base pairs. Functional groups have exhibited great flexibility to generate varying proximity and orientation, which can be tuned to enhance catalyst performance [20], [28]. Therefore, understanding the cooperative effect through the use of model catalytic reactions and quantifying the kinetics of these phenomena is of great fundamental importance. More importantly, water, which is ubiquitous in aldol condensation reactions, can significantly affect this cooperative effect by forming H-bond networks between active sites [16]. Accordingly, in this contribution, the effect of water addition has been investigated for the acid-catalyzed CPO self-condensation on a commercial MCM-41 substrate functionalized with sulfonic acid groups, synthesized by a novel high temperature grafting method recently published [29]. Structural characterization of these samples demonstrates that the organosilane groups are grafted to the surface of MCM-41 by multi-bonded functionalization. As reported in our recent work, in comparison with catalysts synthesized by conventional functionalization methods, the high temperature grafting results in catalysts with reduced leaching and enhanced stability in liquid-phase reactions at high temperatures [29].
In previous studies, Herrmann and Iglesia [18] investigated the mechanisms of acetone condensation on several acid catalysts in vapor phase, including FER, TON, MFI, BEA, FAU, and MCM-41. In situ selective titration of the active sites with 2,6-di-tert-butyl pyridine demonstrated that protons are responsible for the catalytic activity of these materials. A detailed analysis of the reaction mechanism indicated that an enol intermediate is formed at the acid site and then it reacts with another reactant molecule via nucleophilic addition. Clearly, the former is a first-order step while the latter is second-order. The absence of a strong isotopic effect when deuterated acetone was used as feed indicated that the kinetically-relevant step was the bimolecular CC bond formation. This step can occur between the surface enol and an electrophile in the fluid (Eley-Rideal, E-R model) or between the two species on the surface (Langmuir-Hinshelwood, L-H model). When the surface coverage of the H-bonded acetone is saturated (), the observed reaction order would be first for the former and zero for the latter. Accordingly, the observed first order on all acid catalysts indicated that the reaction followed a E-R model. To observe the second-order kinetics one would need to operate at lower coverages ().
In this study, we have measured liquid phase condensation turnover frequencies starting at very low cyclopentanone (CPO) concentrations, i.e. low surface coverage, and then increasing concentration to evaluate the evolution of rate with coverage and clearly differentiate between true first order vs. second order rate limiting steps. Therefore, the specific rate data obtained on this wide concentration range was fitted with first- and second-order L-H and E-R models and we found that CC coupling is rate-limiting on these MCM-41 catalysts functionalized with sulfonic acid groups. Interestingly, when the density of sulfonic acid groups is high (∼0.4/nm2), the reaction follows a bimolecular surface reaction L-H model, favored by the proximity of active sites on the surface. Ab initio molecular dynamics (AIMD) simulations of CPO adsorption at the -SO3H acidic moiety clearly shows that CO double bond of CPO molecule can be polarized by acid site via forming an H-bond, which favors the attack of the surface enol. By contrast, when the density of sulfonic acid groups is lower (∼0.2/nm2), the best fit is obtained with the E-R model. A remarkable effect on kinetic behavior was observed when water was added. Indeed, it was found that in the presence of water the best fitting for the low-acid-density catalyst shifted from the E-R model to the L-H model, which can be explained in terms of a water-assisted remote polarization between a CPO molecule in the liquid near the adsorbed enol and a surface acid site.
Section snippets
Chemicals and materials
Mesoporous MCM-41-Type A was obtained from ACS Materials. 3-Mercaptopropyl Trimethoxysilane (MPTMS, 95% purity), anhydrous methanol (99.8%), anhydrous ethanol (≥99.5%), anhydrous toluene (99.8%), hydrogen peroxide aqueous solution (30 wt%), cyclohexane (for HPLC, ≥99.9%) and pyridine (>99.9% Sigma-Aldrich) were purchased from Sigma-Aldrich and used as provided. Cyclopentanone, (CPO, ≥99%) was purchased from Sigma-Aldrich and distilled before used as the reactant.
Catalyst synthesis
Before functionalization, the
Effect of acid density on cyclopentanone (CPO) self-condensation reaction
Table 1 summarizes the structural properties of the SO3H-functionalized mesoporous MCM-41 catalysts with varying acid density. N2 adsorption-desorption isotherms and pore size distributions of these catalysts are shown in Fig. S1 (Supplementary Material). After grafting the sulfonic acid groups on MCM-41, the surface areas and pore volumes of the catalysts decrease with increasing acid density, which may indicate a small fraction of pore blockage. Fig. S1b (Supplementary Material) shows the
Conclusions
In this contribution, we have investigated the role of water in cyclopentanone self-condensation reaction catalyzed by MCM-41 functionalized with sulfonic acid groups. Both Langmuir-Hinshelwood and Eley-Rideal models were used in the kinetic analysis. It has been shown that the reaction follows a second-order kinetics, in which the formation of the CC bond is the rate-limiting step.
On a low-acid-density catalyst in the absence of added water, the activated enol does not have an adjacent CPO
Acknowledgements
This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award Number DE-SC0018284. The computational simulations used the supercomputer resources at DOE National Energy Research Scientific Computing Center (NERSC). We also would like to thank Drs. Duong T. Ngo and Tuong V. Bui for valuable discussions.
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