Acid And Superacid Solid Materials As Noncontaminant Alternative Catalysts In Refining

Intituto Tecnología Química, UPV-CSIC, Valencia, Spain

INTRODUCTION

New and pending industrial legislation throughout the world requires more stringent environmental protection. Thus, it will soon become illegal to release into the atmosphere some products (benzene, volatile hydrocarbons, carbon oxides, corrosive and reactive oxides of sulfur, and nitrogen) that contribute to the greenhouse effect. Due to new environmental legislation, many refiners will be required to make significant processing changes. In this way, new catalytic technologies will help to protect the ozone layer, to combat the greenhouse effect, and to solve environmental problems of energy, and so forth.

However, the catalyst is in other cases the noxious agent. A clear example is the use of liquid acids, that is, H2SO4, H3PO4, HClO4, or HF, as catalysts in commercial processes: phenol production from cumene hydroperoxide (diluted H2SO4), production of caprolactame (H2SO4 oleum), isobutene/butanes alkylation (HF, H2SO4), polymers derived from aniline, and hydration of olefins. In 1989, 12 million tons of phosphoric acids and 44 million tons of sulphuric acid were used in United States. Albeit not all of these acids were used for catalysis, so the

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amount of liquid acids as catalysts should be greatly reduced in the succeeding years.

Classic superacid solutions are mixtures of strong acids like FSO3H or HF with Lewis acids: SbF5, TaB5, or BF3.: Besides their high acid strength, such liquids also present some difficulties (corrosion of the reactor and other parts of the plant, difficulty in catalyst regeneration, difficulty in separating the catalysts from the reaction mixture, high cost of environmentally safe disposal) that considerably limit their large-scale use.

The first step to reduce the negative impact of the liquid acids is to use the acid, superacid (as triflic acid, CF3SO3H), or Lewis acid (especially AlCl3 or BF3) impregnated on silica.2 This is the case for paraffin isomerization (old technology), alkylation of benzene with olefins, production of polygasoline, some of the new isobutene/butanes alkylation technologies, and olefin hydration. This way not only could decrease the hazards, but also increase the catalyst life. However, the formation of the corresponding acid indicates that this is not a productive route.

The tendency in the past decades has been to replace them with solid acids (Figure 13.1).3-5 These solid acids could present important advantages, decreasing reactor and plant corrosion problems (with simpler and safer maintenance), and favoring catalyst regeneration and environmentally safe disposal. This is the case of the use of zeolites, amorphous silico-aluminas, or more recently, the so-called superacid solids, that is, sulfated metal oxides, heteropolyoxometalates, or nafion (Figure 13.1).3-12 It is clear that the well-known carbocation chemistry that occurs in liquid-acid processes also occurs on the solid-acid catalysts (similar mechanisms have been proposed in both catalyst types) and the same process variables that control liquid-acid reactions also affect the solid catalyst processes.

Increasing acid strength

1 1 1

Zeolites

Halides

1 1 1 ci/ai2o3 f/ai2o3

I 3

Liquid Acids

1

h2o

Applications

Reforming

Isomerization

Isomerization

Isomerization

Alkylation

Alkylation

Oligomerization

Cracking

Increasing acid strength

Increasing acid strength

Figure 13.1 Industrial processes carried out on oxides, halides, and liquid acid catalysts (After Ref. 5).

However, the use of polyfunctional solid catalysts (involving both red-ox and acid properties) could modify, in some cases, the characteristics of some of the actual industrial processes.

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1).3,5 This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nafion, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions: alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth.3-6

Nafion resin, which is a prefluorinated resinsulfonic acid-based catalyst (represented by -CF2-CF2-SO3H groups), has an acid strength similar to 100% sul-furic acid and shows interesting catalytic properties in acid reactions (Figure 13.2). However, they present an important disadvantage, the low surface area (0.02 m2 g"1 ), which makes it difficult for molecules to access their active sites.7 The accessibility of acid sites in nafion was increased by entrapping nano-sized particles of nafion resin within a higly porous silica network using a sol-gel techniques.8 However, silica with high surface areas are not required in this case, since the sulfonic groups of nafion could interact to a greater extent with the silanol groups of the silica, resulting in a decrease in their catalytic activity.9 So, supported nafion catalysts present acid sites with high acid strength and relatively good stability, although with some problems in their ability to regenerate.

Sulfated

Zeolites Nafion Heteropolyacids metal oxides

Sulfated

Zeolites Nafion Heteropolyacids metal oxides

Heteropolyacids

h2s04 fsojh fsojh-tafs

Figure 13.2 Acid strength of acid and superacid solids determined by Hammet method. For a comparable purpose, the acid strengths of some liquid acids are also included. (After Ref. 12.)

h2s04 fsojh fsojh-tafs

Figure 13.2 Acid strength of acid and superacid solids determined by Hammet method. For a comparable purpose, the acid strengths of some liquid acids are also included. (After Ref. 12.)

Heteropolyacids and related compounds have attracted increasing interest in cat-

They present strong acidities (the pH values of aqueous solutions of heteropolyacids indicate that they are strong acids) both in solid and in liquid solution (Figure 13.2). In addition, they can be prepared in an wide range of surface areas (partially salified heteropolyoxometalates permit the modification of the surface areas of these materials) or be supported in metal oxides.

At the end of the 1970s, new solids materials (especially sulphated metal oxides) with a hypothetical superacid character were reported (Figure 13.2).13 Among these, zirconium oxide treated with sulfate anions is one of the most interesting catalysts.14-16 However, it is now clear that the so-called superacid character of these materials is really related to the presence of both acid and red-ox properties. New catalytic reactions also have been proposed for these catalytic systems. However, it appears quite clear that the presence of sulfate will always be a limitation for their practical use. However, instead of sulphate, tungstate or hetepolyacids could also be interesting alternatives depending on the acidity required.14-16

In the case of C4-hydrocarbons, the use of acid or superacid solids will depend on both the acid strength required in each reaction and the reaction conditions required to optimize the thermodynamic equilibrium (Figure 13.3). For example, catalysts with very high acid strength could be substituted for a solid with a lower acidity by increasing reaction temperature. This has been proposed in both the isomerization of lineal alkanes and in the alkylation of isobutene with olefins, although the thermodynamic equilibrium should also be considered.

The modern gasolines are produced by blending products from crude oil distillation, that is, fluid catalytic cracking, hydrocraking, reforming, coking, polymerization, isomerization, and alkylation.17 Two clear examples of the possible use of solid-acid catalysts in refining processes are the isomerization of lineal alkanes and the alkylation of isobutene with butanes. In both these cases, and due to the octane alysis owing too their ability to catalyze both acidic and red-ox processes.

3,10-12

Catalyst

Reaction

Isobutane + butenes to C^^ alkylates

I H3PMo12O40/SiO2

Isobutane + butenes to C^^ alkylates n-Butane to isobutane

I H3PMo12O40/SiO2

Isobutene + methanol to MTBE

Ferrierite

Butene to isobutene

Figure 13.3 Possible catalytic uses of acid and superacid solids in the selective transformation of C4-hydrocarbons by acid reactions.

requirements, the research octane numbers (RON) is an important factor to be considered. For this reason, there are very good reasons for replacing the liquid-acid catalysts with solids acids that have fewer adverse effects on the environment.

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Responses

  • maya
    How many grams of sulfuric acid are in 44 million tons?
    8 years ago
  • miika
    How many grams of sulfur is in 44 million tons of sulfuric acid?
    8 years ago

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