Sustainable heterogeneous acid catalysis by heteropoly acids

doi:10.1016/j.molcata.2006.08.072

Journal of Molecular Catalysis A: Chemical

Volume 262, Issues 1–2, 1 February 2007, Pages 86-92

https://doi.org/10.1016/j.molcata.2006.08.072 Get rights and content

Abstract

Heterogeneous acid catalysis by heteropoly acids (HPAs) has the potential of substantial economic and green benefits. Its application, however, has been limited because of the difficulty of catalyst regeneration due to a relatively low thermal stability of HPAs. The aim of this paper is to discuss the perspectives of acid catalysis by solid HPAs, in particular focusing on several approaches that could help overcome the deactivation of HPA catalysts to achieve sustainable catalyst performance. These approaches include: developing novel HPA catalysts possessing high thermal stability, modification of HPA catalysts to enhance coke combustion, inhibition of coke formation on HPA catalysts during operation, reactions in supercritical fluids and cascade reactions using multifunctional HPA catalysis.

Graphical abstract

Heterogeneous acid catalysis by heteropoly acids offers substantial economic and green benefits. This paper describes approaches that could help overcome the deactivation of HPA catalysts: developing novel HPA catalysts possessing high thermal stability; enhancing coke combustion; coke inhibition on HPA catalysts; reactions in supercritical fluids; and cascade reactions using multifunctional HPA catalysis.

Introduction

Today, catalysis by heteropoly acids (HPAs) is a well-established area [1], [2], [3], [4], [5]. Arguably, it is one of the most successful areas in contemporary catalysis, where systematic studies of HPA catalysts at the molecular level have led to a string of large-scale industrial applications [4]. HPAs possess unique physicochemical properties, with their structural mobility and multifunctionality the most important for catalysis [1], [2], [3], [4]. HPAs, unlike metal oxides and zeolites, have discrete and mobile ionic structure. They possess, on one hand, a very strong Brønsted acidity and, on the other hand, appropriate redox properties, which can be tuned by varying the chemical composition of HPA. Consequently, acid catalysis and catalytic oxidation are the two major areas of catalysis by HPAs.

Although there are many structural types of HPAs [1], [4], [6], the majority of the catalytic applications use the most common Keggin type HPAs, especially for acid catalysis. The Keggin HPAs comprise heteropoly anions of the formula [XM₁₂O₄₀]ⁿ⁻ (α-isomer), where X is the heteroatom (P⁵⁺, Si⁴⁺, etc.) and M is the addenda atom (Mo⁶⁺, W⁶⁺, etc.). The structure of the Keggin anion is composed of a central tetrahedron XO₄ surrounded by 12 edge- and corner-sharing metal-oxygen octahedra MO₆ (Fig. 1). Most typical HPAs, such as H₃PW₁₂O₄₀, H₄SiW₁₂O₄₀, H₃PMo₁₂O₄₀ and H₄SiMo₁₂O₄₀, are commercially available.

Presently, HPAs are used as catalysts in several industrial processes [4], the most important shown in Table 1. The top two are heterogeneously catalysed selective oxidations in gas phase: the oxidation of methacrolein to methacrylic acid and ethylene to acetic acid. The rest are acid-catalysed reactions including the homogeneous liquid-phase hydration of olefins, the biphasic polymerisation of THF to poly(tetramethylene glycole) and the gas-phase synthesis of ethyl acetate from ethylene and acetic acid (BP's AVADA process).

Heterogeneous acid catalysis by HPAs has the potential of a great economic reward and green benefits—hence much interest in it [1], [2], [3], [4], [5]. The acidity of HPAs is stronger than that of the conventional solid acid catalysts (e.g., acidic oxides and zeolites), decreasing in the order: H₃PW₁₂O₄₀ > H₄SiW₁₂O₄₀ > H₃PMo₁₂O₄₀ > H₄SiMo₁₂O₄₀ [1], [2], [4]. The acid sites in HPA are more uniform and easier to control than those in other solid acid catalysts. Being stronger acids, HPA are generally more active catalysts than the conventional solid acid catalysts, which allows efficient operating under milder conditions. However, there is a serious problem to HPA catalysts—their low thermal stability, hence limited reaction temperature and, especially, difficulty of regeneration of solid HPA catalysts (decoking) [2], [4]. The thermal stability of Keggin HPAs, defined as the temperature at which all acidic protons are lost, decreases in the order: H₃PW₁₂O₄₀ (465 °C) > H₄SiW₁₂O₄₀ (445 °C) > H₃PMo₁₂O₄₀ (375 °C) > H₄SiMo₁₂O₄₀ (350 °C), the strongest acid H₃PW₁₂O₄₀ being the most stable [2], [4].

Fig. 2 shows the TGA profile for H₃PW₁₂O₄₀ hydrate [4]. Three main peaks can be observed: (1) a peak at a temperature below 100 °C corresponding to the loss of physisorbed water (a variable amount depending on the number of hydration waters in the sample); (2) a peak in the temperature range of 100–280 °C centred at about 200 °C accounted for the loss of ca. 6H₂O molecules per Keggin unit, corresponding to the dehydration of a relatively stable hexahydrate H₃PW₁₂O₄₀·6H₂O, in which the waters are hydrogen-bonded to the acidic protons to form the [H₂O⋯H⁺⋯OH₂] ions; and (3) a peak in the range of 370–600 °C centred at 450–470 °C due to the loss of 1.5H₂O molecules corresponding to the loss of all acidic protons and the beginning of decomposition of the Keggin structure. For tungsten HPAs, the latter loss is practically irreversible, which causes irreversible loss of catalytic activity. The decomposition is complete at about 610 °C to form P₂O₅ and WO₃, which is shown by an exotherm in DTA and DSC [4], [7]. Therefore, the thermal decomposition of H₃PW₁₂O₄₀ follows the course:

Coke formation is the most frequent cause of catalyst deactivation in heterogeneous acid-catalysed organic reactions [8], [9], [10], [11]. Therefore, much research has been carried out on coke formation on the catalysts for petrochemical processes (cracking, reforming, hydrotreatment, etc.). The most studied catalysts include amorphous silica–alumina, zeolites and acidic alumina, as well as those doped with metals such as palladium, platinum and nickel [8], [9], [10], [11]. Catalyst regeneration (decoking) is usually carried out by coke combustion at 450–550 °C [8], [9], [10], [11]. For the oxide and zeolite catalysts possessing sufficient thermal stability, the combustion is an effective method to recover catalyst activity. Solid HPA catalysts in organic reactions, like the conventional solid acid catalysts, suffer from deactivation by coking. Little information is available about coke formation on HPA catalysts though. For the coked HPA catalysts, the problem is that the standard catalyst regeneration by coke combustion is not applicable due to the low thermal stability of HPAs, which makes coking the most serious problem for heterogeneous acid catalysis by HPAs [2], [4]. Other possible causes of HPA deactivation, such as poisoning, aggregation, dehydration and decomposition of HPA, could also play a role, but these are not as crucial as the coking, at least at moderate reaction temperatures 100–300 °C.

The question is how to overcome the problem of coking and make heterogeneous acid catalysis by HPA sustainable? Several directions that may be instrumental to achieve this goal will be discussed here. These are: developing novel HPA catalysts possessing high thermal stability; modification of HPA catalysts to enhance coke combustion; inhibition of coke formation on HPA catalysts during operation; reactions in supercritical fluids; and cascade reactions using multifunctional HPA catalysis.

Section snippets

Development of novel HPA catalysts possessing high thermal stability

There has been considerable activity in this direction recently, with the main focus on oxide composites comprising W(VI) polyoxometalates and niobia or zirconia ([12], [13] and references therein). The composites are usually prepared by wet chemical synthesis, followed by calcination at 500–750 °C, i.e., at temperatures higher than the temperature of HPA decomposition. The materials thus made contained HPA precursors or HPA decomposition products, possessing Brønsted and Lewis acid sites of

Modification of HPA catalysts to enhance coke combustion

Doping of solid acid catalysts with platinum group metals (PGM) such as Pd and Pt is well known to enhance catalyst regeneration by coke combustion. For example, this method has been used for zeolite and alumina catalysts for alkane isomerisation and cracking [8], [9]. We have found that the PGM doping is also effective for enhancing the regeneration of solid HPA catalysts [14], [15], [16], [17].

The effect of Pd doping on coke combustion is apparent from the TGA/TPO for the coked 20% H₃PW₁₂O₄₀

Inhibition of coke formation on HPA catalysts

Obviously, it is much better to prevent the catalyst from coking to avoid its regeneration in the first place. In collaboration with BP, we studied coke inhibition in HPA catalysed propene oligomerisation as a model reaction [14], [15]. The reaction occurs via the carbenium ion mechanism yielding propene oligomers and coke (Scheme 1). The oligomers may be considered as coke precursors. Addition of nucleophilic molecules, such as water, methanol and acetic acid, was found to greatly affect the

Reactions in supercritical fluids

Heterogeneous catalysis in supercritical fluids (SCFs) offers considerable benefits (for a review, see [22]). Use of SCFs in heterogeneous catalysis can enhance the reaction rate, selectivity control and product separation, intensify mass and heat transfer, facilitate catalyst regeneration and increase catalyst lifetime. SCFs possess unique solvent properties which have long been utilized in separation technology (extraction and chromatography) and are now gaining increasing interest for

Cascade reactions using multifunctional HPA catalysts

The development of one-pot cascade processes without intermediate separation steps using multifunctional catalysts is an important strategy to carry out sustainable organic synthesis with a high atom and energy efficiency [27], [28]. Multifunctional catalysts contain two or more catalytic functions (acid, base, metal, etc.) acting synergistically to carry out a cascade reaction. There is evidence that a combination of HPA acid catalysis and a redox catalysis in a heterogeneous cascade process

Conclusions

Heterogeneous acid catalysis by heteropoly acids offers substantial economic and environmental benefits. However, the relatively low thermal stability of HPAs is a serious problem to the HPA catalysis due to the difficulty of regeneration of solid HPA catalysts (decoking). Several approaches can be instrumental in overcoming the deactivation of HPA catalysts to achieve sustainable catalyst performance. One of these may be the development of novel HPA materials possessing high thermal stability.

References (29)

T. Okuhara et al.
Adv. Catal.
(1996)
M.N Timofeeva
Appl. Catal. A
(2003)
J. Barbier
Stud. Surf. Sci. Catal.
(1987)
E. Furimsky et al.
Catal. Today
(1993)
M. Guisnet et al.
Stud. Surf. Sci. Catal.
(1994)
H.G. Karge
Stud. Surf. Sci. Catal.
(2001)
K. Okumura et al.
J. Catal.
(2005)
B.M. Devassy et al.
J. Catal.
(2005)
I.V. Kozhevnikov et al.
Appl. Catal. A
(2001)
E.F. Kozhevnikova et al.
Appl. Catal. A
(2004)

M. Musawir et al.

J. Mol. Catal. A

(2007)

I.V. Kozhevnikov

Appl. Catal. A

(2003)

M.J. Howard et al.

Stud. Surf. Sci. Catal.

(1999)

P.Y. Gayraud et al.

Catal. Today

(2000)

Cited by (244)

Synthesis and structural characterization of HPW-doped niobium pillared Brazilian clay
2024, Microporous and Mesoporous Materials
Natural clay lamellar structure can be controlled through various treatments, such as pillaring with metallic oxides. In this work, lamellar-structured Brazilian montmorillonite was a raw material for developing a new Nb-pillared clay dopped with phosphotungstic acid (HPW). The pristine clay was exchanged with CTAB to obtain an organophilic intermediate, which was further submitted to an Nb source treatment, followed by HPW doping and calcination. XRD, textural and thermal analysis, electronic scanning spectroscopy (SEM) with EDS microscopy, ²⁷Al and ³¹P solid-state nuclear magnetic resonance spectroscopy, and infrared of pyridine adsorption-desorption acidity properties were employed in the characterization of this new class of pillared clay. The XRD analyses showed the maintenance of the lamellar structure after the calcination step at 400 °C, as planes 001, 110, 004, and 130 are visible. The ³¹P MAS NMR spectrum of the ARGPil-Nb-HPW sample showed a single signal centered at −14.9 ppm, the same chemical shift of the phosphotungstic acid. The absence of additional peaks confirmed the maintenance of the Keggin hetero-polyacid structure in the Nb pillared clay. The pillarization process led to a significant increase in the specific area from 58 m²/g to 111 m²/g in ARGPil-Nb-HPW, suggesting that niobium acts as a pillar of Nb₂O₅, covalently binding to the tetrahedral sheets, as evidenced by the FTIR spectra, and keeping them separated, which may favor catalytic processes that require more significant space for molecules to access the internal surfaces of materials and, therefore, to the Brønsted sites and especially to the strong Lewis sites. The catalysts showed strong Lewis sites by retaining chemically adsorbed pyridine after heating at 250 °C. The pillared catalyst showed activity in the esterification of mandelic acid with methanol at ambient temperature, which was maintained after three cycles.
Role of citric acid in enhancing aromatic hydrogenation performance in Ni-HPW/SiO<inf>2</inf> catalysts
2024, Applied Catalysis A: General
In this study, we utilized a sol-gel method to prepare Ni-HPW/SiO₂ catalysts with varying citric acid (CA):Ni molar ratios ranging from 0 to 5.0. The catalysts demonstrated different levels of activity in the hydrogenation of ethylbenzene, which correlated to their respective CA:Ni ratios. Introducing citric acid in the preparation process of nickel-based catalysts yielded mesopore formation and reduced NiO particle size, producing a more uniform NiO dispersion on SiO₂, which was confirmed by BET, XRD, and TEM analyses. The results of H₂-TPR, H₂-TPD, NH₃-TPD, and Py-IR analyses suggested that the addition of citric acid augmented the interaction between phosphotungstic acid (HPW), Ni species, and SiO₂. Citric acid improved Ni species and HPW dispersion, fostering increased HPW-Ni species formation. This, in turn, generated additional electron-deficient Ni ions, stemming from semi-reduced HPW-Ni species, which acted as Lewis acid sites, enhancing Ni⁺ species effectiveness in activating aromatic rings. The better-dispersed, non-HPW-bound NiO, which, upon complete reduction to Ni, led to a substantial rise in H₂ bond cleavage, thereby facilitating more hydrogen spillover. Overall, the infusion of citric acid into fabricating Ni-HPW/SiO₂ catalysts sparked a significant surge in their aromatic hydrogenation ability.
Green synthesis of novel pyrano[2,3-c]pyrazole-5-carbonitrile analogues by using Fe<inf>5</inf>(PW<inf>10</inf>V<inf>2</inf>O<inf>40</inf>)<inf>3</inf> nanocatalyst through a one-pot Knoevenagel condensation and Michael addition mechanism
2024, Inorganic Chemistry Communications
A swift and green one-pot method was improved for the preparation of new pyranopyrazole analogues using iron-doped heteropoly acid [(Fe₅(PW₁₀V₂O₄₀)₃] (FePWV) as an efficient catalyst at room temperature and ethanol as the solvent. The one-pot, multi-component reaction combining substituted aldehyde, malononitrile,ethyl acetoacetate, and hydrazine hydrate produces remarkable yields (varying from 88 % to 98 %) for this highly selective technique. Using P-XRD, TGA, BET, FT-IR, TEM, and SEM studies, the synthesized FePWV catalyst was fully characterized. Several spectroscopic methods, including ¹H NMR, ¹⁵N NMR, ¹³C NMR, and HRMS, were used to confirm the target molecules' structural information.This environmentally benign method also has other benefits, such as operational simplicity, a stable catalyst with great reusability (up to 7 times), quick reaction times (<20 min), and the absence of chromatographic separations. The proposed method is sustainable and affordable as a result of these properties taken together.
Polydopamine modified heteropolyacids catalyst for methacrolein oxidation: Organic ammonia functionalized and regulation of morphology
2023, Molecular Catalysis
The oxidation of methacrolein (MAL) to methacrylic acid (MAA) catalyzed by Keggin-type phosphomolybdovanadic acid catalyst belongs to the acid-catalyzed and oxygen-catalyzed co-promoted reactions. It is a challenge to design catalysts with suitable acidity and redox properties. In this paper, a strategy using organic ammonia to modify CsH₃PMo₁₁VO₄₀ (CsPAV) was proposed. Core-shell polymerization precursors were synthesized by using polydopamine (PDA) microspheres as modifiers and templates, and regularly arranged PDA-CsPAV bowl-shaped catalysts were obtained via one-step calcination. The morphology, structure, thermal stability, acidity and redox properties of PDA-CsPAV catalysts were characterized by XRD, FT-IR, TG-DTA, XPS, SEM, H₂-TPR and NH₃-TPD, and the catalytic performance was evaluated by oxidation of MAL to MAA. The rich functional groups on the surface of PDA microspheres combine with H₄PMo₁₁VO₄₀ (HPAV) through strong coordination bonds to form a catalyst film. The uniform spherical structure of PDA can adjust the dispersion and morphology of CsPAV. NH₄⁺ was formed and rearranged with heteropoly anions during calcination, which positively regulated the acidity and redox property of the catalyst. The addition of PDA accelerated electron transfer and strengthened the self-reduction process of HPAV. Compared with CsPAV, PDA-CsPAV has more acid sites and oxidation catalytic active centers (VO²⁺), thus exhibiting better MAL oxidation performance and good thermal stability.
Graphite supported heteropolyacid as a regenerable catalyst in the dehydration of 1-butanol to butenes
2023, Catalysis Today
1-butanol dehydration reaction has recently emerged as a sustainable route to produce butenes which can be further oligomerized to be applied as jet fuel. However, the high catalyst deactivation rates observed during this reaction due to coke deposition is still a pending matter. As promising catalysts for this reaction, we have supported two heteropolyacids (HPA), i.e. H₄SiW₁₂O₄₀ (STA) and H₃PW₁₂O₄₀ (TPA), on two commercial carbon materials: an activated carbon (AC) and a high surface area graphite (HSAG). Aiming to evaluate the role of HPA-support interactions, the STA was also dispersed over metallic oxides of different acidic nature, namely SiO₂, Al₂O₃ and ZrO₂. An exhaustive physicochemical characterization revealed that after the HPA dispersion thorough the support, the Keggin structure was maintained and an increase in the amount and strength of acid sites was provoked, but in different degree according to the HPA type and support’s nature. While the TPA-based catalysts developed less quantity of total acid sites, but higher strength than their STA-carbon counterparts, the STA/AC and TPA/AC samples exhibited a slight major amount of acid sites than STA/HSAG and TPA/HSAG. The HPA-support interactions have ultimately modulated to some extent the activity, selectivity, stability and regeneration ability of the synthesized catalysts, when applied in the gas phase butanol dehydration reaction at 275 °C. The higher STA decomposition temperature prompted by the graphitic support, among other factors, allowed the total regeneration of the highly active (39 mmol_BuOH∙min⁻¹∙g_a.p) and n-butenes selective (>98 %) STA/HSAG catalyst by means of combustion of carbon deposits at 400 °C.
Recent advances in polyoxometalates acid-catalyzed organic reactions
2023, Chinese Chemical Letters
Polyoxometalates (POMs) have conducive properties such as controlled Brønsted and Lewis acidity, high thermal stability, nontoxic nature, tunable solubility, and less corrosiveness. POMs have been extensively applied in catalytic organic reactions and have an exciting prospect for industrial applications. This review summarized recent progress in the application of POMs as acid catalysts for various organic reactions including CC bond formation, CN bond formation, CO bond formation, heterocyclic synthesis reactions, cyanosilylation and hydrolysis reactions. Various POMs catalysts including heteropoly acids (HPAs) and cationic functionalized HPAs with Brønsted acidity, HPAs supported on non-precious metal support with Brønsted acidity (or both Brønsted and Lewis acidity), transition metal substituted POMs with Lewis acidity were applied in above reactions. This review attempts to provide up-to-date information about POMs acid-catalyzed organic reactions and propose future prospects.

View all citing articles on Scopus

View full text

Sustainable heterogeneous acid catalysis by heteropoly acids

Abstract

Graphical abstract

Introduction

Section snippets

Development of novel HPA catalysts possessing high thermal stability

Modification of HPA catalysts to enhance coke combustion

Inhibition of coke formation on HPA catalysts

Reactions in supercritical fluids

Cascade reactions using multifunctional HPA catalysts

Conclusions

Adv. Catal.

Appl. Catal. A

Stud. Surf. Sci. Catal.

Catal. Today

Stud. Surf. Sci. Catal.

Stud. Surf. Sci. Catal.

J. Catal.

J. Catal.

Appl. Catal. A

Appl. Catal. A

J. Mol. Catal. A

Appl. Catal. A

Stud. Surf. Sci. Catal.

Catal. Today