Modification of silicalite-1 with ammonium compounds aimed at preparation of acidic catalyst for acetalization of glycerol with acetone
Graphical abstract
Introduction
Silicalite-1, the alumina-free MFI structure, known of its rather negligible acidity, recently gains an interest as a potential catalyst. Even though the acidity of these materials is low, they show relatively high activity for the reactions requiring weak acidic sites such as toluene alkylation [1], propionic acid ketonization [2], Beckmann rearrangement [3] and some others [4,5]. Activity of silicalite-1 in acid catalyzed reaction is associated with the formation of framework defects (silanol nests) created by removal of some silicon atom [2]. The remaining oxygen atoms interact with hydrogen atoms forming the silanol nests (Scheme 1). According to some authors, the silanol nests behave as weak Brønsted sites [6,7] in contrast to isolated external hydroxyl groups, which are not acidic. Syntheses of silicalite-1 presented in the literature were performed under various conditions [8] and in the presence of diverse modifiers, which influenced the nature of resulting silicalite-1 surface active sites, including the acidic ones [9]. Preparation of silicalite-1 in the presence of fluoride medium (NH4F used as one of the reactants for silicalite-1 synthesis) resulted in negligible amounts of defects, whereas hydroxide route causes to obtain silicalite-1 with a higher amount of defects [9]. On the other hand, Lanzafame et al. [6] and also Janiszewska et al. [10] indicated that post-synthesis modification with an aqueous solution of ammonium compounds with different pH (NH4NO3/NH4OH or NH4Cl) influenced the nature, number and strength of acidic sites generated in silicalite-1. Kitamura et al. [11] has shown that treatment of silicalite of MFI structure, with aqueous solution of ammonium salts and at least one basic compound (e.g. aqueous ammonia solution, alkylamines, allylamines or alkylammonium hydroxide) generates acid sites, efficient in the catalytic Beckmann rearrangement reaction. The increase in the activity was connected with the formation of H-bonded silanol defect groups of a different kind indicating an acidic nature (Scheme 1) [10,12]. It was also shown that the treatment of zeolites with a small amount of NH4F with subsequent calcination to introduce the fluorine atoms resulted in increased acid strength of existing acid sites, due to the inductive effects of highly electronegative fluorine, and influenced their catalytic activity [13]. Recently, the treatment of zeolites with concentrated solution of NH4F allowed to increase the micropore volume in zeolite with small cages (sodalite) by opening these cages [14] or to obtain the hierarchical zeolite [15,16] with improved catalytic activity due to increased accessibility to the active sites. The advantage of such method of preparation of hierarchical zeolites is lack of change of framework composition during modification (due to similar rate of Si and Al extraction) as well as preferential extraction of defect zones in the crystals.
Regarding the above, we have performed a modification of pristine silicalie-1, by means of post-synthesis treatment with solutions of various ammonium agents (NH4NO3, NH4Cl, NH4F, NH4OH) at elevated temperature in order to induce the generation of the acidic sites. Contrary to earlier papers [6,10,11] the modification has been carried out without any additional agents (basic compounds). The obtained materials were examined as catalysts for acetalization of glycerol with acetone to produce solketal (Scheme 2), that is considered as very important oxygenate fuel additive enebling to reduce the emission of particles and improve the cold flow properties of liquid transportation fuels. Solketal helps also to reduce the gum formation, improves the oxidation stability, and enhances the octane number when added to gasoline [17]. The role of acidity of the catalysts dedicated to acetalization of glycerol has been widely discussed in many papers [[18], [19], [20], [21]]. The number [22] and strength [23], as well as the nature of acidic sites (both Lewis and Brønsted) [20,21] were indicated as parameters influencing both glycerol conversion and selectivity to solketal.
Section snippets
Synthesis
Silicalite-1 was prepared according to our earlier procedure [10] with some modification. Tetrapropylammonium bromide (TPABr, 98% Lancaster) and water glass (sodium metasilicate with weight percentage ratio: Na2O/SiO2/H2O = 11.1/27.9/60.8, Chempur, Poland) were used as a template and silica source, respectively. Phosphoric acid (85%, POCh, Poland) was used for adjusting the pH of the initial gel. The molar composition of the synthesis gel was 0.08 TPABr/1 SiO2/20 H2O. Template was dissolved in
Structural and textural characterization
Both pristine silicalite-1 (Sil-1) and the ammonium modified samples (Sil-1_OH; Sil-1_F; Sil-1_Cl and Sil-1_NO3) were characterized by XRD (Fig. 1) and showed correct MFI structure. High crystallinity was also confirmed by IR measurements (Fig. 2, Table 1). It indicates that the used modification procedure allows to maintain the MFI structure. The intensities of the reflections of the modified samples are comparable with those of the unmodified sample, which indicates the similar crystallinity.
Conclusions
- 1
Modification of silicalite-1 by treatment with ammonium salt solutions at a higher temperature (without any additional alkaline agents) results in partial removal of external silanols and in a formation of acidic internal OH groups.
- 2
Structure defects formed during the above transformation play a role of Lewis sites.
- 3
Silanols linked by hydrogen bonds or isolated silanols connected with defected silicon atoms are the source of Brønsted centers of very low strength.
- 4
A synergy of the Lewis and
Acknowledgement
K. Góra-Marek acknowledges the Grant No. 2015/18/E/ST4/00191 from the National Science Centre, Poland.
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