Highly accessible catalytic sites on recyclable organosilane-functionalized magnetic nanoparticles: An alternative to functionalized porous silica catalysts
Introduction
The immobilization of homogeneous catalysts to facilitate easy catalyst recovery and recycling, as well as product separation, is a longstanding pursuit of catalysis science [1]. Various support matrices such as organic polymers and inorganic silica, especially porous inorganic materials with high surface areas, have been employed [2], [3], [4]. However, a substantial decrease in activity of the immobilized catalyst is commonly observed, especially under low temperature liquid phase conditions, due to the problem of reactant diffusion to the surface-anchored catalyst [5]. Nanoparticles have emerged as efficient alternative support materials for homogeneous catalyst immobilization [6]. When the size of the support material is decreased to the nanometer scale, high surface areas can be obtained, with all of the area on the external surface of the particle when non-porous nanoparticles are used. As a consequence, the activity of nanoparticle-supported catalysts could be improved compared to homogeneous catalysts immobilized on conventional, porous support matrices under conditions where internal pore diffusion can represent a rate limiting step. However, in this case, facile separation and recycling of nanoparticle materials from reaction media still remains a challenge, as they are often colloidal and therefore easily dispersed in liquid media by Brownian motion. This issue can be addressed by using magnetic supports, allowing the catalyst to be easily separated from the liquid reaction media with application of an external magnetic field. Recently, we demonstrated that the combination of functionalized, superparamagnetic nanoparticles with traditional gravimetrically recovered catalysts allows for the promotion of one-pot, multi-step catalytic reactions with complete recovery of the various catalysts in pure form [7]. The synthesis and use of superparamagnetic nanoparticles such as spinel ferrite nanoparticles has been intensively investigated over the last few years due to their tunable magnetic properties [8], [9]. In the field of catalysis, magnetic nanoparticles have recently been utilized as catalyst supports for organic transformations such as olefin hydroformylation [10], nitrobenzene hydrogenation [11], olefin hydrogenation [12], Suzuki cross-coupling [13], asymmetric hydrogenation [14], and biocatalytic transformations [15].
The Knoevenagel condensation of aldehydes with compounds containing activated methylene groups is one of the most useful and widely employed methods for carbon–carbon bond formation with numerous applications in the synthesis of fine chemicals [16] as well as heterocyclic compounds of biological significance [17]. Conventionally, this reaction is catalyzed by weak bases like primary, secondary, and tertiary amines under homogeneous conditions, which often requires upwards of 40 mol% catalyst with the attendant difficulties in catalyst recovery and recycling [18]. Over the last decades, various solid-supported catalysts have been applied to this reaction such as aminoalkylsilane functionalized silica [19], tetraalkylammonium hydroxide-immobilized MCM-41 [20], guanidine-immobilized MCM-41 or SBA-15 [21], [22], ammonia-grafted FSM-16 [23], ammonia-treated zeolites [24], basic metal-exchanged zeolites [25], MCM-48 with silicon oxynitride frameworks [26], and silicate-organic composite materials [27]. Additionally, Knoevenagel reactions catalyzed by Lewis acids have also been reported [28], [29].
As noted above, we recently demonstrated a combination of catalysts recovered by magnetic, gravimetric, and membrane methods in multi-step, one-pot reactions with recovery of each individual catalyst [7]. In this work, we wish to report the utilization of diamine-functionalized superparamagnetic spinel ferrite nanoparticles as efficient heterogeneous catalysts for low temperature liquid phase reactions with, in principle, no rate contribution associated with internal diffusional resistances. The Knoevenagel reaction of malononitrile with aromatic and heteroaromatic aldehydes is utilized as a well-known model reaction under very mild conditions. High activity is observed and the magnetic catalyst is easily isolated from the reaction mixture by simple magnetic decantation and reused without significant degradation in activity. The performance of the magnetic base catalyst in the Knoevenagel reaction is directly compared with diamine-functionalized hexagonal mesoporous SBA-15 materials of different pore diameters and cubic mesoporous MCM-48 as typical porous silica catalysts that represent a current benchmark in low temperature, liquid phase fine chemical catalysis.
Section snippets
Materials and instrumentation
Cobalt(II) chloride (Alfa Aesar, anhydrous, 99.5%), iron(II) chloride (Alfa Aesar, anhydrous, 99.5%), acetone (Acros, 99%), ammonium hydroxide (Fisher, 29%, v/v, aqueous solution), benzaldehyde (Aldrich, anhydrous, 99%), benzene (Aldrich, anhydrous, 99%), 4-chlorobenzaldehyde (Acros, 98%), ethanol (Fisher, 99%), 2-furaldehyde (Acros, 99%), n-hexadecyltrimethylammonium (Aldrich), hexamethyldisilazane (HMDS) (Aldrich, 99%), hexanes (Fisher, 99%), hydrochloric acid (HCl) (JT Baker; A.C.S.
Catalyst synthesis and characterization
Cobalt spinel ferrite nanoparticles were synthesized following a microemulsion method [30]. It was previously reported that magnetic nanoparticles synthesized in basic aqueous media are covered with a number of surface hydroxyl (–OH) groups [34]. The hydroxyl groups on the surface of the magnetic nanoparticles were then enriched with an aqueous solution of ammonia, facilitating the surface modification step. The resulting nanoparticles were functionalized via silane chemistry with N
Conclusions
In conclusion, cobalt spinel ferrite magnetic nanoparticles were readily synthesized and functionalized with a diamine moiety via silane chemistry to create surface basic sites. The basic magnetic nanoparticles were used as efficient heterogeneous catalysts for the Knoevenagel condensation reaction of several aromatic aldehydes with malononitrile under mild conditions. Modification of the acidity of the magnetic nanoparticle support resulted in a significant effect on catalytic activity. It was
Acknowledgements
The US DOE Office of Basic Energy Sciences is acknowledged for financial support through Catalysis Science Contract No. DE-FG02-03ER15459. DuPont is also thanked for a Young Professor Award. CSG thanks Dr. Yolande Berta for the TEM images and Mehmet Kutukcu for the XRD pattern.
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2022, Inorganica Chimica ActaCitation Excerpt :It is a promising method, especially from an industrial perspective, due to its simplicity, high efficiency and environmental friendliness, given that only water is produced as a by-product [43]. This reaction is usually catalyzed by weak bases [39–41], such as NH3, pyridine, piperidine [43] and other amines [39,40,43], urea [43] and basic salts [42], under homogeneous conditions [39,40,43]. Knoevenagel reactions catalyzed by Lewis acids [39,40,43] have also been reported.