High-efficiency and durable V–Ti–Nb ternary catalyst prepared by a wet-solid mechanochemical method for sustainably producing acrylic acid via acetic acid–formaldehyde condensation

Based on the precise phase control V species adjustment of vanadium phosphorus oxides (VPOs), a series of metal oxides (Nb2O5, MoO3, WO3, and Bi2O3) were selected as modification agents to further enhance the catalytic activity and retain the excellent durability of VPO–TiO2-based catalysts for the new procedure of producing acrylic acid via acetic acid–formaldehyde condensation. At an elevated liquid hourly space velocity (LHSV), the (AA + MA) selectivity reached 92.3% with a (MA + AA) formation rate of 63.8 μmol−1 gcat−1 min−1 over the Nb-decorated catalyst (catalyst VTi–Nb), and it maintained good durability for up to 100 h. The detailed characterization results of XRD, Raman, XPS, NH3-TPD, CO2-TPD, and H2-TPR, demonstrated that the addition of Nb2O5 could observably enhance the catalytic efficiency of the VPO–TiO2 catalyst. It not only improved the catalyst durability by enhancing prereduction of the V5+ species, but also enhanced the active site density to improve the catalytic activity.

V 4+ /V 5+ ratio of different samples is obtainable for comparison.
H 2 -TPR. Hydrogen temperature-programmed reduction (H 2 -TPR) was performed from room temperature (RT) to 850 °C at a rate of 10 °C/min in a flow of 5% H 2 /Ar (v/v, flow rate = 40 mL/min) and isothermally held at 850 °C until reduction was complete.
NH 3 and CO 2 -TPD. Catalyst of 50 mg was first heated in an Ar flow (40 mL/min) to 200 °C and kept at this temperature for 1 h. Then the sample was cooled to 100 °C in the Ar flow. After that, NH 3 or CO 2 adsorption was performed at 100 °C for 1 h. Finally, NH 3 and CO 2 -TPD was carried out in an Ar flow (40 mL/min) with the sample being heated to 500 °C at a rate of 10 °C/min. The amount of desorbed NH 3 (in μmol/g) was determined by a titration, in which a HCl solution (0.01 mol/L) was used to absorb the released NH 3 . A NaOH solution (0.01 mol/L) was used as the titrant. And the calcium oxalate was used as a Standard to calculated the amount of desorbed CO 2 (in μmol/g).

Catalyst evaluation details
All the catalyst powders were pressed, crushed, and sieved to 20-40 mesh for activity evaluation. Two reactors were used for catalyst evaluation, one has an ID of 10 mm without a thermocouple jacket, and the other has an ID of 12 mm with a thermocouple jacket whose outside diameter is 3 mm. The reaction data derived from the two reactors were proved to be reproducible. Catalyst of 3 g was charged into the reactor, and the space above the catalyst bed was filled with quartz chips to preheat the in-coming liquid. Before feedstock introduction, the sample was heated up in a flow of N 2 (30 mL/min) to a desired temperature (360 °C) at a rate of 10 °C/min and kept at this temperature for 2.5 h. When a mixed HAc and FA solution (2.5/1, n/n) was fed into the reactor with a LHSV of 1.33 mL·h -1 ·g-1 cat (15.25 mmol·h -1 ·g-1 cat, HAc-based), a mixture of N 2 and air (50 mL/min, 3 vol.% O 2 in N 2 ) was served as carrier gas. The products were collected in a cold trap. After 2.5-h reaction, the collected liquid sample was analyzed using a gas chromatograph equipped with a flame ion detector (FID) and a HP-FFAP capillary column (0.32 mm × 25 m). Valeric acid and iso-butyl alcohol were used as internal standards for component quantification. All the catalysts were first evaluated by screening their performances in terms of the (MA + AA) Yield in the collected liquid sample based on FA input. In these circumstances, the off-gas was online analyzed by a GC equipped with TCD and TDX-01 packed column. It is worth noting that the formaldehyde component cannot be measured by GC analysis, therefore, the formaldehyde conversion cannot be directly determined by using the GC analysis data. In some cases, the unreacted FA content was analyzed by the iodometry method.
Note that HAc is usually fed significantly excessive in amount over FA (in the current study the molar HAc/FA is 2.5:1) to obtain an overall better performance, the byproducts such as acetone and CO x are mainly originated from HAc, thus the data associated with HAc conversion and particularly (AA + MA) selectivity based on the converted HAc is informative and meaningful to evaluate process economy. In addition, MAc was not regarded as a harmful by-product. In fact, it can continue to react with FA to produce AA/MA, thus there is no negative impact on a recycling manufacture process. For this reason, the molar quantity of generated MAc was treated as unreacted HAc when calculating the HAc conversion and (MA+AA) Selectivity (HAc-based).
Formation rate of AA+MA (FR AA+MA ) is defined by equation S1:

FR AA+MA = n (AA+MA) /(m VPO × t) (S1)
Where n AA+MA is the sum of molar quantity of (AA+MA), m VPO is the mass quantity of VPO component in the sample, and t is the reaction time (150 min).
Selectivity of (AA+MA) (S AA+MA ) based on HAc is defined by equation S2:

S AA+MA = n (AA+MA)equ /(n (HAc)0 -n (HAc)measured -n (MAc)measured ) × 100% (S2)
Where n (AA+MA)equ is the molar quantity of HAc equivalent to (AA+MA), n (HAc)0 is the molar quantity of HAc fed into the reactor, n (HAc)measured is the molar quantity of unreacted HAc, and n (MAc)measured is the molar quantity of generated MAc.