Functional Groups Catalytic Agents

The most studied catalytic functional group is the sul-fonic acid group (-SO3H) attached to styrene-divinylbenzene copolymers of both gel and macro-porous morphologies. Many reactions catalysed by

Table 1 Intrinsic properties of a family of sulfonated porous aromatic polymers: the relationship of surface capacity, specific surface area, crosslinking density, and the working arena

Sulfonated

Theory

Measured

Rings on

Calculated

Crosslinking

Specific surface area, S

Working

porous

weight

weight

internal

theory wt.

density

phase in

polymer

capacity, (meqg-1)

capacity, (meqg-)

surface

(meqg-1)

(wt.%DVB)

(m2g-)

(m2mL -)

catalysis

Amberlyst

XN-1008a

5.299

5.26

2.76

0.146

12

40

60

Gel phase

Amberlyst 15

5.210

5.00

3.72

0.197

20

55

82

Gel phase

Amberlyst

XN-1005b

4.854

3.50

11.95

0.683

50

120

180

Gel phase

Amberlyst

XN-1010

4.749

3.60

37.92

1.827

85

615

850

Surface phase

a Amberlyst XN-1008 is no longer marketed commercially. It has been replaced by Amberlyst 16 which has a similar specific surface area but is slightly more porous.

b Amberlyst XN-1005 is no longer commercially available.

a Amberlyst XN-1008 is no longer marketed commercially. It has been replaced by Amberlyst 16 which has a similar specific surface area but is slightly more porous.

b Amberlyst XN-1005 is no longer commercially available.

Table 2 Qualitative relationship between polymer morphology of ion exchanger and change in reaction system

Polymer morphology

Change in the reaction system

Effect on the kinetics of catalysis

Gel polymer

Increasing level of crosslinking from 2 to 12wt.% divinylbenzene

Increasing bead diameter from 250 to 1000 |im

Poorly solvating reaction medium for catalyst

Decreasing rate of reaction. Gel copolymers of styrene-divinylbenzene with greater than about 6% divinylbenzene perform poorly as catalysts even in good swelling solvents

Decreasing rate of reaction. In good solvating reaction medium, the smaller the bead diameter, the better the catalyst performance Ineffective catalysis at any particle diameter and any crosslinking level

Increasing size of reactants and/or products Decreasing effectiveness as a catalyst

Macroporous polymer with small specific surface area (S) (S < 400-500 m2 mL~1 bead)

Increasing level of crosslinking from 6 to 25 wt.% divinylbenzene

Increasing bead diameter from 250 to 1000 |im

Poorly solvating reaction medium for catalyst

Increasing size of reactants and/or products

Moderate decline in catalytic effectiveness from compensating changes. As the crosslinking level increases, the S increases and the microgel diameter decreases

For small to moderate sized molecules, almost no change in catalyltic effectiveness

Very small impact on catalytic performance

Provided the pore system is sufficient for ingress of reactants and egress of products, only a moderate decline in effectiveness

Macroporous polymer with large specific surface area (S) (S>600 m2mL^1 bead)

Increasing level of crosslinking from 60 to 100wt.% divinylbenzene

Increasing bead diameter from 250 to 1000 |im

Poorly solvating reaction medium for catalyst

Increasing size of reactants and/or products

Increasing catalytic effectiveness, provided pore dimensions remain large enough to accommodate entrance of reactants and exit of products. As level of DVB increases, the surface area increases Little or no impact on catalytic effectiveness

No impact upon catalytic effectiveness

No impact upon catalytic effectiveness provided pore system allows influx of reactants and efflux of products homogeneous strong acids have been examined for effective catalysis by strong acid ion exchangers because of their lack of corrosiveness and their ease of separation from the reaction liquor by filtration (Table 3). Strong acid resins with crosslinking by divinylbenzene of 8 wt.% or less work well only where the reaction medium is very polar (water, dimethylformamide, N-methylpyrrolidinone or dimethyl sulfoxide) and the reactants or resulting products are small (MW < 250 Da).

Strong acid macroporous polymers work well in essentially all reaction media, especially the large specific surface area, strong acid macroporous polymers where the surface phase is the catalytic arena. Here solvation of the gel phase is unimportant. The surface phase sulfonic acid groups, however, are not as powerful in protonating reactants as those buried in the gel phase - the surface sulfonic acid moiety is a weaker acid than those within the gel. By placing two sulfonic acid groups on each surface ring, by sulfonation with fuming sulfuric acid, both the acid strength and the thermal stability are increased. Sul-fonated aromatic polymers with the sulfonic acid group attached directly to the aromatic ring begin to desulfonate at about 120°C, since sulfonation is acid catalysed and reversible. By attaching an electron withdrawing group, such as -SO2-, -SO3H, Cl, Br, F, etc., to the aromatic ring, in addition to the sulfonic acid group, the thermal stability of the strong acid resin is boosted to about 150°C.

Ion exchangers have their functional groups anchored in space relative to each other and the neighbouring groups can be used to enhance the rates of reactions of appropriately structured molecules. As an example of this effect, the rate of hydrolysis of olefinic esters is greatly enhanced by loading silver cations on to a portion of the sulfonic acid groups. At 50% loading of the cation exchanger with silver cations, the maximum rate of hydrolysis of allyl acetate is observed even though the concentration of acid

Table 3 Transformations catalysed by ion exchangers

A. Reactions catalysed effectively by strong acid resins Acetal and ketal synthesis

Addition of carboxylic acids to olefins

Alkylation of aromatic molecules, especially activated rings such as phenols, toluene, etc.

Cumene hydroperoxide conversion to phenol and acetone

Dehydration of alcohols into olefins

Epoxidation of olefins with H2O2

Esterification

Etherification

Hydrolysis

Hydrolysis of starch, cellulose and saccharides

Olefin acylation

Olefin alkylation

Olefin hydration

Olefin isomerization

Olefin oligomerization

Solvolysis of epoxides

Transesterification

B. Reactions catalysed effectively by anion exchangers Active methylene condensation reactions

Aldol condensation Cannizzaro reaction Cyanoethylation

Epoxide addition to carboxylic acids Michael addition reactions Nitrile hydrolysis to amides

C. Reactions catalysed effectively by heterogenized solid phase transition and noble metal catalysts

Epoxidation of olefins

Hydroformylation

Hydrogenation sites is halved. The silver ion with its propensity to complex with double bonds pulls into the resin phase a higher concentration of allyl acetate, thereby increasing the rate of hydrolysis over that of the strong acid resin without silver cations. This is a polymeric matrix effect which is not possible with a homogeneous catalyst. Polymeric matrix effects are an added advantage of solid phase over liquid phase catalysts for enhancing the catalytic effectiveness of ion exchangers.

Anion exchangers are the second most studied group of solid phase catalysts. The catalytic agents that are attached to the polymeric matrices are quaternary ammonium and tertiary amine groups. The positively charged nitrogen of the quaternary ammonium group is effective in catalysing some reactions, but most often it is the associated anion that is varied to achieve an effective catalytic agent. The positively charged nitrogen of the quaternary group is effective in catalysing epoxide addition reactions, as one example. Base catalysis is carried out with either the quaternary ammonium hydroxide or the tertiary amine group. For reactions whose products are sensitive to the base strength, a less powerful basic anion such as carbonate or acetate can be employed when the quaternary ammonium agent is the catalyst. The quaternary ammonium resin in the hydroxide form begins to decompose when used above 60°C. Other anionic forms are stable to about 150°C. The tertiary amine resins are thermally stable to about 150°C.

Phase transfer catalysis is accomplished by anion exchangers with any one of a number of appropriate nucleophilic anions associated with the quaternary ammonium group. The appropriateness of the nuc-leophilic anion is controlled by the nature of the chemical reaction undergoing catalysis. Phase transfer catalysts can also be designed by attaching a quaternary phosphonium group to the polymeric matrix in place of the quaternary ammonium group. Spacer arms that move the onium group further from the crosslinked polymer backbone enhance catalytic activity in phase transfer catalysis.

Crosslinked styrene-divinylbenzene copolymers have been and continue to be actively investigated as solid supports to heterogenize homogeneous catalytic agents. These solid phase catalysts have transition metals and noble metals anchored to the solid polymeric matrix through appropriate ligands. In heterogeneous form, they promote the same chemical reactions as in solution, albeit with the imposed mass transport limitations of the solid support.

Seealso: II/Ion Exchange: Historical Development; Inorganic Ion Exchangers; Novel Layered Materials: Phosphates; Novel Layered Materials: Non-Phosphates; Organic Ion Exchangers; Theory of Ion Exchange. III/Catalyst Studies: Chromatography.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment