Zirconium Phosphate Nanoparticles and Its Application in the Protection of Aldehydes

Introduction

Layered zirconium hydrogen phosphate with an α- type structure, Zr(HPO4)2∙H2O, is one of the most important inorganic materials.

The α-zirconium phosphate (ZrP) is a suitable heterogeneous catalyst, excellent ion exchanger, an interesting intercalating agent, which also has been used in nanocomposite, proton conductor for fuel cells, drug delivery and immobilization of biological materials.

The crystalline ZrP possesses weak and strong Brönsted and Lewis acid centers which are attributed to P-OH groups and the Zr4+, respectively. Due to the presence of high hydroxyl group density on ZrP surface which can be assumed as hooks, various organic functional groups could adsorb on, allowing to control both the reactivity and selectivity of the reaction(Fig. 1). 

 

The protection of aldehydes, as acetals, acylals, oxathioacetals, or dithioacetals, is a common practice for manipulation of other functional groups during multisteps synthesis. Geminal diacetates (acylals) are one of the essential carbonyl protecting groups due to their stability under neutral, basic, and acidic conditions.

Besides, they can be easily converted into parentaldehydes, which are frequently used as protecting groups for aldehydes. Furthermore, the acylals functionality can be converted into other useful functional groups by reaction with appropriate nucleophiles, the diacetates of α, β-unsaturated aldehydes serve as important precursors for Diels–Alder

reactions and also are useful intermediates in industries, such as cross linking agent for cellulose in cotton or used as stain-bleaching agents .

Because of their unique properties as protective groups as well as important synthons, methods for efficient, mild, and facile synthesis and cleavage of acylals have received considerable attention.

 

A novel method for the preparation of α-zirconium phosphate (ZrP) nanoparticles as an eco-friendly and recyclable heterogeneous catalyst was studied. 

Characterization of the catalysts

Pyridine has been verified to be an excellent FTIR spectroscopy probe to characterize the nature of the acid sites of catalyst and the resulting IR spectrum is shown in Fig. 2.

The pyridine‐desorbed FTIR spectra of ZrP showed characteristic strong bands about 1446 cm-1 which is assigned to the coordinated pyridine in Lewis acid sites. The pyridinium ions are formed by the transfer of protons from the P-OH groups in the ZrP to the organic base. The absorption peak about 1630 and 1541 cm-1are caused by pyridine adsorbed on Brönsted acid sites. Additionally, the band at 1488 cm-1 indicates the combination band between those adjacent Lewis and Brönsted acid sites at 1541 and 1446 cm-1respectively. 
 

It is obvious from the figure that the prepared ZrP shows a higher number of Brønsted acid sites accompanied by low amounts of Lewis acid sites. The morphology of the ZrP nanoparticles was characterized by SEM  and TEM. Typically, the SEM micrograph of ZrPs shows the hexagonal plates in which the disks had well-defined shapes with very smooth surface, solid layers are placed parallel, showing good crystallinity (Fig. 3 a and b).

 

Synthesis of 1,1-diacetates

In order to find the most appropriate reaction conditions and evaluate the catalytic efficiency of ZrP on the protection of aldehydes to the corresponding 1,1-diacetates, we tried to convert benzaldehyde (5 mmol) to its corresponding acylal with ZrP (0.33 mol%) and AA (10 mmol) in various solvents and also under solvent-free condition. We observed that the yield of the reaction under solvent-free condition is higher and the reaction time is shorter as compared to the other methods (Table 1).

 

A comparative studies with some solid acids
 

In order to compare the catalytic potentiality of ZrP nanoparticles, we have shown the results of the synthesis of acylal from benzaldehyde in the presence of various catalysts with respect to the amounts of AA, reaction time, and the yield of the products.

The results show that, ZrP is an equally or competitive more efficient catalyst for this reaction with regards to
reaction conditions and yield.Moreover, this procedure offers advantages over some of the methodologies in terms of efficiency, deprotection, as well as protection,the reusability of the catalyst. Thus, the results suggest that ZrP can be considered as one of the best choices for selecting an economically convenient, user friendly catalyst.