The mechanism of a TiO2-catalyzed n-valeraldehyde self-condensation reaction was first investigated using in situ Fourier transform–infrared spectroscopy (FT-IR) analysis. The result shows that the n-valeraldehyde molecule is adsorbed in two ways separately to Ti4+ and Ti–OH active sites: one involving a strong interaction between the surface Ti4+ and the carbonyl oxygen of n-valeraldehyde molecule, causing a red shift of ν(C═O) and the other involving an interaction between the TiO2 surface hydroxyl group Ti–OH and the carbonyl oxygen of n-valeraldehyde molecule via a hydrogen bond, causing a significant shift of Ti–OH peaks to a lower wavenumber. On the basis of the in situ FT-IR analysis, a TiO2-catalyzed n-valeraldehyde self-condensation reaction mechanism was proposed. In the process of n-valeraldehyde adsorption, the infrared characteristic peaks of 2-propyl-3-hydroxyheptanal were not observed, indicating that the dehydration of 2-propyl-3-hydroxyheptanal to 2-propyl-2-heptenal proceeded very quickly. In the process of desorption of products, the infrared characteristic peaks of carboxylates were detected on the surface of TiO2, suggesting that n-pentanoic acid is generated in the reaction system. In addition, we speculate that n-pentanoic acid has a strong interaction with the surface of TiO2 and the generated carboxylates are hardly desorbed, leading to the deactivation of TiO2 catalyst. In order to get a better understanding of the process of TiO2-catalyzed n-valeraldehyde self-condensation, the liquid-phase reaction was monitored in real time by an in situ IR (React-IR). In the whole course of the reaction, 2-propyl-3-hydroxyheptanal was not detected, confirming that this intermediate cannot exist stably, not only on the TiO2 surface but also in the reaction liquid. In order to further verify the reaction mechanism and to confirm the rate-determining step, several possible Langmuir–Hinshelwood models were assumed. By the model identification, the Langmuir–Hinshelwood model with the surface reaction as the rate-determining step is found to be the most probable one.Keywords: in situ FT-IR; kinetics; mechanism; n-valeraldehyde; React-IR; TiO2;

2-Ethylhexanol (2EHO) is an important organic chemical. The industrial production of 2EHO comprises three units: propylene hydroformylation to n-butyraldehyde, n-butyraldehyde self-condensation to 2-ethyl-2-hexenal (2E2H), and 2E2H hydrogenation to 2EHO. In the present work, 2EHO was synthesized by one-pot sequential aldol condensation and hydrogenation of n-butyraldehyde. Among a series of metal–solid acid bifunctional catalysts, Ni/La–Al2O3 showed a better catalytic performance. The effect of reaction conditions on the one-pot sequential synthesis of 2EHO catalyzed by Ni/La–Al2O3 was investigated, and the suitable reaction conditions were obtained as follows: weight percentage of Ni/La–Al2O3 = 15%, self-condensation reaction conducted at 180 °C for 8 h, and then hydrogenation reaction conducted at 180 °C for 6 h under 4 MPa H2 pressure. Under the above reaction conditions, n-butyraldehyde conversion attained 100% at a 2EHO selectivity of 67.0%. The inhibition of Ni to n-butyraldehyde self-condensation reaction is responsible for the low selectivity of 2EHO. On the basis of the analysis of the reaction system, some side reactions in the one-pot sequential synthesis of 2EHO were proposed. The deactivation of Ni/La–Al2O3 was due to the agglomeration of Ni and La2O3 particles and the occurrence of γ-Al2O3 hydration. Introduction of some hydrophobic groups on the surface of γ-Al2O3 could effectively inhibit the hydration of γ-Al2O3.

Several TiO2 samples with different morphologies and structures, nano/microcomposite of TiO2 (anatase), TiO2 whisker (anatase), and nano-TiO2 (anatase), were prepared and characterized by means of N2 adsorption–desorption, CO2-TPD, and NH3-TPD, and their catalytic performance for n-valeraldehyde self-condensation was investigated. The results indicated that the conversion of n-valeraldehyde was correlated with their acid amount while the selectivity of 2-propyl-2-heptenal was associated with their base amount. Since the catalytic performance of nano-TiO2 (anatase) was the best, its preparation process was further studied and the suitable preparation conditions were obtained. Then the effect of reaction conditions on the catalytic performance of nano-TiO2 (anatase) for n-valeraldehyde self-condensation was investigated and the suitable reaction conditions were obtained as follows: a weight percentage of TiO2 catalyst of 15 wt %, a reaction temperature of 190 °C, and a reaction time of 10 h. Under the above reaction conditions, the conversion of n-valeraldehyde, 2-propyl-2-heptenal yield, and selectivity were 94.6%, 93.7%, and 99.1%, respectively. The TiO2 catalyst could be reused four times without a significant loss in its catalytic performance, which was different from most of the literature. The catalytic stability of TiO2 catalyst was associated with the properties of the active sites, especially acid–base property. Not as some of the literature claimed that their TiO2-catalyzed reactions were base-catalyzed reactions, the TiO2 catalyst used in this work possessed much greater acid amount than base amount. To assess the role of acidic and basic sites in n-valeraldehyde self-condensation, ammonia and carbon dioxide were separately used as a probe molecule for poisoning the corresponding active sites. The results confirmed the key role of acid sites in n-valeraldehyde self-condensation. Therefore, we were convinced that the TiO2-catalyzed n-valeraldehyde self-condensation was mainly an acid-catalyzed reaction.

Direct synthesis of 2-ethylhexanol from n-butanal via the reaction integration of n-butanal self-condensation with 2-ethyl-2-hexenal hydrogenation is of crucial interest for industrial production of 2-ethylhexanol. Furthermore, as an important and versatile chemical, n-butanol can be produced simultaneously by reaction integration. In the present work, several bifunctional catalysts based on γ-Al2O3 were prepared by the impregnation method and were characterized by means of H2-TPR, XRD, TEM and H2-TPD, and their catalytic performance for direct synthesis of 2-ethylhexanol from n-butanal was investigated. The results showed that Co/Al2O3 had a low activity for hydrogenation and Cu/Al2O3 had a high selectivity for the hydrogenation of the CO group while a Ru/Al2O3 catalyst only favored the hydrogenation of n-butanal to n-butanol. Among them, the Ni/Al2O3 catalyst showed the best catalytic performance and the yield of 2-ethylhexanol was the highest (49.4%). Ce-modified Ni/Al2O3 enhanced the competitiveness of aldol condensation versus hydrogenation of n-butanal and improved the selectivity of 2-ethylhexanol; the yield of 2-ethylhexanol rose to 57.8%. Then the influence of preparation conditions on the catalytic performance of Ni/Ce-Al2O3 was investigated and the suitable preparation conditions were obtained as follows: Ni loading = 10%, calcined at 550 °C for 5 h, and reduced at 570 °C for 4 h. The effect of reaction conditions on the integration reaction catalyzed by Ni/Ce-Al2O3 was investigated and the suitable reaction conditions were obtained as follows: weight percentage of Ni/Ce-Al2O3 = 15%, reaction temperature = 170 °C, reaction pressure = 4.0 MPa and reaction time = 8 h. Under the above reaction conditions, the yield of 2-ethylhexanol attained 66.9% and that of n-butanol was 18.9%. In addition, the components existing in the integration reaction system were identified by GC-MS analysis, and the main by-products were n-butyl butyrate, 2-ethylhexyl butyrate, n-butyric acid, etc. Based on the analysis of the reaction system, a reaction network for the direct synthesis of 2-ethylhexanol from n-butanal was proposed. Finally, an evaluation of the reusability of Ni/Ce-Al2O3 showed that the recovered Ni/Ce-Al2O3 catalyst lost its catalytic activity for the hydrogenation of the CO group. The main reason for deactivation was that Ni species were covered by the flaky boehmite γ-AlO(OH) formed from the hydration of γ-Al2O3 in the reaction process.

A series of Ca–Zn–Al oxides were prepared by different methods and their structures, compositions, and basicities were characterized by means of X-ray diffraction (XRD), N2 adsorption–desorption, inductively coupled plasma-atomic emission spectrometry (ICP-AES), and CO2 temperature-programmed desorption (TPD). CO2 TPD analysis showed that all of the prepared Ca–Zn–Al oxides had both weak base sites and strong base sites, but their base amounts were different. The weak base sites were provided by the Zn–Al oxides, whereas the strong base sites were due to CaO. Ca–Zn–Al oxides with different base distributions could be prepared by varying the molar ratios of Zn/Al and Ca/Al. The Ca–Zn–Al oxides showed excellent catalytic performances for the one-pot synthesis of dimethyl carbonate (DMC) from urea, 1,2-propylene glycol (PG), and methanol: The yield of DMC could reach 82.9% under appropriate reaction conditions. However, the catalytic performance of the recovered catalyst decreased dramatically, mainly because of the transformation of CaO into CaCO3 during the reaction process.

4,4′-Methylenedianiline (4,4′-MDA) is a key intermediate for the production of polyurethanes. A series of SO3H-functionalized ionic liquids (SFILs) were synthesized, and their catalytic performance was evaluated in the condensation reaction of aniline with formaldehyde for the synthesis of 4,4′-MDA. The result showed that SFILs had excellent catalytic activity, and their catalytic activity was consistent with their acid strength. [HSO3-bmim]CF3SO3 (SImTf) with the highest acid strength showed the best catalytic activity. Then, the influence of reaction conditions on the condensation reaction of aniline with formaldehyde to 4,4′-MDA was investigated using SImTf as the catalyst, and suitable reaction conditions were obtained as follows: molar ratio of aniline to formaldehyde = 5, mass ratio of SImTf to formaldehyde = 3.5, reaction temperature of 80 °C, and reaction time of 8 h. Under the above reaction conditions, the conversion of aniline was 36.3%, and the yield and selectivity of 4,4′-MDA were 79.4% and 87.9%, respectively. However, the catalytic activity of the recovered SImTf declined dramatically. The results of UV–vis and FT-IR spectroscopy analyses showed that there was a chemical interaction between SImTf and aniline. The deactivated SImTf could be regenerated by acidization with trifluoromethanesulfonic acid, and the acidized SImTf could be reused four times with a tolerable loss of its catalytic activity. Meanwhile, a plausible catalytic reaction mechanism for the synthesis of 4,4′-MDA was proposed.

Self-condensation of n-butyraldehyde is an important process for the industrial production of 2-ethylhexanol. The catalytic performance of some solid acids such as γ-Al2O3 and molecular sieves for the self-condensation of n-butyraldehyde was investigated and the results showed that γ-Al2O3 was the best one. Then the effect of preparation conditions on the catalytic performance of γ-Al2O3 and the effect of reaction conditions on the self-condensation of n-butyraldehyde were discussed. In order to improve the catalytic performance, γ-Al2O3 was modified by different substances and Ce–Al2O3 was found to show the best catalytic performance; the conversion of n-butyraldehyde and the yield of 2-ethyl-2-hexenal could reach 93.8% and 88.6%, respectively. Moreover, the Ce–Al2O3 catalyst had excellent reusability. The XPS analysis of Ce3d demonstrated that the valence state of cerium affected the catalytic performance of Ce–Al2O3 to some extent but not predominantly. Instead the acid–base property of Ce–Al2O3 played a dominant role in the catalytic performance. The reaction components formed over the Ce–Al2O3 catalyst were identified by GC-MS and then some side-reactions were speculated and a reaction network for n-butyraldehyde self-condensation catalyzed by Ce–Al2O3 was proposed. Subsequently, the research on the intrinsic kinetics of n-butyraldehyde self-condensation catalyzed by Ce–Al2O3 showed that both the forward and backward reactions are second order and the corresponding activation energy is separately 79.60 kJ mol−1 and 74.30 kJ mol−1, which is higher than that of the reaction catalyzed by an aqueous base or acid.

Self-condensation of n-butyraldehyde to 2-ethyl-2-hexenal is one of the important processes for the industrial production of 2-ethylhexanol. In the present work, several sulfonic acid functionalized ionic liquids (SFILs) were synthesized. Their acid strengths were determined by the Hammett method combined with UV–vis spectroscopy, and their catalytic performances in n-butyraldehyde self-condensation were investigated. The results show that the conversion of n-butyraldehyde correlated well with the acid strength of the SFILs with the same cation. The SFILs with triethylammonium cations showed a better catalytic performance than those with imidazolium cations or pyridinium cations, and [HSO3-b-N(Et)3]p-TSA (“b”, butyl) exhibited the highest selectivity. Under the optimal reaction conditions of the mass ratio of [HSO3-b-N(Et)3]p-TSA to n-butyraldehyde = 0.1, reaction temperature = 393 K, and reaction time = 6 h, the conversion of n-butyraldehyde was 89.7% and the selectivity to 2-ethyl-2-hexenal was 87.8%. [HSO3-b-N(Et)3]p-TSA could be reused four times without a significant loss in its catalytic performance. A kinetic analysis result showed that this is a reversible second-order reaction. Compared with the kinetic parameters from the reaction catalyzed by an aqueous base or acid catalyst, the pre-exponential factor is lower due to the restriction of the high viscosity of [HSO3-b-N(Et)3]p-TSA. Finally, a possible reaction mechanism for n-butyraldehyde self-condensation catalyzed by [HSO3-b-N(Et)3]p-TSA was proposed.

One-pot synthesis of methyl N-phenyl carbamate (MPC) from aniline, urea, and methanol is a green reaction route. However, along which reaction path the reaction proceeds is still unclear and thereby the reaction mechanism cannot be determined. So the present work put much more emphasis on the analysis of the reaction path of MPC synthesis from aniline, urea, and methanol. First, the components in this reaction system were identified by means of gas-phase chromatograph (GC), high-performance liquid chromatograph (HPLC), and HPLC–mass spectrometry (MS) analyses. On the basis of the change tendency of the reaction components with reaction time, five plausible reaction paths were conjectured: one proceeds via a dimethyl carbonate intermediate, another utilizes methyl carbamate as an intermediate, still another passes a 1-phenyl biuret intermediate, yet another takes place through a phenylurea intermediate, and the last one carries on using diphenylurea as an intermediate. Then on the basis of thermodynamic analysis of the five possible reaction paths in combination with necessary experimental verifications, the one via a phenylurea intermediate was determined as the major reaction path in the absence of catalyst. Lastly, the influence of catalyst on the reaction path was studied and the result showed that the reaction path changed from through the phenylurea intermediate to the methyl carbamate intermediate in the presence of γ-Al2O3 catalyst.

Dimethyl toluene-2,4-dicarbamate (TDC) is an important intermediate for the nonphosgene manufacture of toluenediisocyanate. The synthesis of TDC from 2,4-diaminotoluene (TDA), urea, and methanol is a green route with a good industrialization prospect. So, the study on its reaction kinetics is of great significance. First, the reaction network was structured and simplified on the basis of experimental result analyses. Second, the experimental data for kinetics were measured on an autoclave. Last, the reaction kinetics model was established by means of parametric estimation. Model test results showed that the prediction values of the reaction kinetics model agreed well with the experimental data. Therefore, the reaction kinetics model can be used in the process analyses and scale-up of the reaction of TDA, urea, and methanol to TDC.

A series of ionic liquid-promoted zinc acetate catalysts (Zn(OAc)2-ILs) were prepared by separately mixing anhydrous zinc acetate with one of some ionic liquids, and their catalytic performance for the synthesis of methyl N-phenyl carbamate (MPC) from aniline and dimethyl carbonate (DMC) was evaluated. Among the catalysts, Zn(OAc)2-[bmim]PF6 showed the best catalytic performance. Under the suitable reaction conditions, aniline conversion and MPC selectivity were 99.8% and 99.1%, respectively. Moreover, Zn(OAc)2-[bmim]PF6 could be used five times without a significant loss in its catalytic performance. The mechanism for the promotion of ionic liquid [bmim]PF6 on the catalytic performance of zinc acetate was investigated by means of FT-IR analysis. The results show that a hydrogen bond is formed between the carbonyl oxygen of zinc acetate and the C2–H on an imidazoiume ring of [bmim]PF6. As a result, the zinc acetate changes from a bidentate to a unidentate coordination structure. Simultaneously, this change promotes the coordination of the carbonyl oxygen in a DMC molecule with the zinc atom in the zinc acetate molecule, faciliting the attack of the nitrogen atom in an aniline molecule on the carbonyl carbon in the coordinated DMC molecule and then the formation of MPC.

The catalytic performance of mixed metal oxides was evaluated for the synthesis of diethylene glycol bis(allyl carbonate) (ADC) by transesterification of diethylene glycol (DEG), dimethyl carbonate (DMC). and allyl alcohol (AAH), and MgO–PbO was found to show the highest catalytic activity. The influence of preparation conditions on the catalytic performance of MgO–PbO was studied and its suitable preparation conditions were as follows. A calcination process was used, with Mg(OH)2 and Pb(CH3COO)2·3H2O as precursors, molar ratio of Mg:Pb = 6:1, and calcination temperature of 650 °C. Under the following reaction conditions, molar ratio of DEG:DMC:AAH = 0.08:1:2, weight percentage of MgO–PbO = 1.5% of total weight of all reactants, 100 °C, 6 h, and vacuum of 0.08 MPa, the yield of ADC was 97.3%. Moreover, MgO–PbO showed a good recyclability; ADC yield of 95.4% could be achieved after the catalyst was reused for two times. The results of N2 adsorption–desorption measurement and XPS analyses for MgO–PbO indicate there exists an interaction between MgO and PbO, which promotes the catalytic performance and recyclability of MgO–PbO. Furthermore, the reaction pathway for ADC synthesis was elucidated by means of GC-MS analysis and experimental verification.

Dimethyl toluene-2,4-dicarbamate (TDC) was directly synthesized from 2,4-toluene diamine (TDA), urea, and methanol in order to overcome the drawbacks of other technological routes to TDC. First the thermodynamic analysis for this reaction was made and the results show that the reaction is endothermic and can occur spontaneously beyond 413.8 K. Then the effects of catalyst and reaction conditions were studied. TDA conversion of 98.8% and TDC selectivity of 41.6% were attained in the presence of zinc chloride catalyst and under the suitable conditions of molar ratio of TDA/zinc chloride/urea/methanol = 1/0.07/5/80, reaction temperature of 190 °C, reaction pressure of 3.0 MPa, and reaction time of 9 h. Low TDC selectivity is attributed to the difficulty in the conversion of the intermediates, methyl 2-methyl-5-amino N-phenylcarbamate (TMC1) and methyl 3-amino-4-methyl-N-phenylcarbamate (TMC2), to TDC. Finally on the basis of analyses of HPLC–MS, HPLC, and GC, three possible reaction paths were proposed. One path is the reaction of TDA with urea to toluene-2,4-bisurea (TBU) and then the formation of TDC from the succeeding reaction of TBU with methanol. Another path is the generation of TMC1 and TMC2 from the reaction of TDA with first urea and then methanol, and the succeeding reaction of the formed TMC1 and TMC2 with first urea and then methanol to TDC. Still another path is the formation of methyl carbamate (MC) by the reaction of urea and methanol and then the reaction of TDA with MC to TMC1 and TMC2, and finally a further reaction of TMC1 and TMC2 with MC to TDC.

To facilitate the recovery of Pb/SiO2 catalyst, magnetic Pb/Fe3O4/SiO2 samples were prepared separately by emulsification, sol-gel and incipient impregnation methods. The catalyst samples were characterized by means of X-ray diffraction and N2 adsorption-desorption, and their catalytic activity was investigated in the reaction for synthesizing propylene carbonate from urea and 1,2-propylene glycol. When the gelatin was applied in the preparation of Fe3O4 at 60°C and the pH value was controlled at 4 in the preparation of Fe3O4/SiO2, the Pb/Fe3O4/SiO2 sample shows good catalytic activity and magnetism. Under the reaction conditions of a reaction temperature of 180°C, reaction time of 2 h, catalyst percentage of 1.7 wt-% and a molar ratio of urea to PG of 1:4, the yield of propylene carbonate attained was 87.7%.

The synthesis of methylene diphenyl dimethylcarbamate (4,4′-MDC) from methyl N-phenyl carbonate (MPC) and formaldehyde (HCHO) was conducted in the presence of sulfonic acid-functionalized ionic liquids (ILs) as dual solvent-catalyst. The influences of the kind of anion in the ionic liquids, reaction conditions and the recycle of the ionic liquid on 4,4′-MDC synthesis reaction were investigated. In addition, the acid strength of ILs was determined by the Hammett method with UV-visible spectroscopy, and the acid strength-catalytic activity relationship was correlated. The activity estimation results showed that [HSO3-bmim]CF3SO3 was the optimal dual solvent-catalyst. Under the suitable reaction conditions of 70°C, 40 min, molar ratio of nMPC/nHCHO10/1 and mass ratio of WILs/WMPC4.5/1, the yield of 4,4′-MDC based on HCHO was 89.9 % and the selectivity of 4,4′-MDC with respect to MPC was 74.9%. Besides, [HSO3-bmim]CF3SO3 was reused four times after being purified and no significant loss in the catalytic activity was observed.

Methyl N-phenyl carbamate (MPC), an important organic chemical, can be synthesized from aniline, CO2 and methanol. Catalyst Cu-Fe/ZrO2-SiO2 was first prepared and its catalytic performance for MPC synthesis was evaluated. Then the influence of solvent on the reaction path of MPC synthesis was investigated. It is found that the reaction intermediate is different with acetonitrile or methanol as a solvent. With acetonitrile as a solvent, the synthesis of MPC follows the reaction path with diphenyl urea as the intermediate, while with methanol as a solvent the reaction occurs via the reaction path with dimethyl carbonate as the intermediate. The catalytic mechanism of cooperative catalysis comprising metal sites, Lewis acid sites and Lewis base sites is proposed according to different reaction intermediates.

Toluene-2,4-bisurea (TBU) is an important intermediate for urea route to dimethyl toluene-2,4-dicarbamate and the study on TBU synthesis via the reaction of 2,4-toluene diamine (TDA) and urea is of great significance. Firstly, thermodynamic analysis shows that the reaction is exothermic and a high equilibrium conversion of TDA is expected due to its large reaction equilibrium constant. Secondly, under the suitable reaction conditions, 130 °C, 7 h, and molar ratio of TDA/zinc acetate/urea/sulfolane=1/0.05/3.5/10, TDA conversion is 54.3%, and TBU yield and selectivity are 39.8% and 73.3% respectively. Lastly, the synthesis of TBU is a 1st order reaction with respect to TDA and the reaction kinetics model is established. This work will provide useful information for commercializing the urea route to toluene-2,4-dicarbamate (TDC).