**3. Potential application of kaolin-based ZSM-5**

#### **3.1 Catalytic oligomerisation**

The performance of the zeolites as catalysts was tested in the transformation of 1-hexene, used as a model compound for the oligomerisation of alkenes; reaction was performed at the following conditions: T = 350°C, pressure = 1 atm, WHSV = 8 h<sup>−</sup><sup>1</sup> . The activities of the catalysts were determined by assuming all

**89**

**Figure 7.**

*Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry*

activity as a function of time on stream is shown in **Figure 7**.

the influence of quartz on the performance of the catalyst.

further and correlated with their physicochemical properties.

1-hexene present in the product was due to unconverted feed. The graph showing

The catalytic tests were conducted over a time period of 420 min. All catalysts showed some deactivation over the total time on stream although some were more rapidly deactivated than others. All catalysts had a conversion not less than 90% in the first hour on stream. The highest conversion achieved was approximately 98%. It is clearly noticed that the conversion of the catalysts is highly dependent on the acidity, in particular the Bronsted acidity. Si-Al 42 RK and Si-Al 70 BK with similar acidities and possessing the highest acidities of the catalysts had the highest conversions and both showed only slight deactivation as the conversion remained above 90% for the duration of the reaction. Si-Al 70 RK and Si-Al 150 BK having identical amounts of strong acid sites and tetrahedral alumina (**Tables 3** and **4** respectively) have the same conversion and deactivation for the time on stream. Si-Al 42 BK which had the least amount of Bronsted sites showed the lowest conversion and the most rapid deactivation. Interestingly, Si-Al 150 RK having a much lower acidity compared to Si-Al 42 RK and Si-Al 70 BK has a very similar activity and shows even better stability (**Figure 7**). This was attributed to the effect of quartz coating the surface of the ZSM-5 which prevents deactivation. Therefore it is seen that this catalyst functions better than catalysts with more than double its acidity and further validates

The catalysts tested in the transformation of 1-hexene all possess a wide product distribution. The products are grouped accordingly: (C2–C5) range, (C6–C9) gasoline range and (C10+) diesel range. This indicates multiple types of reactions may occur at the set reaction conditions such as oligomerisation, cracking, isomerisation and alkylation to name a few. The selectivities to these ranges however change with time on stream and it is noticed that small changes in conversion can have large changes in selectivity to products. The selectivities of the catalysts are discussed

Si-Al 150 BK and Si-Al 70 RK are compared to each other since they have similar

acidities as well as conversions over time on stream as shown in **Figure 7**. The selectivities however differ from each other. The selectivities to gasoline (C6–C9)

and diesel (C10+) ranges are shown in **Figure 8(a)** and **(b)** respectively.

*Graph showing hexane conversion over different catalysts (P = 1 atm, T = 350°C, WHSV = 8 h<sup>−</sup><sup>1</sup>*

*).*

*DOI: http://dx.doi.org/10.5772/intechopen.81375*

#### *Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry DOI: http://dx.doi.org/10.5772/intechopen.81375*

1-hexene present in the product was due to unconverted feed. The graph showing activity as a function of time on stream is shown in **Figure 7**.

The catalytic tests were conducted over a time period of 420 min. All catalysts showed some deactivation over the total time on stream although some were more rapidly deactivated than others. All catalysts had a conversion not less than 90% in the first hour on stream. The highest conversion achieved was approximately 98%. It is clearly noticed that the conversion of the catalysts is highly dependent on the acidity, in particular the Bronsted acidity. Si-Al 42 RK and Si-Al 70 BK with similar acidities and possessing the highest acidities of the catalysts had the highest conversions and both showed only slight deactivation as the conversion remained above 90% for the duration of the reaction. Si-Al 70 RK and Si-Al 150 BK having identical amounts of strong acid sites and tetrahedral alumina (**Tables 3** and **4** respectively) have the same conversion and deactivation for the time on stream. Si-Al 42 BK which had the least amount of Bronsted sites showed the lowest conversion and the most rapid deactivation. Interestingly, Si-Al 150 RK having a much lower acidity compared to Si-Al 42 RK and Si-Al 70 BK has a very similar activity and shows even better stability (**Figure 7**). This was attributed to the effect of quartz coating the surface of the ZSM-5 which prevents deactivation. Therefore it is seen that this catalyst functions better than catalysts with more than double its acidity and further validates the influence of quartz on the performance of the catalyst.

The catalysts tested in the transformation of 1-hexene all possess a wide product distribution. The products are grouped accordingly: (C2–C5) range, (C6–C9) gasoline range and (C10+) diesel range. This indicates multiple types of reactions may occur at the set reaction conditions such as oligomerisation, cracking, isomerisation and alkylation to name a few. The selectivities to these ranges however change with time on stream and it is noticed that small changes in conversion can have large changes in selectivity to products. The selectivities of the catalysts are discussed further and correlated with their physicochemical properties.

Si-Al 150 BK and Si-Al 70 RK are compared to each other since they have similar acidities as well as conversions over time on stream as shown in **Figure 7**. The selectivities however differ from each other. The selectivities to gasoline (C6–C9) and diesel (C10+) ranges are shown in **Figure 8(a)** and **(b)** respectively.

**Figure 7.** *Graph showing hexane conversion over different catalysts (P = 1 atm, T = 350°C, WHSV = 8 h<sup>−</sup><sup>1</sup> ).*

*Nanofluid Flow in Porous Media*

possible that BK forms more Al rich ZSM-5 than RK for two reasons. The first being that RK causes a higher degree of depolymerisation due to the presence of quartz (as discussed above) of the remaining Na-silicate which is broken down to silicate species. This then forms more networked silicate and aluminosilicate and has less formation of close Al atoms resulting in higher ratios. Second for the BK samples, which were prepared with a smaller amount of additional Na-silicate, was not well depolymerised due to a lower alkalinity i.e. the silicate remained non-transformed

the highly reactive Al species resulting in aluminosilicate with a much lower

The performance of the zeolites as catalysts was tested in the transformation of 1-hexene, used as a model compound for the oligomerisation of alkenes; reaction was performed at the following conditions: T = 350°C, pressure = 1 atm,

. The activities of the catalysts were determined by assuming all

of aluminosilicate species with close Al atoms [37].

*27Al MAS NMR spectra of the RK and BK samples with different Si/Al ratios.*

**3. Potential application of kaolin-based ZSM-5**

Si/Al ratio having also more Alefr and in the case of Si-Al 42 BK, in a high probability

and only a much lower concentration of Si was available for

**88**

WHSV = 8 h<sup>−</sup><sup>1</sup>

and balanced by the Na<sup>+</sup>

**Figure 6.**

**3.1 Catalytic oligomerisation**

*Selectivity of Si-Al 150 BK and Si-Al 70 RK to gasoline (C6–C9) (a) and diesel (C10+) (b) range products over time on stream.*

Both catalysts have a similar trend to selectivity of diesel range products as noticed in the graph and decrease with time as the conversion decreased. However Si-Al 150 BK has a slightly higher selectivity throughout the reaction. Both show good selectivity (40–55%) to gasoline products with the selectivity of Si-Al 70 RK possessing a steady increase over the reaction time. Si-Al 150 BK was more unstable and the selectivity varied over time and was slightly less than Si-Al 70 RK. It was thought Si-Al 70 RK was more selective to reactions pertaining to chain growth such as oligomerisation and alkylation but a closer look at the product selectivity of the gasoline range revealed that Si-Al 70 RK was highly selective to C6 hydrocarbons as compared to Si-Al 150 BK as shown in **Figure 9**. An increase from ~12% to above 30% was observed. The C2–C5 selectivity trend is also shown and decreases over time for the Si-Al 70 RK catalyst but shows an overall increase in the C2–C5 products

**Figure 9.**

*Selectivity of Si-Al 150 BK and Si-Al 70 RK to C6 (black curves) and C2–C5 (blue curves) range products over time on stream.*

**91**

**Figure 10.**

*RK, Si-Al 70 BK and Si-Al 42 BK as a function of time.*

*Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry*

synthesised from RK are shown to affect its catalytic performance.

**Figure 10** shows the selectivity of Si-Al 150 RK, Si-Al 42 RK, Si-Al 70 BK and Si-Al 42 BK to diesel and gasoline range hydrocarbons. The three catalysts with similar conversions i.e. Si-Al 150 RK, Si-Al 42 RK and Si-Al 70 BK can be compared

From **Figure 10** it is noticed that after the first hour on stream the selectivity to C10+ is the highest for the catalyst Si-Al 42 RK showing ~16% selectivity. This may indicate that oligomerisation to long chain hydrocarbons largely depends on the acidity since Si-Al 42 RK which has a high Bronsted acidity had the highest selectivity. The more acidic catalysts have better selectivity over the first 3 h on stream. As the reaction proceeds however, all catalysts show a large decrease in selectivity to C10+ hydrocarbons except Si-Al 150 RK which shows a greater stability. The decrease for Si-Al 42 RK is almost linear over the reaction time and drops to ~3%. The decrease in selectivity to C10+ suggests the deactivation of certain acid sites which are most likely strong Bronsted acid sites as oligomerisation reactions are favoured at these catalytic centres [65]. Si-Al 70 BK which has similar acidity to Si-Al 42 RK shows a slightly better selectivity to C10+ hence less deactivation of Bronsted sites. This is most likely due to the higher surface area and increased mesoporosity which may inhibit the build-up of carbonaceous deposits usually responsible for deactivation and hence further highlights the effects of the physicochemical properties of the zeolites on its catalytic behaviour. As mentioned in our previous work [30], the effect of quartz deposits on

*Selectivities to diesel range (C10+) and gasoline range (C6–C9) hydrocarbons of catalysts Si-Al 150 RK, Si-Al 42* 

for Si-Al 150 BK. Thus there may be a greater selectivity to isomerisation reactions over cracking reactions for the Si-Al 70 RK catalyst whereas Si-Al 150 BK has a better selectivity to chain growth reactions as its total selectivity to C7+ hydrocarbons is higher. This may suggest a slight difference in strength of acid sites available as oligomerisation and cracking occur on stronger acid sites than isomerisation reactions which occur on sites of intermediate acidity [65]. The main difference between the two catalysts however is the crystal morphology and size. The Si-Al 70 RK possessing nanocrystals as shown in HRSEM, this may reduce the effects of diffusion limitation. As shown by Buchanan et al. [66] larger crystals showed a higher selectivity to C3/C4 products and a lower isomerization/cracking of olefin ratio due to diffusion limitation. Therefore it is possible that as the strength of the acidic sites are reduced over time due to deactivation which happens more in Si-Al 70 RK, isomerization reactions which occur faster than cracking reactions [66], increase and due to the nanocrystals, the isomers which form are able to diffuse out before any secondary cracking leading to more C6 isomers, less cracked C2–C5 products and a better gasoline selectivity. Therefore the physical properties of the catalyst

*DOI: http://dx.doi.org/10.5772/intechopen.81375*

in terms of selectivity.

#### *Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry DOI: http://dx.doi.org/10.5772/intechopen.81375*

for Si-Al 150 BK. Thus there may be a greater selectivity to isomerisation reactions over cracking reactions for the Si-Al 70 RK catalyst whereas Si-Al 150 BK has a better selectivity to chain growth reactions as its total selectivity to C7+ hydrocarbons is higher. This may suggest a slight difference in strength of acid sites available as oligomerisation and cracking occur on stronger acid sites than isomerisation reactions which occur on sites of intermediate acidity [65]. The main difference between the two catalysts however is the crystal morphology and size. The Si-Al 70 RK possessing nanocrystals as shown in HRSEM, this may reduce the effects of diffusion limitation. As shown by Buchanan et al. [66] larger crystals showed a higher selectivity to C3/C4 products and a lower isomerization/cracking of olefin ratio due to diffusion limitation. Therefore it is possible that as the strength of the acidic sites are reduced over time due to deactivation which happens more in Si-Al 70 RK, isomerization reactions which occur faster than cracking reactions [66], increase and due to the nanocrystals, the isomers which form are able to diffuse out before any secondary cracking leading to more C6 isomers, less cracked C2–C5 products and a better gasoline selectivity. Therefore the physical properties of the catalyst synthesised from RK are shown to affect its catalytic performance.

**Figure 10** shows the selectivity of Si-Al 150 RK, Si-Al 42 RK, Si-Al 70 BK and Si-Al 42 BK to diesel and gasoline range hydrocarbons. The three catalysts with similar conversions i.e. Si-Al 150 RK, Si-Al 42 RK and Si-Al 70 BK can be compared in terms of selectivity.

From **Figure 10** it is noticed that after the first hour on stream the selectivity to C10+ is the highest for the catalyst Si-Al 42 RK showing ~16% selectivity. This may indicate that oligomerisation to long chain hydrocarbons largely depends on the acidity since Si-Al 42 RK which has a high Bronsted acidity had the highest selectivity. The more acidic catalysts have better selectivity over the first 3 h on stream. As the reaction proceeds however, all catalysts show a large decrease in selectivity to C10+ hydrocarbons except Si-Al 150 RK which shows a greater stability. The decrease for Si-Al 42 RK is almost linear over the reaction time and drops to ~3%. The decrease in selectivity to C10+ suggests the deactivation of certain acid sites which are most likely strong Bronsted acid sites as oligomerisation reactions are favoured at these catalytic centres [65]. Si-Al 70 BK which has similar acidity to Si-Al 42 RK shows a slightly better selectivity to C10+ hence less deactivation of Bronsted sites. This is most likely due to the higher surface area and increased mesoporosity which may inhibit the build-up of carbonaceous deposits usually responsible for deactivation and hence further highlights the effects of the physicochemical properties of the zeolites on its catalytic behaviour. As mentioned in our previous work [30], the effect of quartz deposits on

#### **Figure 10.**

*Selectivities to diesel range (C10+) and gasoline range (C6–C9) hydrocarbons of catalysts Si-Al 150 RK, Si-Al 42 RK, Si-Al 70 BK and Si-Al 42 BK as a function of time.*

*Nanofluid Flow in Porous Media*

**90**

**Figure 9.**

**Figure 8.**

*time on stream.*

*time on stream.*

*Selectivity of Si-Al 150 BK and Si-Al 70 RK to C6 (black curves) and C2–C5 (blue curves) range products over* 

Both catalysts have a similar trend to selectivity of diesel range products as noticed in the graph and decrease with time as the conversion decreased. However Si-Al 150 BK has a slightly higher selectivity throughout the reaction. Both show good selectivity (40–55%) to gasoline products with the selectivity of Si-Al 70 RK possessing a steady increase over the reaction time. Si-Al 150 BK was more unstable and the selectivity varied over time and was slightly less than Si-Al 70 RK. It was thought Si-Al 70 RK was more selective to reactions pertaining to chain growth such as oligomerisation and alkylation but a closer look at the product selectivity of the gasoline range revealed that Si-Al 70 RK was highly selective to C6 hydrocarbons as compared to Si-Al 150 BK as shown in **Figure 9**. An increase from ~12% to above 30% was observed. The C2–C5 selectivity trend is also shown and decreases over time for the Si-Al 70 RK catalyst but shows an overall increase in the C2–C5 products

*Selectivity of Si-Al 150 BK and Si-Al 70 RK to gasoline (C6–C9) (a) and diesel (C10+) (b) range products over* 

**Figure 11.** *Selectivity to C6 hydrocarbons and C2–C5 selectivity of Si-Al 42 RK (blue curve) as a function of time.*

the external surface of the crystals in Si-Al 150 RK plays a major role in inhibiting the formation of carbonaceous material causing deactivation. Thus even when compared to catalysts with greater acid site density and strength (almost double its acidity), Si-Al 150 RK possesses greater stability and selectivity is almost double that of the more acidic catalysts. This clearly suggests that the acid sites are prevented from being deactivated by the deposition of quartz and continue to catalyse oligomerisation reactions for the duration of the reaction. Si-Al 42 BK had the lowest selectivity to C10+. This is most likely due to its low Bronsted acidity but also the high content of extra-framework aluminium may cause pore blockage and prevent access to acid sites for oligomerisation. The closeness of Al atoms in the structure may also have an effect on the acidity and types of catalytic reactions that are favoured.

All catalysts showed good selectivity to the gasoline range. Si-Al 42 RK shows the opposite trend for selectivity to gasoline as this increase over the reaction time. The selectivity spikes in the last 2 h from 45 to 74%. **Figure 11** shows the selectivity to C6 and C2–C5 products.

The spike in gasoline activity is again due to an increase in C6 selectivity. This further confirms our observations with the Si-Al 70 RK catalyst. The decrease in C10+ selectivity indicates deactivation of strong acid sites. Thus isomerisation reactions are then favoured when only less acidic site are available and due to Si-Al 70 RK and Si-Al 42 RK possessing the same morphology the isomerisation products diffuse out before cracking. **Figure 11** clearly shows the contrasting selectivities between isomerisation (C6) and cracking reactions (C2–C5) for the Si-Al 42 RK catalyst. Si-Al 42 BK also shows good isomerisation activity which further indicates the reaction occurring on weaker acid sites.

#### **4. Conclusion**

In conclusion, this chapter discusses some of the key factors affecting the synthesis of kaolin-based ZSM-5 such as kaolin crystallinity, kaolinite content,

**93**

*Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry*

crystallisation parameters and Si/Al ratio. Additionally, the catalytic performance of the ZSM-5 derived from kaolin was evaluated. These factors are important considerations when attempting to synthesise ZSM-5 with high purity and crystallinity. However, ZSM-5 can also be synthesised from impure kaolin sources and these impurities may act as poisons or promoters during synthesis and catalytic application. The requirements to develop synthesis conditions that are optimised for specific sources of kaolin are established. The physicochemical properties such as porosity, morphology and acidity of ZSM-5 can be controlled by choosing the right synthesis procedures. Kaolin-based ZSM-5 zeolites are promising as catalysts for petrochemical reactions such as oligomerisation. High activity and selectivity to gasoline and diesel range hydrocarbons was attainable. The activity of the catalysts correlated well with the acidity of ZSM-5 samples. The catalytic performance of the zeolites also correlated well with the physical properties such as morphology and surface area which were shown to influence selectivity to certain products by favouring isomerisation and oligomerisation reactions respectively. Impurities in the kaolin precursor may also have positive effects on catalytic performance, in this case quartz deposition on ZSM-5 inhibiting deactivation and increasing catalyst stability. Therefore, ZSM-5 zeolites can be successfully synthesised from cheaper, more environmentally friendly alternative starting materials that have satisfactory

The authors would like to thank the Petroleum, Oil and Gas Corporation of South Africa (PetroSA) for their financial support and technical discussions, the National Research Foundation (NRF) for granting Ebrahim Mohiuddin a scholarship, Makana Brick for supplying the raw kaolin, the electron microscope unit, Physics department, University of the Western Cape for the SEM images, Mrs. E. Antunes, Chemistry department, University of the Western Cape and Dr. D.J. Brand, University of Stellenbosch for the solid state NMR work and Ithemba

*DOI: http://dx.doi.org/10.5772/intechopen.81375*

performances in catalytic application.

**Acknowledgements**

labs for the XRD work.

*Synthesis and Application of Porous Kaolin-Based ZSM-5 in the Petrochemical Industry DOI: http://dx.doi.org/10.5772/intechopen.81375*

crystallisation parameters and Si/Al ratio. Additionally, the catalytic performance of the ZSM-5 derived from kaolin was evaluated. These factors are important considerations when attempting to synthesise ZSM-5 with high purity and crystallinity. However, ZSM-5 can also be synthesised from impure kaolin sources and these impurities may act as poisons or promoters during synthesis and catalytic application. The requirements to develop synthesis conditions that are optimised for specific sources of kaolin are established. The physicochemical properties such as porosity, morphology and acidity of ZSM-5 can be controlled by choosing the right synthesis procedures. Kaolin-based ZSM-5 zeolites are promising as catalysts for petrochemical reactions such as oligomerisation. High activity and selectivity to gasoline and diesel range hydrocarbons was attainable. The activity of the catalysts correlated well with the acidity of ZSM-5 samples. The catalytic performance of the zeolites also correlated well with the physical properties such as morphology and surface area which were shown to influence selectivity to certain products by favouring isomerisation and oligomerisation reactions respectively. Impurities in the kaolin precursor may also have positive effects on catalytic performance, in this case quartz deposition on ZSM-5 inhibiting deactivation and increasing catalyst stability. Therefore, ZSM-5 zeolites can be successfully synthesised from cheaper, more environmentally friendly alternative starting materials that have satisfactory performances in catalytic application.

### **Acknowledgements**

*Nanofluid Flow in Porous Media*

the external surface of the crystals in Si-Al 150 RK plays a major role in inhibiting the formation of carbonaceous material causing deactivation. Thus even when compared to catalysts with greater acid site density and strength (almost double its acidity), Si-Al 150 RK possesses greater stability and selectivity is almost double that of the more acidic catalysts. This clearly suggests that the acid sites are prevented from being deactivated by the deposition of quartz and continue to catalyse oligomerisation reactions for the duration of the reaction. Si-Al 42 BK had the lowest selectivity to C10+. This is most likely due to its low Bronsted acidity but also the high content of extra-framework aluminium may cause pore blockage and prevent access to acid sites for oligomerisation. The closeness of Al atoms in the structure may also have an effect

*Selectivity to C6 hydrocarbons and C2–C5 selectivity of Si-Al 42 RK (blue curve) as a function of time.*

All catalysts showed good selectivity to the gasoline range. Si-Al 42 RK shows the opposite trend for selectivity to gasoline as this increase over the reaction time. The selectivity spikes in the last 2 h from 45 to 74%. **Figure 11** shows the selectivity to C6

The spike in gasoline activity is again due to an increase in C6 selectivity. This further confirms our observations with the Si-Al 70 RK catalyst. The decrease in C10+ selectivity indicates deactivation of strong acid sites. Thus isomerisation reactions are then favoured when only less acidic site are available and due to Si-Al 70 RK and Si-Al 42 RK possessing the same morphology the isomerisation products diffuse out before cracking. **Figure 11** clearly shows the contrasting selectivities between isomerisation (C6) and cracking reactions (C2–C5) for the Si-Al 42 RK catalyst. Si-Al 42 BK also shows good isomerisation activity which further indicates

In conclusion, this chapter discusses some of the key factors affecting the synthesis of kaolin-based ZSM-5 such as kaolin crystallinity, kaolinite content,

on the acidity and types of catalytic reactions that are favoured.

**92**

and C2–C5 products.

**Figure 11.**

**4. Conclusion**

the reaction occurring on weaker acid sites.

The authors would like to thank the Petroleum, Oil and Gas Corporation of South Africa (PetroSA) for their financial support and technical discussions, the National Research Foundation (NRF) for granting Ebrahim Mohiuddin a scholarship, Makana Brick for supplying the raw kaolin, the electron microscope unit, Physics department, University of the Western Cape for the SEM images, Mrs. E. Antunes, Chemistry department, University of the Western Cape and Dr. D.J. Brand, University of Stellenbosch for the solid state NMR work and Ithemba labs for the XRD work.

*Nanofluid Flow in Porous Media*
