Alkali Earth Metal Molybdates as Catalysts for the Selective Oxidation of Methanol to Formaldehyde — Selectivity, Activity, and Stability

Alkali earth metal molybdates (MMoO4, M = Mg, Ca, Sr, and Ba) were investigated as catalysts for the selective oxidation of methanol to formaldehyde in the search for more stable alternatives to the current industrial iron molybdate catalyst. The catalysts were prepared by either sol-gel synthesis or co-precipitation with both stoichiometric ratio (Mo:M = 1.0) and 10 mol% to 20 mol% excess Mo (Mo:M = 1.1 to 1.2). The catalysts were characterized by X-ray diffraction (XRD), nitrogen physisorption, Raman spectroscopy, temperature programmed desorption of CO2 (CO2-TPD), and inductively coupled plasma (ICP). The catalytic performance of the catalysts was measured in a lab-scale, packed bed reactor setup by continuous operation for up to 100 h on stream at 400 °C. Initial selectivities towards formaldehyde of above 97% were achieved for all samples with excess molybdenum oxide at MeOH conversions between 5% and 75%. Dimethyl ether (DME) and dimethoxymethane (DMM) were the main byproducts, but CO (0.1%–2.1%) and CO2 (0.1%–0.4%) were also detected. It was found that excess molybdenum oxide evaporated from all the catalysts under operating conditions within 10 to 100 h on stream. No molybdenum evaporation past the point of stoichiometry was detected.


Motivation for choice of catalyst synthesis methods
It was planned to synthesize all the alkali earth metal molybdates by the sol-gel method. However, as discussed below, the syntheses of SrMoO4 and BaMoO4 by this method resulted in low phase purity according to XRD. Hence, the co-precipitation method was used, which in contrast to the sol-gel method gave good phase purity for SrMoO4 and BaMoO4 by XRD, but the exact Mo:M ratios were difficult to control. For SrMoO4 and BaMoO4 slightly over stoichiometric Mo:M ratios were achieved, but for CaMoO4 the co-precipitation method gave under stoichiometric Mo:Ca ratio with poor selectivity for formaldehyde in the catalytic activity and selectivity test. MgMoO4 was not possible to precipitate under the employed conditions.
The full XRD and BET characterizations and activity measurements on both the phase pure and the non-phase pure catalysts are disclosed in this supplementary material.

Characterization by XRD
In the following, XRD results for all the prepared catalysts as function of preparation method, Mo:M ratio and calcination temperature are given. As can be seen from Table S1 and Figure   S1, MgMoO4 was formed by calcination already at 400°C. Increasing the temperature sintered the material somewhat.  Figure S1. XRD of the fresh MgMoO4 samples prepared by sol-gel synthesis after calcination at 400°C and 500°C.
The co-precipitated CaMoO4 sample was found by ICP to have a substoichiometric ratio of Mo:Ca = 0.90, however no Ca phases, e.g. CaO, Ca(OH)2, CaCO3, were found by XRD.  Figure S2. XRD of the fresh CaMoO4 samples prepared by sol-gel synthesis and co-precipitation after calcination at 400°C and 500°C.
SrMoO4 was difficult to form, as calcination temperatures of 600°C was needed to get only SrMoO4 and MoO3 by XRD for the samples prepared by the sol-gel method (Table S3 and Figure S3), possibly arising from an inhomogeneous solid formed by sol-gel. In contrast, SrMoO4 was readily formed after calcination at 400°C from the co-precipitated sample.  Figure S3. XRD of the fresh SrMoO4 samples prepared by sol-gel synthesis and co-precipitation after calcination at 400°C, 500°C and 600°C.
For the BaMoO4 samples, the pure phases were difficult to obtain by sol-gel synthesis (Table   S4, Figure S4). Many phases were observed (including non-decomposed precursor material), uncertain phases and non-identified phases. BaMoO4 was, as for SrMoO4, readily formed by the co-precipitation route.   Figure S4. XRD of the fresh BaMoO4 samples prepared by sol-gel synthesis and co-precipitation after calcination at 400°C, 500°C and 600°C.

Characterization by BET
Increasing calcination temperature was found to decrease the surface area (Table S5).

Activity Tests
All activity and selectivity data for the catalysts calcined at 400 and 500°C are given in Table   S6 to Table S9 and Figure S5 to Figure S6. The trends of selectivity and activity as function of temperature were very similar irrespective of calcination at 400°C or 500°C. The main difference was the conversion due to the difference in surface area. The samples calcined at 500°C were used for the further investigation (up to 100 h on stream) to decrease the effect of sintering during the long measurements. The Arrhenius plots all gave good correlations with R 2 >0.99.         Cryst. 43, 1126Cryst. 43, -1128 using the VoigtA peaks.

Results
The samples calcined at 500°C were investigated by CO2-TPD (See Figure S1). The Temperature ramp was conducted until 600°C, but only results until 450°C (the drying temperature) was used as it could be seen from the other tracked species (particularly water) that the large peaks there stemmed from decomposition.   The total surface basicity of the samples calculated from the peak fits is shown in Table S14. 2.9⋅10 -2 9.4⋅10 -3 Mo:Ba = 1.12 -- Figure S9 and Table S14 show that the sample with Mo:Ca = 1.0 had the highest mass based basicity of 0.27 µmol CO2/g, while the other samples had much lower (3.5-9 times) values.
This sample also showed significantly lower formaldehyde selectivity (Figure 4 and 7) and was the only one with desorption of CO2 at low temperature, with major peaks at 100 and 150°C. On surface area basis, see Table S14 column 2, the Mo:Ca = 1.0 sample still had the second highest value, only surpassed by the Mo:Sr = 1.20 sample which had a low surface area of 3.1 m 2 /g and a noisy desorption signal, due to the low total amount of adsorbed CO2 being close to the detection limit for the samples with low surface basicity on mass basis. No fit was made for Mo:Ba = 1.12 in Figure S9(d) as the apparent peak was believed to be a measurement artefact.

Sample preparation
There were used two different preparation methods in this article: A Sol-gel synthesis and a co-precipitation method. The exact amounts of chemicals used are shown in Table S15 and   Table S16. calcination. Precipitation with stoichiometric and 10 % excess of Mo compared to Sr had also previously been tried at pH = 10.4, but only yielded SrCO3.
MgMoO4 was tried co-precipitated by the route described in the article, but no precipitation took place due to too high solubility of MgMoO4. Addition of 100 mL of ethanol gave a precipitate but with extreme excess of Mo. No further efforts was made to precipitate MgMoO4.

Corrected selectivity and activity
The selectivity and conversion were corrected for reversible byproducts (DME, DMM and MF), as these will mostly be converted under industrial operation. DME was treated as two MeOH, DMM as two MeOH and one CH2O, and MF as one MeOH and one CO, as they would be converted back to these species at high MeOH conversion. The reversible byproducts corrected selectivity and conversion made the comparison of the catalysts clearer. This is illustrated in Figure S10 for the industrial reference (MoO3/Fe2(MoO4)3), where the reversible byproducts corrected selectivity remains constant even though the specific selectivities undergo large changes as function of increasing temperature and conversion. Particularly the DMM selectivity, but also the DME selectivity, decreased when the conversion increased.