PROGRAMA: ACCESO Y EDUCACIÓN PERTINENTE

Nb2O5 calcined at 110-300 °C was found to possess Brønsted and Lewis acidity, exhibiting ΔH values in the range of -115 to -129 kJ mol-1. The acid strength of niobia predictably increased as the calcination temperature increased.

In this study, it was found that niobium oxide and Zn-Cr oxides differed in the nature of their acidity, making it impossible to quantitatively characterise the strength of each type of acid site in these catalysts by ammonia adsorption microcalorimetry. The results for the acidity of niobium oxide catalysts are presented in Figures 3.31-3.33 and summarised in Table 3.7. It has been reported that increasing the niobium oxide calcination temperature above 500 °C reduced the acidity of the resulting catalysts.33 This can be explained by the transformation of protonic sites to Lewis sites with the elimination of water. The loss of acidity can probably be attributed to the change from amorphous to crystalline structure.33 This can be explained by the transformation of protonic sites to Lewis sites with the elimination of water. The loss of acidity can probably be attributed to the change from amorphous to crystalline structure.44, 45

Table 3.7 Acid properties of catalysts

Catalysta Acid site typeb ΔH (kJ mol-1)c

Nb2O5 (110 °C) B + L41 -115

Nb2O5 (200 °C) B + L41 -123

Nb2O5 (300 °C) B + L41 -129

a

Nb2O5 at 110-300 °C in air for 3 h. b B and L acid sites from DRIFT spectra of adsorbed pyridine. c Initial enthalpy of NH3 adsorption at zero adsorption coverage.

_________________ *

Figure 3.31 Plot of enthalpy of NH3 adsorption versus NH3 uptake for Nb2O5 calcined at 110 °C.

Figure 3.32 Plot of enthalpy of NH3 adsorption versus NH3 uptake for Nb2O5 calcined

at 200 °C. 0 30 60 90 120 150 0.00 0.10 0.20 0.30 Heat of ad sor p tion (kJ m ol -1) NH₃ uptake (mmol g-1 _Nb 2O5) 0 30 60 90 120 150 0.00 0.10 0.20 0.30 Heat of ad sor p tion (kJ m ol -1) NH₃ uptake (mmol g-1 _Nb 2O5)

Figure 3.33 Plot of enthalpy of NH3 adsorption versus NH3 uptake for Nb2O5 calcined at 300 °C.

3.8 C, H, N analysis

To examine catalyst stability over longer time on stream, the silicalite was tested for at least 28 h TOS (time on stream) using 0.2 g of catalyst, 500 oC and 20 mL min-1 N2 flow rate. While the Zn-Cr oxide and Al2O3 supported catalysts were tested at 380 oC, and 20 mL min-1 N2 flow rate for 18 and 24 h TOS respectively. Table 3.8 shows the total amount of carbon deposits in the spent catalysts used in the gas phase ketonisation of propionic acid using C and H combustion analysis.

0 30 60 90 120 150 0.00 0.04 0.08 0.12 Heat of ad sor p tion (kJ m ol -1) NH₃ uptake (mmol g-1 _Nb 2O5)

Table 3.8 C and H combustion analysis for spent catalysts used in the gas phase deoxygenation of propionic acid

Catalyst C (%) H (%)

Silicalite chemically treated by NH3(aq)/NH4NO3a 6.60 0.94 Zn-Cr (10:1)b 4.10 0.67 a After 28 h reaction at 500 °C. b After 18 h reaction at 380 °C. 3.9 Conclusion

This chapter has addressed in detail the catalyst texture, XRD and FTIR spectroscopy, and the acidity of catalyst used in the ketonisation of carboxylic acids. First, the catalysts were characterised using N2 physisorption. The specific surface area, pore diameter and pore volume were calculated for silicalite, zinc-chromium oxides and for niobium oxide catalysts, which were calcined at different temperatures. It was found that the texture of zeolite catalysts was not affected by chemical treatment, as their surface area, pore diameter and pore volume remained largely unchanged. The surface area was, however, positively affected by the Cr/Zn ratio in Zn-Cr catalyst, while the pore size decreased as the ratio increased. XRD measurements showed that silicalite was crystalline, while the crystallinity of zinc chromium oxides depended on their chromium and zinc content. Increasing the amount of chromium made the catalyst more amorphous, while those with high zinc content were crystalline.

The nature of acid sites was measured using DRIFT spectroscopy of pyridine adsorption. It was found that in Cr2O3, ZnO and Zn-Cr (1:6) oxide catalysts, Lewis acid sites predominated, while Cr2O3 and Zn-Cr (1:6) oxide catalysts also had Brønsted acid sites. In addition, niobium oxides catalysts were found to possess both Brønsted and

Lewis acid sites. The acidity of niobium oxides increased as the calcination temperature increased from 100 to 300 °C.

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Chapter 4

Gas-phase ketonisation of propionic acid