Beato, Pablo

Chevallier, Jacques

Damsgaard, Christian Danvad

Grundwaldt, Jan-Dierk

Høj, Martin

Jensen, Anker Degn

Nielsen, Kenneth

Raun, Kristian Viegaard

Schumann, Max

Thorhauge, Max

This work presents a study of the deactivation behavior of Fe-Mo oxide catalyst during selective oxidation of methanol to formaldehyde in a period of 5 days. The structural changes in the catalyst have been investigated in situ for the initial 10 h by Raman spectroscopy, and the structure after 5 days on stream in a fixed bed reactor have been determined by comprehensive characterization.<br/

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Downloaded from orbit.dtu.dk on: Dec 16, 2017Studies of Deactivation of Methanol to Formaldehyde Selective Oxidation CatalystRaun, Kristian Viegaard; Schumann, Max; Høj, Martin; Thorhauge, Max; Beato, Pablo; Damsgaard,Christian Danvad; Chevallier, Jacques; Nielsen, Kenneth; Grundwaldt, Jan-Dierk; Jensen, Anker DegnPublication date:2017Document VersionPeer reviewed versionLink back to DTU OrbitCitation (APA):Raun, K. V., Schumann, M., Høj, M., Thorhauge, M., Beato, P., Damsgaard, C. D., ... Jensen, A. D. (2017).Studies of Deactivation of Methanol to Formaldehyde Selective Oxidation Catalyst. Abstract from 13th EuropeanCongress on Catalysis (EUROPACAT 2017), Florence, Italy.  Studies of Deactivation of Methanol to Formaldehyde                                       Selective Oxidation Catalyst   Kristian Viegaard Raun,a Max Schumann,a,b Martin Høj,a Max Thorhauge,c Pablo Beato,c Christian D. Damsgaard,d Jacques Chevallier,e Kenneth Nielsen,d Jan-Dierk Grundwaldt,b Anker Degn Jensena,* aDTU Chemical Engineering, Technical University of Denmark, Kgs. Lyngby, 2800, Denmark bInstitute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Karlsruhe, 76131, Germany   cHaldor Topsøe A/S, Kgs. Lyngby, 2800, Denmark  dDepartment of Physics, Technical University of Denmark, Kgs. Lyngby, 2800, Denmark  eDepartment of Physics and Astronomy, Aarhus University, Aarhus, 2800, Denmark  *Corresponding author: aj@kt.dtu.dk  This work presents a study of the deactivation behavior of Fe-Mo oxide catalyst during selective oxidation of methanol to formaldehyde in a period of 5 days. The structural changes in the catalyst have been investigated in situ for the initial 10 h by Raman spectroscopy, and the structure after 5 days on stream in a fixed bed reactor have been determined by comprehensive characterization.   1. Scope  Formaldehyde (CH2O) may be synthesized industrially by selective oxidation of methanol over an iron-molybdate (Fe-Mo) oxide catalyst according to: CH3OH + ½O2 → CH2O + H2O. The reaction is normally carried out in a multitubular reactor with excess of air at 250-400 °C (yield = 90-95 %), known as the Formox process [1]. The average lifetime of the industrial catalyst is only 1–2 years depending on the operating conditions. The catalyst consists of a bulk Fe2(MoO4)3 with a surface layer of MoO3. The MoO3 surface is selective towards formaldehyde while the iron in the sublayer increases the activity of the catalyst [2]. Pure MoO3 in itself has low activity. Literature from the last decades agrees that the major reason for the deactivation is loss of molybdenum from the catalyst due to formation of volatile species with methanol, which can leave behind Mo poor zones. The catalyst is usually prepared with excess MoO3 (Mo/Fe > 1.5) to counter the loss of Mo. This work focuses on understanding the structural changes occurring in the catalyst and its behavior during deactivation via prolonged activity testing and spectroscopic investigations.   2. Results and discussion Fe-Mo oxide with Mo/Fe molar ratio = 2.0 was prepared by hydrothermal synthesis. The fresh catalyst was characterized by ICP-OES, XRD, TEM and SEM-EDX. The catalyst (150-300 µm) was tested in a fixed-bed reactor and in an in situ Raman cell [3]. The feed gas consisted of 10 % O2 and ~5 % MeOH in N2. Before each experiment the catalyst was thermally treated at 400 °C in air for two hours. The reactors were heated to 375 °C during operation. At the end of the experiment in the fixed-bed reactor, the catalyst was thermal-treated in air followed by reapplying reaction conditions. The spent catalyst from the fixed-bed experiment was characterized by XRD, XPS, Raman spectroscopy, SEM-EDX and STEM-EDX.    Figure 1 shows the conversion of MeOH, the selectivity towards formaldehyde and the catalyst bed temperature in the fixed-bed reactor for 116 h on stream. Figure 2 shows Raman spectra of the catalyst and conversion of MeOH including the relative presence of α-MoO3 and Fe2(MoO4)3 (weight parameters determined by linear optimization for selected Raman bands). For the experiment performed in the fixed-bed reactor, the selectivity was constantly above 90 %. The conversion decreased during the first 53 hours on stream from 80 to 44 % followed by a slight increase to 54 % (100 h). The conversion increased to 64 % after thermal-treatment at 400 °C, and further increased to 81-84 % after thermal-treatment at 500 °C.         Figure 1. MeOH conversion, formaldehyde selectivity and catalyst bed temp. in the fixed bed reactor.  Operating conditions: 25 mg catalyst diluted in 170 mg SiC, ~157.5 NmL/min gas feed: 10 % O2, ~5 % MeOH in N2.  Other products: DME and small amounts of CO and CO2 (< 1 C-mol.%).   600 650 700 750 800 850 900 950 1000 1050Fe2(MoO4)3693661h-MoO3-MoO3  IntensityRaman shift [cm-1] 375 °C, synthetic air375 °C, 5 % MeOH in air after 20 min after 5 h after 10 hFe2(MoO4)3-MoO39649910 2 4 6 8 100,00,51,01,52,0 Component weightTime (h) weight "-MoO3" weight "Fe2(MoO4)3" Conversion020406080100Conversion [%]  Figure 2. In situ Raman spectra after 20 min, 5 h and 10 h on stream during methanol oxidation.  Insert: Corresponding MeOH conversion and weight parameters representative for α-MoO3 and Fe2(MoO4)3.  Operating conditions: TOS = 10 h, 25 mg catalyst diluted in 18 mg Si, ~42 NmL/min gas feed: 10 % O2, ~5 % MeOH in N2.  Laser: CVI Melles HeNe laser. A 10x objective was used with 5 mW at 633 nm. For the experiment performed in the Raman reaction cell, the conversion decreased from 85 to 67 % (10 h experiment). The weight parameter assigned to α-MoO3 decreased to ~0 (Raman bands disappeared) and that assigned to Fe2(MoO4)3 increased with a factor of ~2.   The catalyst was prepared by hydrothermal synthesis, which is a mild synthesis (temp. = 180 °C). The intensity increase in Raman bands related to Fe2(MoO4)3 over time was probably due to crystallization of Fe2(MoO4)3. Surface oxygen is consumed during the reaction and the catalyst must be reoxidized by oxygen from the gas phase. Partial reduction of the catalyst could explain the decrease in Raman bands related to MoO3 and the decreasing conversion. The SEM-EDX analysis showed that different spent catalyst particles were subject to slightly different degrees of Mo-loss. The XRD analysis showed a composition of mainly Fe2(MoO4)3 with small amounts of FeMoO4. The XPS analysis showed a surface ratio of Mo/Fe of about 1.6, compared to 5.6 for the fresh catalyst. Raman analysis showed bands from α-MoO3 and Fe2(MoO4)3. TEM and STEM-EDX analysis showed particles of ~50 nm in size with few larger particles 200 – 500 nm and a decrease in the Mo/Fe ratio between the fresh and spent catalyst from 2.0 to 1.4. Additionally some iron oxide particles were observed.  3. Conclusions The results show that Fe2(MoO4)3 is reduced to Mo depleted phases (FeMoO4 and Fe2O3) due to loss of Mo. However, excess MoO3 (observed with SEM-EDX) acts as a replenishing reservoir, which partly stabilizes the MoO3 layer on top of the Fe2(MoO4)3 crystals. The reactivation of the catalyst after thermal-treatment is probably due to re-establishment of the molybdenum rich catalyst surface, by replenishing with Mo oxide from the excess MoO3 crystals (observed with XPS).   References  1. C. Brookes, P. P. Wells, G. Cibin, N. Dimitratos, W. Jones, D. J. Morgan, M. Bowker, ACS Catal. 2014, 4, 243-250.  2. M. Rellán-Piñeiro, N. López, ChemSusChem. 2015, 8, 2231-2239. 3. P. Beato, E. Schachtl, K. Barbera, F. Bonino, S. Bordiga, Catal. Today. 2013. 205, 128-133. 

Studies of Deactivation of Methanol to Formaldehyde Selective Oxidation Catalyst

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Studies of Deactivation of Methanol to Formaldehyde Selective Oxidation Catalyst

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