Preparation Of Solid Acid And Base Catalysts For Transesterification And Alkylation Reaction

With the continuous development of environmentally friendly chemistry, greatefforts have been made to develop and improve green chemical process, especially inthe fine chemicals. Based on some defects of liquid acid and base in chemicalprocess (e.g., corrosion problem, pollution problem, separation and reuse problem),more attentions are paid to explore solid acid and base catalysts as a substitute.Heterogeneous acid and base catalysts show high catalytic activity and selectivityunder a mild condition, and they can be easily separated from reaction systems.In this work, solid acid and base catalysts were synthesized and applied in thetransesterification reaction. The structure and surface properties of solid acid and basematerials were studied. And the effect of the support on the active site was alsoinvestigated. In addition, the catalytic reaction dynamics were studied to determinethe main kinetic parameters, including the reaction rate constant, apparent activationenergy, pre-exponential factors and so on. The main experimental results andconclusions are as follows:1. Transesterification of carbonate with alcohols over porous carbon supportedMgO catalystsPorous carbon-supported catalysts (MgO/NC-2), prepared by wet impregnationtechnique, showed remarkably high activity, recoverability and stability for thetransesterification of dialkyl carbonate with alcohols to unsymmetrical organiccarbonates. Besides, some characterization means were carried out in order to clarifythe role of surface functional groups of carbon materials in the formation of activesupported MgO catalysts.Combined with different characterizations relusts (XRD, N₂adsorption/desorption,CO₂-TPD, TPD-MS, DRIFT and XPS), it is found that well-dispersed MgO/NC-2 material contains a large amount of ion pairs of Mg2+-O2-, attributed to the interactionbetween the rich oxygen-containing surface groups on the NC-2carbon support withMg species. The abundant ion pairs of Mg2+-O2-on the surface of MgO/NC-2materialshould play a critical role in the activation of the reactants in the transesterificationreactions. Besides, other surface functional groups like nitrogen-containing groupsmay also play positive role in activating reactants to produce unsymmetrical organiccarbonates.2. Amorphous mesoporous aluminophosphate as highly efficient heterogeneouscatalysts for transesterification of diethyl carbonate with dimethyl carbonateMesoporous aluminophosphate (AlPO), prepared by citric acid route, were appliedto the transesterification of DEC with DMC in comparison with other solid acidand/or base catalysts. It was found that mesoporous AlPO showed high activity andstability. And the catalysts can be easily recycled without any special treatment (e.g.,heat-activation).Combined with different characterizations relusts (XRD, N₂adsorption/desorption,and CO₂-TPD), a mechanism for the transesterification of DMC with DEC overAlPO materials was proposed. First, chemisorption of DEC and DMC could occuron the neighboring acid-base pairs simultaneously. Then the two adjacentchemisorbed species (e.g.,+COOC2H5and CH3O-) react directly to form EMC thatdesorbs easily. In addition, the abundant weak acid-base pairs on the surface ofmesoporous AlPO materials should play a critical role in the activation of thereactants in the transesterification reaction.3. Side-chain alkylation of toluene with methanol over ion-exchanged Cs-IM-5zeolitesCs-IM-5catalysts were prepared by ion-exchange method with cesium acetateaqueous solution. Compared with other basic zeolites (e.g., Cs-Y, Cs-MCM-56),Cs-IM-5zeolites exhibited remarkably high activity and selectivity for side-chainalkylation of toluene with methanol. Cs-IM-5and Cs-IM-5(d) prepared withdealuminated IM-5showed21.6%and26.2%conversion of toluene with8.1%and 6.0%yield of styrene, respectively. Moreover, the two catalysts showed relativelyhigh stability and recyclability.By using a variety of characterization means, including XRD, TPD, IR spectra ofCO₂, pyridine and in situ IR spectroscopy, the physicochemical properties ofCs-IM-5zeolites were investigated. It was found that both acidic and basic siteswere present on the surface of Cs-IM-5zeolites. A certain amount Lewis acidic sites(Cs+) could interact with Ï€-electrons of toluene and facilitate the polarization of themethyl C-H bond. And suitable basic centers could dehydrogenate methanol toformaldehyde (a key side-chain alkylating species), thus leading to high selectivitytoward styrene. Besides, the specific topology of IM-5zeolite, such as possessingmultidimensional large-pore system with intersecting12-and10-ring channel, mayprovide enough space for the formation of large intermediates and offershape-selectivity for the side-chain alkylation products.In summary, several solid acid and base catalysts including MgO/NC-2, AlPO andCs-IM-5were prepared and investigated for the transesterification reaction andside-chain alkylation with toluene and methanol. Several excellent solid catalystswere obtained and it provides a profound understanding of the nature of the activesite, reaction mechanism and reaction kinetics, which is in favour of the preparationof high active solid acid and base catalysts for the transesterification reaction andside-chain alkylation reaction.