Temperature In Presence Of Water Research Articles

Evolution of acid and basic sites in UiO-66 and UiO-66-NH2 metal-organic frameworks: FTIR study by probe molecules

Abstract Metal-organic frameworks (MOFs) and MOF-based materials find increasing and diverse applicability. The performance of MOFs in adsorption and catalysis is strongly related to their acid-basic properties. Here we report on the development of acidity (protonic and Lewis) and basicity during thermo-vacuum treatment of two MOFs of practical importance, UiO-66 and UiO-66-NH2. Residual DMF is more strongly bound to UiO-66-NH2 but is practically removed from both samples at 473 K. The structural μ3-OH groups (belonging to the Zr6 cluster) for both samples are observed at 3678-3672 cm−1. Their intrinsic frequency was estimated to be at 3682 cm−1 and the lower values detected are due to a very weak H-bonding to the MOF walls. Both samples are practically dehydroxylated at 523 K but easily re-hydroxylated at ambient temperature in presence of water. Three probe molecules (CO, N2 and CD3CN) were utilized to assess the acidity and basicity of the samples. Low-temperature CO adsorption experiments revealed a weak protonic acidity of the UiO-66 sample evacuated at 298 K: the CO induced shift of the O H modes (ΔtνOH, calculated on the basis of the intrinsic frequency) was −83 cm−1 with a small fraction of more acidic groups (ΔtνOH = −93 cm−1). Evacuation at 473 K leads to a strong (and reversible) decrease in the population of the structural OH groups in UiO-66 and creation of a fraction of more acidic hydroxyls (ΔtνOH = −108 cm−1). Similar results were obtained with the DMF-free UiO-66-NH2: the ΔtνOH was −91 cm−1 for a sample evacuated at 298 K while for a sample evacuated at 473 K two shifts were observed, −98 and −117 cm−1. These results were fully confirmed by adsorption of N2. In this case an additional N N band was detected at 2324 cm−1 (2246 cm−1 after adsorption of 15N2) and attributed to N2 polarized by O2− basic sites. The band developed with the pre-evacuation temperature evidencing creation of basic sites. No Lewis acidity was established by CO and N2 probes on samples evacuated up to 573 K. However, with samples evacuated at 473 K or higher temperature, Zr4+ Lewis acid sites were unambiguously monitored by CD3CN through a ν(CN) band at 2299 cm−1. The existence of this “hidden” Lewis acidity is explained by structural re-arrangement of the Zr4+ environment induced by relatively strong bases as CD3CN. Subsequent re-hydroxylation of the sample provokes almost full disappearance of the Lewis acid sites at the expense of OH groups formed.

Bio-oil hydrotreating over conventional CoMo & NiMo catalysts: The role of reaction conditions and additives

Wood-derived bio-oil was investigated as a feedstock for the production of renewable hydrocarbons over commercial hydrotreating CoMoS and NiMoS catalysts. Alumina-supported CoMoS active phase showed better selectivity to diesel-like products than NiMoS phase and higher activity in removal of gaseous intermediates (COx) by hydrogenation. This reaction can play an important role in the reduction of corrosivity at the reactor outlet and determine the process economy. The disadvantage of alumina as a support material was its low stability at high temperatures in presence of water. NiMoS catalyst was found to be more active in decarboxylation and it was possible to reach a steady state production of hydrocarbons having a comparable boiling point distribution with crude oil middle distillates. A test performed with a combination of NiMoS (at the top of the catalyst bed) and CoMoS (at the bottom of the reactor) revealed the key role of the catalyst type and reaction conditions in the first zone of the reactor. The experiments with elevated feeding rates showed the undesirable presence of the not-stabilized reactants behind the first reactor zone. Reaction conditions, catalyst selection and presence of additives (methanol to modify viscosity, DMDS/H2S to stabilize active centers) were combined and optimized to tune the product composition and properties. The catalyst low-temperature activity in the first reactor zone was the limiting factor for the maximum feed rate and reactor capacity. The reaction temperature in this zone strongly affects the boiling point distribution of the product. In case of NiMo catalyst, the reaction temperature above 360°C in the 3rd zone can reduce molecular weight of the product due to mild cracking. In contrast, temperatures above 400°C result in fast deactivation preventing thus the establishment of the steady state operating regime.