Dehydration Of Formic Acid Research Articles

Electrostatic Field Regulate Interfacial Hole Transfer for Photocatalytic Valorization of Biopolyols

AbstractPhotocatalysis has shown power in the valorization of biomass while controlling the selectivity is a long‐standing goal and challenge, especially for the complex and reactive bio‐polyols. The selectivity control of photocatalytic biopolyols reforming to formic acid (FA) or CO is realized via engineering the electrostatic field on TiO2 semiconductor. The electrostatic field is generated by surface modification with anion adsorbates, which can alter the trap states of photoexcited holes and regulate interfacial hole transfer to the surface‐bound species, thereby strongly affecting photocatalytic activity. Taking formic acid dehydration as an example, a shallow trap of photoexcited holes on pristine TiO2 favors the dehydration of FA to CO, while a deep trap of photoexcited holes after introduced anion adsorbates makes FA stable. Based on this result, the selectivity is successfully tuned in the photocatalytic oxidation of biopolyols via controlling electrostatic field. A wide range of biopolyols can be selectively converted into FA or CO. This work presents an effective strategy to manipulate reaction pathways via generating electric field.

Conditions and Mechanism of Formation of the Maillard Reaction Pigment, Furpenthiazinate, in a Model System and in Some Acid Hydrolyzates of Foods and its Biological Properties.

Furpenthiazinate is a yellow pigment formed by the Maillard reaction between cysteine and furfural under strongly acidic conditions. Here, we describe the conditions and mechanism of pigment formation in a model system and in an acid hydrolyzate of food and analyze its biological properties. A reaction solution containing 32 mM cysteine and 128 mM furfural or 64 mM cysteine and 256 mM furfural in the presence of 2-6 M hydrochloric acid that was heated to 110 °C for 1-2 h yielded approximately 3 mM furpenthiazinate. Nuclear magnetic resonance analysis of furpenthiazinate prepared using 1-13C or 5-13C d-ribose suggests that it was formed through the condensation of cysteine and two C5 chains derived from pentose with the dehydration and elimination of formic acid. Furpenthiazinate was detected in mieki, a seasoning, and some acid hydrolyzates of food, and it did not show antibacterial or mutagenic activity.

Hydrogen-free carbon monoxide production through decomposition of formic acid over a HPW/TiO2 catalyst

The dehydration of formic acid over supported phosphotungstic acid (H3PW12O40, HPW) catalysts was investigated for the production of hydrogen-free carbon monoxide (CO). The variations in the support type (SiO2, Al2O3, ZrO2, and TiO2) and HPW content (0.0–20.0 wt.%) affect the acidity of catalysts, which was demonstrated by the temperature-programed desorption of iso-propanol (IPA-TPD) and pyridine-adsorbed infrared spectroscopy (Py-IR). The catalytic activity in formic acid dehydration exhibited a volcano-type distribution in accordance with the HPW content and was maximized at 15 wt.% loading. Furthermore, the catalytic activity was more closely related to the density of Brønsted acids than that of Lewis acids and inversely proportional to the peak temperature of propylene desorption during IPA-TPD. The strongly adsorbed H2O on the catalysts, which is generated during formic acid dehydration, induces deactivation of the catalyst, particularly at lower reaction temperatures (<230 °C).

Kinetics of formic acid dehydration on Pt electrodes by time-resolved ATR-SEIRAS.

The potential dependence of the rate of dehydration of formic acid to adsorbed CO (COad) on Pt at pH 1 has been studied on a polycrystalline Pt surface by time-resolved surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS) with simultaneous recording of current transients after a potential step. A range of formic acid concentrations has been used to obtain a deeper insight into the mechanism of the reaction. The experiments have allowed us to confirm that the potential dependence of the rate of dehydration has a bell shape, going through a maximum around the potential of zero total charge (pztc) of the most active site. The analysis of the integrated intensity and frequency of the bands corresponding to COL and COB/M shows a progressive population of the active sites on the surface. The observed potential dependence of the rate of formation of COad is consistent with a mechanism in which the reversible electroadsorption of HCOOad is followed by its rate-determining reduction to COad.

Pd single atom supported on N-doped egg tray graphene as formic acid dehydrogenation catalysts

Graphene and many 2D carbon allotropes are good support materials for single-atom catalysts (SACs) and have been successfully applied to many catalytic reactions. Herein, based on the egg tray graphene (ETG), a carbon allotrope constructed in our previous report, we designed ETG and three N-doped ETG supported Pd SACs, Pd@ETG-N x (x= 0–3), for dehydrogenation of formic acid (HCOOH) by density functional theory. Our calculations show that ETG is easier for N doping than graphene, and Pd single atom can be stably adsorbed on the ETG with different N doping concentrations. Major pathways of formic acid dehydrogenation and dehydration were identified. We found that HCOOH dehydrogenation proceeds along the COOH-mediated pathway on each catalyst. With the increased N content in the substrate, the activity and H2 selectivity of Pd SACs are greatly improved. Especially, among these four SACs, Pd@ETG-N3 shows the best catalytic performance, which is even better than Pd(111). Furthermore, electronic analysis was made to reveal the metal-support interactions and the origin of the activity trend of Pd@ETG-N x . Our study reveals the unique potential of carbon allotropes in catalyst design, and provides theoretical insights for rational design of efficient catalysts by adjusting the support and the coordination environment.

Facet-Dependent Electron Transfer Regulates Photocatalytic Valorization of Biopolyols.

The electron transfer (ET) from the conduction band of the semiconductor to surface-bound species is a key step in the photocatalytic reaction and strongly affects the reactivity and selectivity, while the effect of catalyst surface structure on this process has rarely been explored due to the lack of an effective method. Herein, we have developed a strategy to detect and measure surface electrons' transfer energy to the adsorbates and disclosed a facet-dependent electron transfer energy over anatase TiO2. The photogenerated electrons are shallowly confined in the five-coordinated Ti atom (Ti5c) on the surface of the (101) facet with a transfer energy below 1.0 eV, while deeply confined in the six-coordinated Ti atom (Ti6c) on the subsurface of the (001) facet with a transfer energy higher than 1.9 eV. The different electron trap states strongly affect the ET process, thus regulating the photocatalytic activity. Taking formic acid (FA) dehydration as the probe reaction, a shallow trap of photoexcited electrons on the (101) facet of anatase TiO2 favors the dehydration of FA to CO, while a deep trap of photoexcited electrons on the (001) facet makes FA stable. Based on this knowledge, we successfully controlled the selectivity in the photocatalytic oxidation of biopolyols via selectively exposing the facet of TiO2. Through controlling the (001)/(101) facet, a wide range of biopolyols can be selectively converted into FA or CO with a selectivity of up to 80%. The present work disclosed a facet-dependent electron transfer process and provides a new horizon to the design of photocatalytic systems.

Production of H2-Free Carbon Monoxide from Formic Acid Dehydration: The Catalytic Role of Acid Sites in Sulfated Zirconia.

The formic acid (CH2O2) decomposition over sulfated zirconia (SZ) catalysts prepared under different synthesis conditions, such as calcination temperature (500–650 °C) and sulfate loading (0–20 wt.%), was investigated. Three sulfate species (tridentate, bridging bidentate, and pyrosulfate) on the SZ catalysts were characterized by using temperature-programmed decomposition (TPDE), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The acidic properties of the SZ catalysts were investigated by the temperature-programmed desorption of iso-propanol (IPA-TPD) and pyridine-adsorbed infrared (Py-IR) spectroscopy and correlated with their catalytic properties in formic acid decomposition. The relative contributions of Brønsted and Lewis acid sites to the formic acid dehydration were compared, and optimal synthetic conditions, such as calcination temperature and sulfate loading, were proposed.

H2 from biofuels and carriers: A kinetic investigation of formic acid decomposition on Rh/Al2O3 in the annular reactor

Vapor-phase decomposition of formic acid on Rh/Al2O3 catalyst was studied performing experiments in an isothermal annular microreactor. Effects of operating conditions on formic acid conversion and composition of products mixture were explored. Formic acid conversion was activated at 150 °C and two parallel decomposition routes were involved: dehydrogenation (predominant reaction) and dehydration (minor reaction). It was shown that water gas shift reaction played a kinetic role only above 300 °C. At T > 500 °C, the system reached chemical equilibrium, with formic acid achieving complete conversion around 400 °C. The kinetic relevance of each reaction step of this triangular network was singled out to derive a molecular kinetic scheme. This kinetic analysis demonstrated that, despite being a minor reactivity, formic acid dehydration is characterized by a lower activation energy with respect to dehydrogenation (50 vs 67 kJ/mol). Unfavorable energetics of the competing reactions and unfavorable low-temperature water gas shift are the kinetic factors behind the impossibility to obtain CO-free H2 in the low temperature region on Rh catalyst. Conversely, a sensitivity analysis on water gas shift kinetics showed that the selectivity can be optimized at intermediate temperatures by selectively promoting the consecutive consumption of CO to CO2, which opens to catalyst design and bifunctional catalysts.

Study on the application of sulfonation catalysis in a new formaldehyde recovery process

This study proposed a new catalytic conversion process using a sulfonated catalyst to promote the condensation reaction of methanol and formaldehyde (FA) to form methylal (DMM). This method can remove FA while generating the valuable chemical product DMM. A series of SiO 2 -xSO 3 H catalysts were developed, and the synergistic effect of the sulfonic acid group vacancy and hydroxyl vacancy contents was evaluated by adjusting the activation temperature and sulfonation ratio. When the activation temperature was 60 °C, the FA yield reached 82.3%, and DMM selectivity was 80.4%. The FA recovery rate was much higher than in other studies. A series of characterization studies revealed that when there were few -SO 3 H groups, the hydroxyl content increased, providing more activation centers for forming the formic acid by-product from FA. The dehydration and condensation of formic acid and methanol formed methyl formate (MF). However, when there were too many -SO 3 H groups, side reactions of methanol and FA occurred preferentially. Therefore, the synergistic effect of the -SO 3 H group and hydroxyl group content was the fundamental reason for the exceptional catalytic performance. Furthermore, in-situ infrared spectroscopy revealed the possible reaction mechanism of methanol and FA on sulfonated silica gel and verified the conclusion drawn from the catalyst characterization. This approach overcomes the disadvantages of the high cost of FA absorption and difficulty of industrialization due to the high reaction temperature and provides the potential to “turn waste into treasure” during FA recovery while providing new avenues to solving the problem of FA pollution.

Study on the optimized Al2O3–SO3H catalysis for a new formaldehyde recovery process

Formaldehyde is a dangerous chemical present in many industrial waste gases. It can be recovered via catalytic processes; however, many of these require costly precious metals operating under limited reaction temperatures, making their industrialization difficult. This paper proposes a novel method of formaldehyde recovery. A sulfonation catalyst is used to promote a condensation reaction of methanol and formaldehyde to form methylal. Hence, a valuable chemical product is obtained while removing formaldehyde. A series of Al2O3-xSO3H catalysts were designed and the effects of surface acidity and hydroxyl groups on catalytic activity were explored by varying the activation temperature and sulfonation ratio. At an activation temperature of 60 °C, the yield of formaldehyde reaches 78.62% and the selectivity to methylal reaches 80.11%. This recovery rate is much higher than those reported in other studies. Through a series of characterizations, it was found that when the content of –SO3H groups is low, the hydroxyl content increases, which provides more activation centers for the formation of by-product formic acid from formaldehyde, and for the dehydration and condensation of formic acid and methanol to form methyl formate. At the same time, when the content of –SO3H groups is too high, side reactions of methanol and formaldehyde occur preferentially. Therefore, the synergistic effect of the contents of –SO3H and hydroxyl groups is the fundamental cause of the excellent catalytic performance. Based on in-situ infrared research, a possible mechanism of the reaction between methanol and formaldehyde on sulfonated silica gel is proposed, which also verifies the characterization of the catalyst. This research presents new possibilities for solving the problem of formaldehyde pollution.

Dehydrogenation and dehydration of formic acid over orthorhombic molybdenum carbide

The dehydrogenation and dehydration of formic acid is investigated on the β-Mo2C (100) catalyst surface using time independent density functional theory. The energetics of the two mechanisms are calculated, and the thermochemistry and kinetics are discussed using the transition state theory. Subsequently, microkinetic modelling of the system is conducted, considering the batch reactor model. The potential energy landscape of the reaction shows a thermodynamically favourable cleavage of HCOOH to form CO; however, the kinetics show that the dehydrogenation mechanism is faster and CO2 is continuously formed. The effect of HCOOH adsorption on the surface is also analysed, in a temperature-programmed desorption, with the conversion proceeding at under 350 K and desorption of CO2 is observed with a selectivity of about 100 %, in line with the experimental reports.

Plasmon enhanced glucose photoreforming for arabinose and gas fuel co-production over 3DOM TiO2-Au

Glucose photoreforming provides a promising alternative strategy for biomass valorization. However, the use of harsh environment (high alkalinity or organic solvents) and low product selectivity due to non-selective free radical oxidative cleavage limit their application in large-scale settings. Here, we show photoreforming of glucose to arabinose with high selectivity in water using gold nanoparticles decorated three dimensionally ordered macroporous TiO2 (3DOM TiO2-Au). We demonstrate that the hierarchically porous 3DOM architecture and Au nanoparticles enhance glucose conversion and arabinose selectivity by improving glucose-photocatalysts interaction, electron generation, charge separation, and localized surface plasmon resonance (LSPR) induced light absorption. The 3DOM TiO2-Au produces arabinose via the direct C1-C2 α-scissions mechanism as evidenced by 13C labeled glucose at C1 position. Our experimental results demonstrate the presence of Au(I) is crucial for glucose to arabinose selective conversion. Aside from arabinose production, 3DOM TiO2-Au also generates considerable amount of gas fuels (H2, CH4 and CO) from transient dehydration and hydrogenation reaction of the by-product formic acid. The present work demonstrates a green and promising approach to convert biomass derived feedstocks into value-added chemicals along with gas fuel production.

New Mechanism for Dehydration of HCOOH on Pt(111)

Existing computed results for formic acid (HCOOH) dehydration on Pt catalysts cannot explain the facile CO poisoning effect. By DFT calculations, this work studied HCOOH dehydration on a Pt(111) su...

Formic Acid Dehydration Rates and Elementary Steps on Lewis Acid–Base Site Pairs at Anatase and Rutile TiO2 Surfaces

Formic acid (HCOOH) decomposition is often used to assess the acid–base properties of oxide surfaces. Its reverse reaction forms HCOOH and formate species that can act as intermediates in CO2/CO/H2...

Elementary steps and site requirements in formic acid dehydration reactions on anatase and rutile TiO2 surfaces

Mechanistic details of HCOOH decomposition routes provide valuable insights into reactions involving bound formates as intermediates or spectators; these routes are also widely used as a probe of the acid-base properties of oxide surfaces. The identity and kinetic relevance of bound intermediates, transition states, and elementary steps are reported here for HCOOH dehydration on anatase and rutile TiO2 surfaces through complementary kinetic, isotopic, spectroscopic and theoretical assessments. Five-coordinate exposed Ti5c centers are saturated with bidentate formates (*HCOO*) at catalytic conditions (423–463 K; 0.1–3 kPa HCOOH), as evident from infrared spectra collected during catalysis and the amounts of HCOOH and CO evolved upon heating the TiO2 samples containing pre-adsorbed HCOOH-derived species. These *HCOO* species are inactive but form a stable “surface template” that contains stochiometric protons onto which HCOOH binds molecularly (HCOOH-H*) to form a coexisting adlayer. H2O elimination from HCOOH-H* is the sole kinetically-relevant step. DFT-derived barriers show that this step involves its reaction with Ti5c-O2c that acts as a Lewis acid-base pair. Such route, in turn, requires the access of HCOOH-H* to a Ti5c center, which is made available through a momentary reprotonation of a *HCOO*. This step is much less facile on rutile than on anatase due to stronger acid strength of its Ti5c centers that binds *HCOO* species more strongly and its shorter Ti5c-Ti5c distances that induce greater repulsions between co-adsorbed HCOOH* formed upon reprotonation step. These differences account for low dehydration reactivity of rutile at these temperatures. This mechanistic interpretation is in full accord with DFT-derived barriers, binding energies, and kinetic isotope effects that quantitatively agree with the values from regressed kinetic and thermodynamic parameters, with in-situ infrared spectra that identify HCOOH-H* species as the sole reactive intermediates, and with the differences in turnover rates between anatase and rutile catalysts. These dehydration routes are also consistent with the surface chemistry expected for Lewis acid-base pairs on stoichiometry TiO2 surfaces without requiring the presence or involvement of reduced centers or titanols in the catalytic cycle. The reaction routes described in this work show how strongly-bound species, evident in presence and unreactive nature from in-situ infrared spectra, provide an organic “permanent” template for reactions of weakly-bound species that are often invisible in spectroscopy.

Enhanced formic acid dehydrogenation by the synergistic alloying effect of PdCo catalysts supported on graphitic carbon nitride

The catalytic ability of graphitic carbon nitride (g-C3N4)-supported composition-controlled PdCo catalysts towards H2 generation from the formic acid dehydrogenation reaction was assessed in this study and a noticeable composition dependence was evidenced. It was seen that the alloying effect combined with the nitrogen functionalities present on g-C3N4 assisted the formation of small and well-distributed nanoparticles. This fact, combined with the electronic promotion of Pd species via charge transfer from Co and basic features of the support, resulted in enhanced catalytic activities compared to that displayed by the counterpart Pd/g-C3N4, reaching a TOF value of 1193 h−1 for the most active catalyst among investigated (PdCo/g-C3N4 (1/0.7)). Furthermore, the present catalytic system showed high selectivity towards formic acid dehydrogenation, suppressing the generation of undesired CO via formic acid dehydration, which makes it a suitable candidate for practical application in fuel cells.

Dehydrogenation of Formic Acid over a Homogeneous Ru-TPPTS Catalyst: Unwanted CO Production and Its Successful Removal by PROX

Formic acid (FA) is considered as a potential durable energy carrier. It contains ~4.4 wt % of hydrogen (or 53 g/L) which can be catalytically released and converted to electricity using a proton exchange membrane (PEM) fuel cell. Although various catalysts have been reported to be very selective towards FA dehydrogenation (resulting in H2 and CO2), a side-production of CO and H2O (FA dehydration) should also be considered, because most PEM hydrogen fuel cells are poisoned by CO. In this research, a highly active aqueous catalytic system containing Ru(III) chloride and meta-trisulfonated triphenylphosphine (mTPPTS) as a ligand was applied for FA dehydrogenation in a continuous mode. CO concentration (8–70 ppm) in the resulting H2 + CO2 gas stream was measured using a wide range of reactor operating conditions. The CO concentration was found to be independent on the reactor temperature but increased with increasing FA feed. It was concluded that unwanted CO concentration in the H2 + CO2 gas stream was dependent on the current FA concentration in the reactor which was in turn dependent on the reaction design. Next, preferential oxidation (PROX) on a Pt/Al2O3 catalyst was applied to remove CO traces from the H2 + CO2 stream. It was demonstrated that CO concentration in the stream could be reduced to a level tolerable for PEM fuel cells (~3 ppm).

From HCOOH to CO on Pd catalyst: theoretical study

The present theoretical and experimental results in the literature for formic acid (HCOOH) oxidation into CO on Pd(111) cannot rationalize the well-known, easy CO poisoning. The present study reexamines HCOOH oxidation on Pd(111) by performing density functional theory calculations. Upon a thorough search, we present a new adsorption configuration of HCOOH on Pd(111). From the calculated energy pathway, we proposed that CO arises from the reduction of the hydrogenation product CO2 and direct dehydration of formic acid during HCOOH oxidation on Pd(111), with latter step being energetically easier. The present theoretical study rationalizes the early experimental findings and provides insights into the deactivation problem of Pd catalyst in the process of HCOOH oxidation.

Combined Ab Initio, Kinetic Modeling, and Shock Tube Study of the Thermal Decomposition of Ethyl Formate.

The potential energy surfaces (PESs) and reaction rate constants of the unimolecular decomposition of ethyl formate (EF) were investigated using high-precision theoretical methods at the CCSD(T)/CBS(T-Q)//M06-2X/6-311++G(d,p) level of theory. The calculated PESs of EF dissociation and molecular decomposition reactions indicate that the intramolecular H-shift to produce formic acid and ethylene is the dominant decomposition pathway. A detailed chemical kinetic mechanism for EF pyrolysis was constructed by incorporating the important reactions of EF and its radicals into an existing mechanism previously developed for small methyl esters. The updated mechanism was first used to reproduce CO, CO2, and H2O concentration time histories during EF pyrolysis in the shock tube reported by Ren et al. [ Ren , W. , Mitchell Spearrin , R. , Davidson , D. F. , and Hanson , R. K. J. Phys. Chem. A 2014 , 118 , 1785 - 1798 ]. The rate of production and sensitivity analyses show that the competing dehydration and decarboxylation channels of the intermediate formic acid control the final product yields of EF pyrolysis. The EF mechanism was further validated against the shock tube data of OH, CO, CO2, and H2O time histories measured during EF oxidation (equivalence ratio Φ = 1.0) at 1331-1615 K and 1.52-1.74 atm. This revised EF mechanism captured all of the species' time histories over the entire temperature range. Such modeling capability was due to the more accurate rate constants of EF reactions determined by high-precision theoretical calculations and a high-fidelity C0-C2 basis mechanism.

Exfoliated HNb3O8 nanosheets of enhanced acidity prepared by efficient contact of K2CO3 with Nb2O5

Although exfoliated HNb3O8 nanosheet (e-HNb3O8) is known to be a powerful catalyst in acid-catalyzed reactions, the preparation technique has limitations in its high-yield production. We herein report the enhanced exfoliation efficiency and recovery of e-HNb3O8 by contacting Nb2O5 solid with a diluted K2CO3 solution before ball milling (mechanochemical activation). The so-obtained e-HNb3O8 showed a larger specific surface area, a higher density of total acid sites and an improved catalytic performance in the dehydration of 2-heptanol and formic acid, compared to the corresponding sample prepared by the conventional solid-state mixing. The ball-milled K2CO3–Nb2O5 mixtures, layered KNb3O8, ion-exchanged HNb3O8 and exfoliated HNb3O8 samples were characterized by PXRD, TEM-EDS, TGA-MS and SEM-EDS. The characterization results revealed more intercalated K+ ions in KNb3O8 particles as well as more homogeneous K2CO3–Nb2O5 mixture when the employed preparation technique was used. This is achieved by more infiltration of K2CO3 into Nb2O5 lattice before ball milling, thereby resulting in the above positive findings. Therefore, our approach has great potential to be extended to other niobate-containing nanosheet-type materials.