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Abstract
The industrial production of formaldehyde from methanol is a major chemical process, with an annual global production of more than 50 million tons of formalin (37 wt% aqueous solution). The majority of the formaldehyde is used in the production of synthetic resins, but also various other products, making formaldehyde an important C1 building block.
Formaldehyde is industrially produced from methanol by two main processes, where the Formox process is responsible for over two thirds of the world’s production. The catalyst used in the Formox process is an iron molybdate catalyst with molybdenum oxide in excess giving a Mo/Fe ratio of 2-3, to increase the selectivity and catalyst lifetime, which due to selectivity loss and increased pressure drop over the reactor from molybdenum volatilization and downstream deposition is only 6-18 months. This gives a huge industrial motivation for improving the catalyst lifetime.
This thesis was dedicated to the development and investigation of the catalytic performance of alternative catalysts, which could mitigate the problem of short catalyst lifetime.
The first part of the thesis screened alternative catalysts reported to have promising behavior in the literature. This was done by synthesizing the catalysts, followed by characterization with XRD nd BET. The samples were then tested for activity and selectivity in a lab scale fixed bed reactor setup with catalyst particle size of 150-250 μm at 250-400°C. The catalysts reported in the literature could generally be categorized as molybdenum containing, vanadium containing, and those without molybdenum and vanadium. Several of these catalysts showed significantly lower performance in these tests, than reported in the literature. In general, the molybdenum containing catalysts were the most selective, vanadium containing catalysts were less selective but more active, and the catalysts without molybdenum or vanadium lacked both selectivity and activity. From the screening experiments it was decided to investigate molybdenum oxide supported on calcium hydroxyapatite (CaHAP) and its alkali earth metal analogues, as well as the respective alkali earth metal molybdates, since CaMoO4 was found by
XRD to form in the molybdenum oxide on CaHAP catalysts.
Hence, the second part of the thesis investigated the alkali earth metal molybdates (MMoO4, M = Mg, Ca, Sr, and Ba), prepared both stoichiometrically and with excess of molybdenum oxide from sol/gel synthesis or co-precipitation. The catalysts were tested at 400°C for up to 100 h. Excess molybdenum was required to reach high selectivity, but this molybdenum was found to evaporate quickly during time on stream, giving decreasing selectivity. However, no molybdenum evaporation past the point of stoichiometry was detected.
The third part of this thesis dealt with the study of molybdenum oxide supported on CaHAP and SrHAP with nominal molybdenum oxide loadings up to 20 wt%. Structural and compositional changes of the catalyst were characterized on both fresh samples and samples used for up to 600 h on stream at 350°C. The HAP based samples were found to be both more active and selective than the respective alkali earth metal molybdates. This was attributed to the promoting effects of the phosphate groups in the HAP supports on the proposed active phase of amorphous [MoO6]-octahedra supported on the HAP surface. Most significantly, the deactivation of the HAP based catalysts was much slower compared to the industrial iron molybdate catalyst. Thus, the samples kept a large degree of the initial active molybdenum oxide species, as well as activity and selectivity.
In the fourth part of the thesis, the focus moved to industrial sized catalyst pellets composed of the 10 wt% MoO3 supported on CaHAP catalyst. Particularly, the influence of catalyst pellet density (1.18 g/cm3-1.76 g/cm3), and thus porosity, on pellet activity, selectivity and durability was evaluated at 250- 400°C and for up 100 h on stream at 350°C. A random pore model together with pore size distribution measurements, and effectiveness factor calculations was used to understand the effects of pellet density. An increase in both activity, selectivity and molybdenum volatilization were observed with decreasing pellet density. The much higher catalytic stability of the powder molybdenum oxide catalyst compared to the iron molybdate was not evident on the pellets. The much lower rate of molybdenum volatilization, however, still gave promise to an application in the front-end of the catalyst bed in an industrial Formox
reactor. This would limit the MoO3 loss from volatilization and the accompanied downstream precipitation of MoO3 and pressure drop increase. In the last part of the thesis, a parametric study was performed on the 10 wt% molybdenum oxide support on CaHAP with the goal of fitting a Langmuir-Hinshelwood kinetic model to the data for later applications in reactor modelling. The model described well the influence of the feed concentrations of methanol, oxygen and water on formaldehyde as well as the by-products dimethyl ether, dimethoxymethane, methyl formate and CO. CO2 as by-product was not well described and the improved description of CO2 would require additional knowledge of the CO2 formation pathways.
This thesis contributes with new knowledge of alternative catalytic materials for the important production of formaldehyde. Most significantly, it has dealt in depth with the characterization, understanding and testing of the molybdenum oxide supported on HAP, including general trends and effects with applicability for other alternative catalytic materials for the oxidation of methanol to formaldehyde. Furthermore, it has discussed the possible application for the much-needed increase in catalyst lifetime in the Formox process.
Formaldehyde is industrially produced from methanol by two main processes, where the Formox process is responsible for over two thirds of the world’s production. The catalyst used in the Formox process is an iron molybdate catalyst with molybdenum oxide in excess giving a Mo/Fe ratio of 2-3, to increase the selectivity and catalyst lifetime, which due to selectivity loss and increased pressure drop over the reactor from molybdenum volatilization and downstream deposition is only 6-18 months. This gives a huge industrial motivation for improving the catalyst lifetime.
This thesis was dedicated to the development and investigation of the catalytic performance of alternative catalysts, which could mitigate the problem of short catalyst lifetime.
The first part of the thesis screened alternative catalysts reported to have promising behavior in the literature. This was done by synthesizing the catalysts, followed by characterization with XRD nd BET. The samples were then tested for activity and selectivity in a lab scale fixed bed reactor setup with catalyst particle size of 150-250 μm at 250-400°C. The catalysts reported in the literature could generally be categorized as molybdenum containing, vanadium containing, and those without molybdenum and vanadium. Several of these catalysts showed significantly lower performance in these tests, than reported in the literature. In general, the molybdenum containing catalysts were the most selective, vanadium containing catalysts were less selective but more active, and the catalysts without molybdenum or vanadium lacked both selectivity and activity. From the screening experiments it was decided to investigate molybdenum oxide supported on calcium hydroxyapatite (CaHAP) and its alkali earth metal analogues, as well as the respective alkali earth metal molybdates, since CaMoO4 was found by
XRD to form in the molybdenum oxide on CaHAP catalysts.
Hence, the second part of the thesis investigated the alkali earth metal molybdates (MMoO4, M = Mg, Ca, Sr, and Ba), prepared both stoichiometrically and with excess of molybdenum oxide from sol/gel synthesis or co-precipitation. The catalysts were tested at 400°C for up to 100 h. Excess molybdenum was required to reach high selectivity, but this molybdenum was found to evaporate quickly during time on stream, giving decreasing selectivity. However, no molybdenum evaporation past the point of stoichiometry was detected.
The third part of this thesis dealt with the study of molybdenum oxide supported on CaHAP and SrHAP with nominal molybdenum oxide loadings up to 20 wt%. Structural and compositional changes of the catalyst were characterized on both fresh samples and samples used for up to 600 h on stream at 350°C. The HAP based samples were found to be both more active and selective than the respective alkali earth metal molybdates. This was attributed to the promoting effects of the phosphate groups in the HAP supports on the proposed active phase of amorphous [MoO6]-octahedra supported on the HAP surface. Most significantly, the deactivation of the HAP based catalysts was much slower compared to the industrial iron molybdate catalyst. Thus, the samples kept a large degree of the initial active molybdenum oxide species, as well as activity and selectivity.
In the fourth part of the thesis, the focus moved to industrial sized catalyst pellets composed of the 10 wt% MoO3 supported on CaHAP catalyst. Particularly, the influence of catalyst pellet density (1.18 g/cm3-1.76 g/cm3), and thus porosity, on pellet activity, selectivity and durability was evaluated at 250- 400°C and for up 100 h on stream at 350°C. A random pore model together with pore size distribution measurements, and effectiveness factor calculations was used to understand the effects of pellet density. An increase in both activity, selectivity and molybdenum volatilization were observed with decreasing pellet density. The much higher catalytic stability of the powder molybdenum oxide catalyst compared to the iron molybdate was not evident on the pellets. The much lower rate of molybdenum volatilization, however, still gave promise to an application in the front-end of the catalyst bed in an industrial Formox
reactor. This would limit the MoO3 loss from volatilization and the accompanied downstream precipitation of MoO3 and pressure drop increase. In the last part of the thesis, a parametric study was performed on the 10 wt% molybdenum oxide support on CaHAP with the goal of fitting a Langmuir-Hinshelwood kinetic model to the data for later applications in reactor modelling. The model described well the influence of the feed concentrations of methanol, oxygen and water on formaldehyde as well as the by-products dimethyl ether, dimethoxymethane, methyl formate and CO. CO2 as by-product was not well described and the improved description of CO2 would require additional knowledge of the CO2 formation pathways.
This thesis contributes with new knowledge of alternative catalytic materials for the important production of formaldehyde. Most significantly, it has dealt in depth with the characterization, understanding and testing of the molybdenum oxide supported on HAP, including general trends and effects with applicability for other alternative catalytic materials for the oxidation of methanol to formaldehyde. Furthermore, it has discussed the possible application for the much-needed increase in catalyst lifetime in the Formox process.
Original language | English |
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Place of Publication | Kgs. Lyngby |
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Publisher | Technical University of Denmark |
Number of pages | 323 |
Publication status | Published - 2020 |
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Novel catalysts for the selective oxidation of methanol to formaldehyde
Thrane, J. (PhD Student), Hulteberg, C. (Examiner), Jensen, A. D. (Main Supervisor), Høj, M. (Supervisor), Thorhauge, M. (Supervisor), Kegnæs, S. (Examiner) & Christensen, K. A. (Examiner)
01/11/2017 → 08/02/2021
Project: PhD