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Zirconium Phosphate Catalysts in the XXI Century
Abstract
An overview on the developments of zirconium phosphate (ZrP) and its organic derivatives in heterogeneous catalysis in recent years is reported in the present review.
Two basic aspects have been emphasized: first, the catalytic properties of zirconium phosphates were discussed, with particular attention to the effect of surface acidity and hydrophobic/hydrophilic character, textural properties, and particle morphology on the catalytic performances. Then, the use of zirconium phosphates as support for catalytic active species was reported, including organometallic complexes, metal ions, noble metal, and metal oxide nanoparticles.
Zirconium phosphate plays, in those cases, a dual role, since it promotes the dispersion and stabilization of the catalysts, thanks to their interaction with the active sites on the surface of ZrP, and facilitates the recovery and reuse of the catalytic species due to their immobilization on the solid support.
1. Introduction
The chemistry of tetravalent metal phosphates and phosphonates has passed through the last sixty years, leaving important footprints in many fields. The key topic of this special issue is the use of zirconium phosphates in catalysis and this minireview aims to collect the main contributions in this field since 2010 to date.
In order to better understand the roles of zirconium phosphates in heterogeneous catalysis, two sections will be devoted to the use of zirconium phosphates in catalysis; the first section deals with the role as catalyst itself in acid-catalyzed reactions, while the second investigates the use of zirconium phosphate as support for catalytic active species.
The main aspects which make zirconium phosphate interesting in catalysis are:
- the presence of a robust inorganic structure which enables its application in relatively high temperature reactions;
- the presence of active sites on the particle surface whose features can be tuned according to the desired properties, by controlling the synthetic conditions or by introducing suitable functional groups covalently bonded the particle surface; they can be Bronsted acid or basic sites, Lewis basic sites, or metal coordinating sites.
- the possibility to control the textural properties, the surface area, and the porosity degree.
All these aspects are fundamental to the design of an efficient solid catalyst. Starting from the state of the art, this special issue gives the opportunity to take a look at the future of zirconium phosphate-based catalysis
2. Zirconium Phosphate: Structural Features and Functionalization Strategies
Inorgano-organic zirconium phosphate/phosphonates, with the general formula Zr(O3PR)x(O3PR’)2-x, where R and R’ are an inorganic (e.g., –H, –OH) and/or organic (e.g.,–CH3, –C6H5, –C6H4SO3H, –O(CH2), –CH3, etc.) groups, are generally synthesized by the same procedures for the synthesis of -ZrP, by using phosphonic acid solutions, or phosphoric/phosphonic
3. Zirconium Phosphate/Phosphonates as Heterogeneous Catalysts
The catalytic properties of ZrP are well known and have been extensively investigated, especially in acid-catalyzed reactions such as dehydration, isomerization, and esterhydrolysis reactions, due to its high thermal stability, high water tolerance ability, and easy recovery.
4. Zirconium Phosphates/Phosphonates as Solid Supports for Catalytically Active Species
An important key issue in catalysis is the development of catalysts with high thermal and chemicalstability, and ease of recovery.
The optimization of these properties can be addressed by immobilizing the catalyst on a specific and suitable solid support.
Zirconium phosphates and phosphonates have been employed as supports for several catalytic active species among which are organometallic complexes and metal ions, noble metal nanoparticles, and metal oxide nanoparticles.
An overview on the developments of zirconium phosphate (ZrP) and its organic derivatives in heterogeneous catalysis in recent years is reported in the present review.
Two basic aspects have been emphasized: first, the catalytic properties of zirconium phosphates were discussed, with particular attention to the effect of surface acidity and hydrophobic/hydrophilic character, textural properties, and particle morphology on the catalytic performances. Then, the use of zirconium phosphates as support for catalytic active species was reported, including organometallic complexes, metal ions, noble metal, and metal oxide nanoparticles.
Zirconium phosphate plays, in those cases, a dual role, since it promotes the dispersion and stabilization of the catalysts, thanks to their interaction with the active sites on the surface of ZrP, and facilitates the recovery and reuse of the catalytic species due to their immobilization on the solid support.
1. Introduction
The chemistry of tetravalent metal phosphates and phosphonates has passed through the last sixty years, leaving important footprints in many fields. The key topic of this special issue is the use of zirconium phosphates in catalysis and this minireview aims to collect the main contributions in this field since 2010 to date.
In order to better understand the roles of zirconium phosphates in heterogeneous catalysis, two sections will be devoted to the use of zirconium phosphates in catalysis; the first section deals with the role as catalyst itself in acid-catalyzed reactions, while the second investigates the use of zirconium phosphate as support for catalytic active species.
The main aspects which make zirconium phosphate interesting in catalysis are:
- the presence of a robust inorganic structure which enables its application in relatively high temperature reactions;
- the presence of active sites on the particle surface whose features can be tuned according to the desired properties, by controlling the synthetic conditions or by introducing suitable functional groups covalently bonded the particle surface; they can be Bronsted acid or basic sites, Lewis basic sites, or metal coordinating sites.
- the possibility to control the textural properties, the surface area, and the porosity degree.
All these aspects are fundamental to the design of an efficient solid catalyst. Starting from the state of the art, this special issue gives the opportunity to take a look at the future of zirconium phosphate-based catalysis
2. Zirconium Phosphate: Structural Features and Functionalization Strategies
Regarding catalysis, many efforts have been devoted to the synthesis of zirconium phosphate with mesoporous textural properties, providing large surface area and a high number of surface active sites.
Inorgano-organic zirconium phosphate/phosphonates, with the general formula Zr(O3PR)x(O3PR’)2-x, where R and R’ are an inorganic (e.g., –H, –OH) and/or organic (e.g.,–CH3, –C6H5, –C6H4SO3H, –O(CH2), –CH3, etc.) groups, are generally synthesized by the same procedures for the synthesis of -ZrP, by using phosphonic acid solutions, or phosphoric/phosphonic
acid solutions as source of ligands for Zr(IV).
Phosphonate groups bearing sulfonic acid groups are generally used to increase the catalyst surface acidity, while hydrophobic phosphonates are covalently linked to the inorganic layer in order to modulate the hydrophobic/hydrophilic character of the catalyst surface.
Phosphonate groups bearing sulfonic acid groups are generally used to increase the catalyst surface acidity, while hydrophobic phosphonates are covalently linked to the inorganic layer in order to modulate the hydrophobic/hydrophilic character of the catalyst surface.
3. Zirconium Phosphate/Phosphonates as Heterogeneous Catalysts
The catalytic properties of ZrP are well known and have been extensively investigated, especially in acid-catalyzed reactions such as dehydration, isomerization, and esterhydrolysis reactions, due to its high thermal stability, high water tolerance ability, and easy recovery.
4. Zirconium Phosphates/Phosphonates as Solid Supports for Catalytically Active Species
An important key issue in catalysis is the development of catalysts with high thermal and chemicalstability, and ease of recovery.
The optimization of these properties can be addressed by immobilizing the catalyst on a specific and suitable solid support.
Zirconium phosphates and phosphonates have been employed as supports for several catalytic active species among which are organometallic complexes and metal ions, noble metal nanoparticles, and metal oxide nanoparticles.
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