The recent advances in the development of heterogeneous catalysts and processes for the direct hydrogenation of CO2 to formate/formic acid, methanol, and dimethyl ether are thoroughly reviewed, with special emphasis on thermodynamics and catalyst design considerations. After introducing the main motivation for the development of such processes, we first summarize the most important aspects of CO2 capture and green routes to produce H2. Once the scene in terms of feedstocks is introduced, we carefully summarize the state of the art in the development of heterogeneous catalysts for these important hydrogenation reactions. Finally, in an attempt to give an order of magnitude regarding CO2 valorization, we critically assess economical aspects of the production of methanol and DME and outline future research and development directions.

The structure and elementary composition of various commercial Fe-based MOFs used as precursors for Fischer–Tropsch synthesis (FTS) catalysts have a large influence on the high-temperature FTS activity and selectivity of the resulting Fe on carbon composites. The selected Fe-MOF topologies (MIL-68, MIL-88A, MIL-100, MIL-101, MIL-127, and Fe-BTC) differ from each other in terms of porosity, surface area, Fe and heteroatom content, crystal density and thermal stability. They are re-engineered towards FTS catalysts by means of simple pyrolysis at 500 °C under a N2 atmosphere and afterwards characterized in terms of porosity, crystallite phase, bulk and surface Fe content, Fe nanoparticle size and oxidation state. We discovered that the Fe loading (36–46 wt%) and nanoparticle size (3.6–6.8 nm) of the obtained catalysts are directly related to the elementary composition and porosity of the initial MOFs. Furthermore, the carbonization leads to similar surface areas for the C matrix (SBET between 570 and 670 m2 g−1), whereas the pore width distribution is completely different for the various MOFs. The high catalytic performance (FTY in the range of 1.9–4.6 × 10−4 molCO gFe−1 s−1) of the resulting materials could be correlated to the Fe particle size and corresponding surface area, and only minor deactivation was found for the N-containing catalysts. Elemental analysis of the catalysts containing deliberately added promoters and inherent impurities from the commercial MOFs revealed the subtle interplay between Fe particle size and complex catalyst composition in order to obtain high activity and stability next to a low CH4 selectivity.

In this combined in situ XAFS, DRIFTS, and Mössbauer study, we elucidate the changes in structural, electronic, and local environments of Fe during pyrolysis of the metal organic framework Fe-BTC toward highly active and stable Fischer–Tropsch synthesis (FTS) catalysts (Fe@C). Fe-BTC framework decomposition is characterized by decarboxylation of its trimesic acid linker, generating a carbon matrix around Fe nanoparticles. Pyrolysis of Fe-BTC at 400 °C (Fe@C-400) favors the formation of highly dispersed epsilon carbides (ε′-Fe2.2C, dp = 2.5 nm), while at temperatures of 600 °C (Fe@C-600), mainly Hägg carbides are formed (χ-Fe5C2, dp = 6.0 nm). Extensive carburization and sintering occur above these temperatures, as at 900 °C the predominant phase is cementite (θ-Fe3C, dp = 28.4 nm). Thus, the loading, average particle size, and degree of carburization of Fe@C catalysts can be tuned by varying the pyrolysis temperature. Performance testing in high-temperature FTS (HT-FTS) showed that the initial turnover frequency (TOF) of Fe@C catalysts does not change significantly for pyrolysis temperatures up to 600 °C. However, methane formation is minimized when higher pyrolysis temperatures are applied. The material pyrolyzed at 900 °C showed longer induction periods and did not reach steady state conversion under the conditions studied. None of the catalysts showed deactivation during 80 h time on stream, while maintaining high Fe time yield (FTY) in the range of 0.19–0.38 mmolCO gFe–1 s–1, confirming the outstanding activity and stability of this family of Fe-based FTS catalysts.Keywords: dispersion; Fischer−Tropsch synthesis; iron; iron carbide phases; metal organic framework; MOF mediated synthesis; pyrolysis; structure−activity relations

In this modeling study, the uses of nitrogen (77.3 K), probe molecule of choice for decades, and argon, opted as alternative in the 2015 IUPAC report on adsorptive characterization, as probe molecules for geometric surface area determination are compared. Graphene sheets possessing slit-shaped pores with varying size (width) are chosen as model porous solids, and different methods for the determination of specific surface areas are investigated. The BET method, which is the most commonly applied analysis, is compared to the Langmuir and relatively recently proposed ESW (excess sorption work) method. We show that either using argon or nitrogen as adsorptive, the physical meaningfulness of adsorption-derived surface areas highly depends on the pore size. When less than two full layers of adsorbate molecules can be formed within slitlike pores of a graphitic material (Dpore < 5.8 Å for Ar/N2), adsorption-derived surface areas are about half that of the geometric surface area. Between two and four layers (6.8 < Dpore < 12.8 Å), adsorption surface areas can be significantly larger (up to 75%) than the geometric surface area because monolayer−multilayer formation and pore filling cannot be distinguished. For four or more layers of adsorbate molecules (Dpore > 12.8 Å), adsorption-derived surface areas are comparable to their geometrically accessible counterparts. Note that for the Langmuir method this only holds if pore-filling effects are excluded during determination. This occurs in activated carbon materials as well. In the literature, this indistinguishability issue has been largely overlooked, and erroneous claims of materials with extremely large surface areas have been made. Both the BET and Langmuir areas, for Dpore > 12.8 Å, correspond to geometric surface areas, whereas the ESW method yields significantly lower values. For the 6.8 Å < Dpore < 12.8 Å range, all methods erroneously overestimate the specific surface area. For the energetically homogeneous graphene sheets, differences between argon and nitrogen for the assessment of surface areas are minor.

Multiphase catalytic processes involve the combination of scale-dependent and scale-independent phenomena, often resulting in a compromised, sub-optimal performance. The classical approach of randomly packed catalyst beds using unstructured catalyst particles may be outperformed by the careful design of the catalyst at the nano-scale and by the judicious choice and design of reactor. Application of structured catalysts and reactor internals and the combination of advanced reactor and catalyst systems with in situ separation allow decoupling the various phenomena involved, opening the way to intensified processes on a large scale. The integral approach of Catalysis and Reaction Engineering discussed here will play a pivotal role in the development of novel, future-proof processes.

A large fraction of global energy is consumed for heating and cooling. Adsorption-driven heat pumps and chillers could be employed to reduce this consumption. MOFs are often considered to be ideal adsorbents for heat pumps and chillers. While most published works to date on this topic have focused on the use of water as a working fluid, the instability of many MOFs to water and the fact that water cannot be used at subzero temperatures pose certain drawbacks. The potential of using alcohol–MOF pairs in adsorption-driven heat pumps and chillers is investigated. To this end, 18 different selected MOF structures in combination with either methanol or ethanol as a working fluid are considered, and their potential is assessed on the basis of adsorption measurements and thermodynamic efficiencies. If alcohols are used instead of water, then (1) adsorption occurs at lower relative pressures for methanol and even lower pressure for ethanol, (2) larger pores can be utilized efficiently, as hysteresis is absent for pores smaller than 3.4 nm (2 nm for water), (3) larger pore sizes need to be employed to ensure the desired stepwise adsorption, (4) the effect of (polar/apolar) functional groups in the MOF is far less pronounced, (5) the energy released or taken up per cycle is lower, but heat and mass transfer may be enhanced, (6) stability of MOFs seems to be less of an issue, and (7) cryogenic applications (e.g., ice making) become feasible. From a thermodynamic perspective, UiO-67, CAU-3, and ZIF-8 seem to be the most promising MOFs for both methanol and ethanol as working fluids. Although UiO-67 might not be completely stable, both CAU-3 and ZIF-8 have the potential to be applied, especially in subzero-temperature adsorption chillers (AC).

•Comparison of a flexible and non-flexible MOF in mixed matrix membranes for CO2/CH4 separation.•Effect operational pressure and temperature on permeability and selectivity.•Higher pressure operation of MMM’s compared to pure polymeric membranes.•MMM’s have higher apparent permeation activation energies than their pure polymeric membranes.•FIB–SEM as a powerful method to analyse the structure of MMM’s.Mixed matrix membranes (MMMs) based on NH2-functionalized MIL-53(Al) and MIL-101(Al) MOFs dispersed in polysulfone (PSF) and polyimide (PI) polymers have been investigated. The MOF loading was varied in the range of 8–25 wt.%, while membranes with different thicknesses were obtained by two casting methodologies. The synthesized membranes were tested in the separation of CO2 from an equimolar CO2/CH4 mixture. At steady operation (T = 35 °C, ΔP = 3 bar), incorporation of the MOF filler has a positive effect on the separation performance which consists of a moderate enhancement of the separation selectivity, in certain cases along with an improvement in CO2 permeability. In general, higher separation factors and CO2 permeabilities are achieved with PI than with PSF. Our study reveals the relevance of the membrane thickness for both the separation performance under given conditions and the sensitivity to other structural and operational variables. The incorporation of NH2-MIL-53(Al) as filler in PI-based MMMs has a larger effect, particularly a beneficial increment in CO2 permeability at constant separation factor, for thinner membranes casted in a Doctor Blade system. This is tentatively attributed to the partial preservation of the original narrow pore configuration of this flexible MOF, unlike in thicker membranes casted in the absence of shear forces. Although improvements in the separation performance remain moderate with respect to the neat PI counterpart, the benefits of the incorporation of MOF as filler become more apparent at high pressures: while for pure polymeric membranes a decrease in the separation factor is observed at increasing ΔP, MMMs maintain large separation factors up to transmembrane pressures as high as 12 bar, highlighting the application potential of these composites. On the other hand, reducing the membrane thickness limits the MOF loading that can be incorporated before the mechanical stability of the membrane becomes compromised. It also enlarges the impact of the temperature and trans-membrane pressure on the separation performance. Overall, this work reveals that an interplay between structural and operational variables determines the performance of MOF MMMs and calls for a multi-variable optimization to advance this technology.

The permeation and separation characteristics of an all-silica DDR zeolite tubular membrane have been studied in the temperature range of 303–773 K and feed pressures up to 500 kPa. The permeation experiments are complemented by single component adsorption isotherms.The permeance of He, H2, CO2, CO and N2 monotonically decreases with increasing temperature. This behaviour could be described accurately for all components by a surface diffusion mechanism. Only in case of N2 and CO small deviations are observed above 600 K. Isobutane is not able to enter the DDR pores and passes only through a very small number of defects in the membrane.The single component permeance is about equal to the permeance of this component in a binary mixture. Only below 473 K the H2 permeance in a mixture with CO2 or isobutane is reduced in comparison with its single component permeance. The ideal H2/CO and CO2/CO selectivities range from 3 to 12 and 10 to 2 between 303 and 673 K, respectively. These mixture selectivities where always below 5 and much lower than the ideal selectivities because of non-differential operation along the membrane tube. The ideal H2/isobutane selectivity is >600 at 101 kPa feed pressure at all temperatures. The mixture selectivities at 101 kPa total feed pressure is ∼400 in an equimolar binary mixture.The high selectivities, high H2 and CO2 fluxes and stable membrane operation, also at high temperatures, makes this membrane a potential candidate for high temperature (reactive) separations that involve removal of H2 and CO2.

The recently introduced relevant site model (RSM) (Van den Bergh et al., J. Phys. Chem. C, 113 (2009), 17840) to describe the loading dependency of diffusion in zeolite DDR is successfully extended to a variety of light gases (CH4, CO2, Ar and Ne) and binary mixtures thereof in other zeolite topologies, DDR, CHA, MFI and FAU, utilizing the extensive diffusivity dataset published by Krishna and van Baten for this variety of zeolite-guest systems (e.g. Chem. Eng. Sci., 63 (2008), 3120 (supplementary material)).The RSM is formulated around the central idea of segregated adsorption in structures consisting of cages connected by windows, distinguishing cage and window adsorption sites. Only the molecules located at the window site (i.e. the relevant site (RS)) are able to make a successful jump to the next cage. The RSM is based on the Maxwell–Stefan framework for mass transport but includes only one extra parameter that describes the adsorption properties of the ‘relevant site’. Key feature of the RSM as applied to mixtures is that competitive adsorption effects and ‘speeding up and slowing down’ (exchange) effects between guest molecules are related to the relevant site loading and composition instead of to the overall loading, which can be very different.From the RSM approach a measure for the level of adsorption segregation is derived: the ratio of the RS and total occupancy. The predicted level of adsorption segregation correlates well with the level of confinement of a molecule at the RS: the molecule diameter to zeolite pore diameter. The predicted degree of adsorption segregation of the studied light gases in DDR is in good agreement with molecular simulations results, indicating the physical meaningfulness of the estimated RS adsorption parameters.The binary mixture diffusivity modelling points out that in case of the small-pore zeolites (DDR and CHA) the data is described best with equal RS saturation loadings for both components. For the large pore zeolite FAU the ratio of the RS saturation loadings equals that of the bulk saturation loadings. The geometry of the RS strongly influences the RS saturation loading: in case of the small-pore zeolites the RS (= window site) is restricted to only one molecule but when the RS becomes larger its saturation loading becomes similar to that of the bulk.

Single component (CO2, CH4, and N2) and equimolar binary mixture (CO2/CH4, N2/CH4, and CO2/Air) permeation data across a disk-shaped all-silica DDR zeolite membrane have been the subject of a thorough modeling study over a challenging broad temperature (220−373 K) and feed pressure (101−1500 kPa) range. The mass transport through the zeolite layer is evaluated for two rival, Maxwell Stefan-based, models: the Relevant Site Model (RSM) and the so-called Reed Ehrlich (RE) approach. Both models have been introduced to account for the strong loading dependency of the diffusivity in small-pore cage-like zeolites like DDR. High pressure adsorption isotherms (up to 7000 kPa) measured on DDR crystals are incorporated to describe adsorption on the zeolite. Both the RSM as the RE approach yield an excellent model fit of the single component permeation data. However, for both models the N2 and CH4 data did not allow an accurate estimation of the model fit parameters. Both models can lead to a good prediction of comparable quality of the mixture permeation data based on the single component model fit parameters. The RE approach is very sensitive toward the model input parameters and the estimated mixture loading, which both can be very hard to determine accurately in practice. The RSM does not suffer from both these issues, which is an evident advantage with respect to application of this model.

A new model is introduced to describe the loading dependency of diffusion in zeolites. The model is formulated around the idea of segregated adsorption in cage-like zeolites, that is, that molecules are located either in the cage or its window site. Furthermore, only the molecules located at the window site are able to make a successful jump to another cage. This so-called relevant site model (RSM) is based on the Maxwell−Stefan framework for mass transport but includes one extra parameter that describes the adsorption properties of the “relevant site”. The RSM describes diffusivity data of N2 and CO2 in DDR (eight-ring cage-like zeolite) very well up to saturation. The observed diffusivity loading dependency is explained from the relative low window site occupancy that is typically much lower than the total occupancy at lower loadings. The model is successfully extended to nonisothermal diffusivity data of CO2 and N2 in DDR. Relating intermolecular correlation effects to the relevant site occupancy instead of the total occupancy leads to a quantitative prediction of the observed correlation effects and, consequently, the self-diffusivity. Analysis of the N2 data suggests positional rearrangements in the DDR cages in a certain loading range. These effects have been incorporated in the model successfully.

The recently introduced relevant site model (RSM) (Van den Bergh, et al. J. Phys. Chem. C, 113, 2009, 17840) to describe the loading dependency of single-component diffusion in zeolites is extended to mixtures. The model is formulated around the central idea of segregated adsorption in structures consisting of cages connected by windows, distinguishing cage and window adsorption sites, and only the molecules located at the window site (i.e., the relevant site RS) are able to make a successful jump to the next cage. The RSM is based on the Maxwell−Stefan framework for mass transport but includes only one extra parameter that describes the adsorption properties of the ‘relevant site’. Key feature of the RSM as applied to mixtures is that competitive adsorption effects and ‘speeding up and slowing down’ (exchange) effects between guest molecules are related to the relevant site loading instead of the overall loading, which can be very different. Analysis of an extensive set of diffusivity data of N2/CO2 and Ne/Ar mixtures in zeolite DDR, directly computed using molecular dynamics, shows that the RSM provides excellent mixture diffusivity predictions from single component data. The results are comparable to the ‘Reed−Ehrlich’ approach as put forward by Krishna and co-workers (e.g., Sep. Purif. Technol. 61, 2008, 414). Although the model predictions are comparable, the two approaches are fundamentally different since in the Reed−Ehrlich approach the loading dependency of diffusion is described by intermolecular repulsions. A clear improvement by the RSM approach is found in the case of the N2 diffusivity in N2/CO2 mixtures, attributed to the specific window blocking effect by CO2 and inherently incorporated in the RSM by relating adsorption to the relevant (window) site.

Permeation of various gases (carbon dioxide, nitrous oxide, methane, nitrogen, oxygen, argon, krypton, neon) and their equimolar mixtures through DD3R membranes have been investigated over a temperature range of 220–373 K and a feed pressure of 101–400 kPa. Helium was used as sweep gas at atmospheric pressure. Adsorption isotherms were determined in the temperature range 195–298 K, and modelled by a single and dual site Langmuir model. The permeation flux is determined by the size of the molecule relative to the window opening of DD3R, and its adsorption behaviour. As a function of temperature, bulky molecules (methane) show activated permeation, weakly adsorbing molecules decreasing permeation behaviour and strongly adsorbing molecules pass through a maximum. Counter diffusion of the sweep gas (helium) ranged from almost zero up to the order of the feed gas permeation and was strongly influenced by the adsorption of the feed gas.DD3R membranes have excellent separation performance for carbon dioxide/methane mixtures (selectivity 100–3000), exhibit good selectivity for nitrogen/methane (20–45), carbon dioxide and nitrous oxide/air (20–400), and air/krypton (5–10) and only a modest selectivity for oxygen/nitrogen (∼2) separation. The selectivity of mixtures of a strongly and a weakly adsorbing component decreased with increasing temperature and pressure. The selectivity of mixtures of weakly adsorbing components was independent of pressure.The permeation and separation characteristics of light gases through DD3R membranes can be explained by taking into account: (1) steric effects introduced by the window opening of DD3R leading to molecular sieving and activated transport, (2) competitive adsorption effects, as observed for mixtures involving strongly adsorbing gases, and (3) interaction between diffusing molecules in the cages of the zeolite.

The permeation of CO2 and CH4 and their binary mixtures through a DDR membrane has been investigated over a wide range of temperatures and pressures. The synthesized DDR membrane exhibits a high permeance and maintains a very high selectivity for CO2. At a total pressure of 101 kPa, the highest selectivity for CO2 in a 50∶50 feed mixture was found to be over 4000 at 225 K. This is ascribed to the higher adsorption affinity, as well as to the higher mobility for the smaller CO2 molecules in the zeolite, preventing the bypassing of the CH4 through the membrane. An engineering model, based on the generalized Maxwell-Stefan equations, has been used to interpret the transport phenomena in the membrane. The feasibility of DDR membranes as applied to CO2 removal from natural gas or biogas is anticipated.

Different amounts of cesium (0.8–1.3 at.%) and lithium (up to 0.5 at.%) were introduced in the tunnel structure of cryptomelane in order to tailor its physical and chemical properties. The mechanism by which cryptomelane accommodates these alkali cations is different; Li occupies mainly empty sites by a redox-type reaction, while Cs is ion-exchanged by H3O+ cations. The adsorption of these alkali metals into cryptomelane was highly selective and is dictated by the solvation energy and ionic radius. Extrinsic defects (e.g. positive holes and oxygen vacancies) are created upon doping, enhancing the redox-properties of cryptomelane. The basicity of the framework also increases when Cs and Li are incorporated. The catalytic properties of this material were tested in the oxidation of ethyl acetate, and greatly improved after doping. The observed activity increase is explained by the redox and basic properties of the modified materials. The amount of cesium loaded has no effect on the activity, as a result of pore mouth catalysis.Graphical abstractCryptomelane-type manganese oxide with cesium and lithium incorporated in its tunnel structure is a highly active catalyst for the oxidation of ethyl acetate.Download high-res image (155KB)Download full-size imageHighlights► Li and Cs were successfully incorporated in the cryptomelane structure. ► Alkali metals doping enhances both the reducibility and the basicity of cryptomelane. ► These parameters have a significant effect on the catalytic properties. ► Li and especially Cs-doped catalysts are highly active for ethyl acetate oxidation. ► The amounts of Cs or Li incorporated do not influence the performance (pore mouth activity).

•Cerium-doped cryptomelane stabilizes polyhedron Au nano-particles smaller than 3 nm.•The obtained Au/Ce/cryptomelane catalyst is highly active for CO oxidation.•Cationic, neutral, and anionic gold species were found on Ce/cryptomelane.•Ce increases the number of defect sites and charge transfer between Au and KOMS-2.•TOF for Au/Ce–K-OMS-2 is about twice that of Au/CeO2 prepared by the same procedure.Cerium-doped cryptomelane stabilizes polyhedron Au nano-particles smaller than 3 nm. The obtained Au/Ce/cryptomelane catalyst is highly active for CO oxidation. Cationic, neutral, and anionic gold species were found on Ce/cryptomelane. The modification of cryptomelane by the addition of cerium is twofold: (i) cerium is incorporated in the tunnels of cryptomelane, increasing the number of defects which function as nucleation sites for gold and (ii) ceria nano-particles at the surface induce charge transfer between gold and cryptomelane. TOF for Au/Ce–K-OMS-2 is about twice that of Au/CeO2 prepared by the same procedure.Graphical abstractThe interplay of stabilized (sub)-nanosized gold species, cryptomelane, and cerium (as dopant and ceria nano-particles) results in a highly active gold catalyst for the low-temperature oxidation of CO.Download high-res image (110KB)Download full-size image

The recently introduced relevant site model (RSM) (Van den Bergh et al., J. Phys. Chem. C, 113 (2009), 17840) to describe the loading dependency of diffusion in zeolite DDR is successfully extended to a variety of light gases (CH4, CO2, Ar and Ne) and binary mixtures thereof in other zeolite topologies, DDR, CHA, MFI and FAU, utilizing the extensive diffusivity dataset published by Krishna and van Baten for this variety of zeolite-guest systems (e.g. Chem. Eng. Sci., 63 (2008), 3120 (supplementary material)).The RSM is formulated around the central idea of segregated adsorption in structures consisting of cages connected by windows, distinguishing cage and window adsorption sites. Only the molecules located at the window site (i.e. the relevant site (RS)) are able to make a successful jump to the next cage. The RSM is based on the Maxwell–Stefan framework for mass transport but includes only one extra parameter that describes the adsorption properties of the ‘relevant site’. Key feature of the RSM as applied to mixtures is that competitive adsorption effects and ‘speeding up and slowing down’ (exchange) effects between guest molecules are related to the relevant site loading and composition instead of to the overall loading, which can be very different.From the RSM approach a measure for the level of adsorption segregation is derived: the ratio of the RS and total occupancy. The predicted level of adsorption segregation correlates well with the level of confinement of a molecule at the RS: the molecule diameter to zeolite pore diameter. The predicted degree of adsorption segregation of the studied light gases in DDR is in good agreement with molecular simulations results, indicating the physical meaningfulness of the estimated RS adsorption parameters.The binary mixture diffusivity modelling points out that in case of the small-pore zeolites (DDR and CHA) the data is described best with equal RS saturation loadings for both components. For the large pore zeolite FAU the ratio of the RS saturation loadings equals that of the bulk saturation loadings. The geometry of the RS strongly influences the RS saturation loading: in case of the small-pore zeolites the RS (= window site) is restricted to only one molecule but when the RS becomes larger its saturation loading becomes similar to that of the bulk.

Improvements in catalyst activity make the heat transport in fixed bed reactors increasingly important. Structured packings operated in two-phase flow are expected to outperform randomly packed beds, but heat transfer data on structured packings is scarce. In this work structured packings such as OCFS (Open Cross Flow Structures), CCFS (Closed Cross Flow Structures), knitted wire, and foam were characterised with respect to the heat transfer performance. A dedicated set-up was designed and built which enabled us to measure the heat transfer rates in two-phase flow at ambient pressure in the absence of reaction. Benchmarking and set-up validation was carried out using glass beads. The structured packings—especially OCFS and CCFS—show heat transfer coefficients that are superior over those of glass beads, at lower energy dissipation.

Multiphase catalytic processes involve the combination of scale-dependent and scale-independent phenomena, often resulting in a compromised, sub-optimal performance. The classical approach of randomly packed catalyst beds using unstructured catalyst particles may be outperformed by the careful design of the catalyst at the nano-scale and by the judicious choice and design of reactor. Application of structured catalysts and reactor internals and the combination of advanced reactor and catalyst systems with in situ separation allow decoupling the various phenomena involved, opening the way to intensified processes on a large scale. The integral approach of Catalysis and Reaction Engineering discussed here will play a pivotal role in the development of novel, future-proof processes.