Precursors for the preparation of vanadyl pyrophosphate (VPO) catalyst were prepared via the sesquihydrate route using 1-butanol as the reducing agent. The presence of P–O and V=O proved that the obtained catalyst is attributed to the vanadyl pyrophosphate phase.
Definitions of Catalysis
Catalysed Reactions and Non-Catalysed Reactions
A catalyst differs from reactants in that it returns to its original state after the reaction. Although a catalyst participates in a chemical reaction, the catalyst only interacts with the reactants to form intermediate species that will go on to form the desired product.
Types of Catalyst
- Homogeneous Catalyst
- Heterogeneous Catalyst
- Biocatalyst
- Desirable Properties of Catalyst
- Importance of Catalyst
In addition to carriers, the selection of active species and promoter also has an influence on the performance of the catalyst. Catalyst regeneration can reduce plant operating costs, as the catalyst can be regenerated multiple times (Bartholomew and Farrauto, 2005).
Problem Statement
As a result, the plant's operating costs increase, as maintenance or replacement of the catalyst must be performed frequently. It was found that doping vanadyl pyrophosphate (VPO) catalyst can actually improve the performance of the catalyst in the oxidation of n-butane.
Aims and Objectives The objectives of this project were
As pointed out by Ballarini et al. 2006), the latest patent review reports that the peak selectivity and conversion to maleic anhydride from n-butane is only about 65% and 86%, respectively. Furthermore, as the conversion of n-butane to maleic anhydride increases to about 70-80%, the selectivity for maleic anhydride will decrease dramatically.
Production of Maleic Anhydride from n-Butane
Vanadyl Pyrophosphate Catalyst
Structure of Vanadyl Pyrophosphate Catalyst
To produce VPO crystal structure, two VO6-distorted octahedra are bonded in a trans-oriented fashion, meaning the two octahedra are in opposite directions. However, the presence of impurities and structural defects make this ideal VPO strand impossible to exist in VPO catalyst in industry (Musa, 2016).
In Route B, dihydrofuran undergoes a dehydrogenation process to form furan(I) intermediate which can then be reacted with (O*) to produce maleic anhydride. Although there have been many other proposed reaction mechanisms of VPO on oxidation of n-butane to maleic anhydride, researchers have not yet found a definitive answer.
Vanadyl Pyrophosphate Catalyst Preparation Route
- Hemihydrate (Aqueous Route)
- Hemihydrate (Organic Route)
- Hemihydrate (Dihydrate Route)
- Sesquihydrate Route
The dihydrate route is a two-step process involving the reduction of the dihydrate, VOPO4·2H2O, with an alcohol to form the hemihydrate, VOHPO4·0.5H2O. Vanadium pentoxide first reacts with phosphoric acid to form VOPO4·2H2O in the presence of water.
Sonochemical Synthesis
A more recent study found that VPO catalyst can also be produced via a sesquihydrate precursor VOHPO4·1.5H2O. The sesquihydrate precursor can be produced by reducing vanadyl hydrogen phosphate dihydrate VOPO4·2H2O by refluxing in less reductive alcohols such as 1-butanol. It was found that the activated VPO catalyst provided showed high specific activity in the oxidation of n-butane to maleic anhydride (Ishimura et al., 2000).
Modified VPO catalyst often shows high activity and selectivity for maleic anhydride (Ishimura et al., 2000).
Parameters of Vanadyl Pyrophosphate Catalyst
- Calcination Duration
- Calcination Temperature
- Calcination Environment
- Doped System
- P/V Atomic Ratio
The selectivity of the VPO catalyst in the oxidation of n-butane to maleic anhydride depends on the Lewis acidity of the catalyst. It was found that high oxidation strength can increase the vanadium valence of the used VPO catalyst and in turn increase the selectivity of n-butane to maleic anhydride. Structural promotion effects improve the performance of VPO catalyst by increasing the surface area of the catalyst.
The promotional effects are achieved by a redox mechanism between the bulk and the surface of the VPO catalyst (Hutchings, 1991). It is observed that VPO catalyst doped with lithium has increased ion conductivity properties (Ballarini et al., 2006).
Materials
Methodology
Preparation of Vanadyl Phosphate Dihydrate Precursor
Calcination
Vanadyl pyrophosphate is then collected at the end of the calcination process and is designated as VPOBulk, VPOCo1%, VPOCu1% and VPOCu1%Co1%.
Characterisation of Catalyst
- X-ray Diffraction Analysis (XRD)
- Redox Titration
- Scanning Electron Microscope (SEM)
- Energy Dispersive X-ray (EDX)
- Temperature Programmed Reduction (TPR)
- Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) is one of the
- Fourier-Transform Infrared (FTIR)
By comparing the sample peak profile with the Joint Committee on Powder Diffraction Standard (JCPDS) database, the phase composition can be determined (Stanjek and Häusler, 2004). When the color of the solution changes from greenish-blue to pink, the end point has been reached. With the help of detectors in the SEM, signals are generated and the SEM image can be viewed via a monitor connected to the SEM.
In addition, the higher the number of photons produced, the higher the concentration of the element present in the sample solution. The purpose of this step is to obtain a calibration curve and allow the unknown concentrations of elements in the sample to be determined using the calibration curve.
Introduction
Thus, based on Figure 4.1, it can be concluded that the produced VPOBulk, VPOCo1%, VPOCu1% and VPOCu1%Co1% are amorphous in nature.
Fourier-Transform Infrared Spectroscopy (FTIR)
Since XRD is unable to analyze amorphous solids, FTIR will instead be used to confirm the identity of the catalysts produced by determining the presence of characteristic bonds for VPO catalyst. As all the IR spectra indicate the presence of P - O and V = O bonding in the samples, it can be confirmed that VPOBulk, VPOCo1%, VPOCu1% and VPOCu1%Co1% contained the active phase of (VO)2P2O7 in the samples.
Scanning Electron Microscope (SEM)
All doped catalysts have an average crystallite size of 10 µm, while the undoped catalyst has an average crystallite size of 20 µm. It is known that crystallite size is inversely related to the surface area and activity of the catalyst. A smaller crystallite size can increase the specific surface area of the catalyst and improve the catalytic performance of the catalyst.
This is consistent with literature findings where copper and cobalt increase the specific surface area of the catalyst (Hutchings and Higgins, 1996).
Energy Dispersive X-ray Spectrometry (EDX)
However, when the P/V atomic ratio is too high, the crystalline phase of the catalyst will decrease, while the formation of amorphous VOPO4 phases will increase (Guliants, Benziger, S. Sundaresan, et al., 1996). This is due to more dominant promotion effect as discussed in SEM analysis where cobalt has a more prominent promotion effect on VPO catalyst than copper. Since EDX analysis is a surface analysis technique, only X-rays generated from the surface atom up to a thickness of a few micrometers can be detected.
Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES) To support the results obtained from EDX analysis, ICP-OES is carried out to analyse
Thus, it can be said that the cobalt and copper dopant can reduce the average P/V atomic ratio in the VPO catalyst. However, all the VPO samples have a much lower average P/V atomic ratio overall compared to the results obtained from EDX analysis. A slightly high P/V atomic ratio can stabilize (VO)2P2O7 phases and prevent oxidation of the phases to V5+.
On the other hand, a low P/V atomic ratio is not able to limit the oxidation of (VO)2P2O7 phases also in VOPO4. However, the formation of this metaphosphate phase should be minimal since the P/V ratio found in EDX is still relatively small compared to literature findings in which the atomic P/V ratio is 4.0.
Redox Titration
Since the average P/V atomic ratio found in EDX is greater than the average P/V atomic ratio determined from ICP-OES, it can be inferred that most phosphorus is segregated near the surface of VPO catalyst due to the presence of an amorphous metaphosphate phase near the surface as pointed out by Ruitenbeek (1999). The former is in fact consistent with the result reported by Cornaglia et al. 2003) where the researchers found that cobalt dopant can reduce the average oxidation number of the VPO catalyst by promoting phosphorus enrichment and limiting the oxidation of V4+. Low P/V atomic ratio will increase the formation of VOPO4 phases which in turn increases the V5+ present in the VPO catalyst.
It can thus be concluded that the addition of cobalt decreases the average oxidation number, while the addition of copper increases the average oxidation number in a VPO catalyst. A higher average oxidation number can translate to a higher number of V5+ present in the VPO catalyst.
Temperature-Programmed Reduction (TPR)
On the other hand, the latter, where addition of copper dopant increases the average oxidation number of VPO catalyst, can also be observed in EDX analysis and ICP-OES analysis, where VPOCu1% has the lowest P/V ratio among the four samples. When comparing the average oxidation number of VPOCu1%Co1% with other VPO samples, it was found that the average oxidation number of VPOCu1%Co1% Since a high number of V5+ can significantly reduce the activity of VPO catalyst, although it increases the selectivity towards oxidation of n-butane to maleic anhydride, it is undesirable in industry.
Using the data obtained from Figure 4.4, reduction activation energy (ER) and total amount of oxygen removed from the VPO catalysts are calculated and tabulated in table 4.4. In terms of total oxygen removed from the VPO catalyst, VPOBulk has the highest amount of oxygen removed, followed by VPOCo1%, VPOCu1%Co1% and VPOCu1%.
Conclusion
The synergistic effect between cobalt and copper dopants can be observed in both redox titration and TPR analysis. In redox titration, cobalt dopant increases the promotion effect of copper dopant, producing a bi-metal doped VPO catalyst with a higher percentage of V5+ present in the catalyst. From TPR analysis, it was found that the reduction activation energy is reduced and this may be related to the synergistic effect between cobalt and copper dopant in VPO catalyst.
Recommendations For further research
Surface dynamics of a vanadyl pyrophosphate catalyst for the oxidation of n-butane to maleic anhydride: An in situ Raman study and reactivity of the P/V atomic ratio effect. Effect of calcination environment on the selective oxidation of n-butane to maleic anhydride over promoted and unpromoted VPO catalysts. Introduction to Catalysis and Industrial Catalytic Processes 1st ed., John Wiley & Sons, Inc., United States.
The effect of the phase composition of model VPO catalysts for n-butane partial oxidation. Effect of promoters and reactant concentration on the selective oxidation of n-butane to maleic anhydride using vanadium phosphorus oxide catalysts.
Preparation of Diphenylamine, Ph 2 NH indicator
Preparation of 0.01 N KMnO 4 Solution
Preparation of Sample Solutions
For the first dilution, transfer 90 cm3 P stock solution to the 100 cm3 volumetric flask and top up with deionized water until the calibration mark gives 45 ppm P standard solution. For fifth dilution, transfer 10 cm3 P stock solution to the 100 cm3 volumetric flask and top up with deionized water until the calibration mark gives 5 ppm P standard solution. For the first dilution, transfer 90 cm3 V stock solution to the 100 cm3 volumetric flask and top up with deionized water until the calibration mark gives 45 ppm V standard solution.
For second dilution, 60 cm3 of V stock solution is transferred to the 100 cm3 volumetric flask and made up to the calibration mark with deionized water to produce 30 ppm V standard solution. For fifth dilution, 10 cm3 V stock solution is transferred to the 100 cm3 volumetric flask and made up to the calibration mark with deionized water to produce 5 ppm V standard solution.
