UC Berkeley

Abstract

Structure and Reactivity of Zeolite- and Carbon-Supported Catalysts for the Oxidative Carbonylation of Alcohols

by

Daniel Neal Briggs

Doctor of Philosophy in Chemical Engineering

University of California, Berkeley

Professor Alexis T. Bell, Chair

The oxidative carbonylation of alcohols to produce dialkyl carbonates is a process that takes place commercially in a slurry of cuprous chloride in the appropriate alcohol. While this process is chemically efficient, it incurs costs in terms of energy (for product separation) and materials attrition (due to the corrosive nature of the chloride anion) that can be alleviated in a gas-phase process. Efforts to develop a supported copper catalyst for making dialkyl carbonates have been undertaken, using carbons or oxidic supports (including zeolites). However, the activity, selectivity and stability of the supported catalysts are not yet competitive with the slurry process. Little is understood regarding the nature of the active species or the mechanism by which carbonates and byproducts are formed. Catalyst properties that lead to favorable activity and selectivity have not been clearly outlined. To improve supported catalysts for this process, we have carried out detailed investigations of the structure and catalytic behavior of zeolite- and carbon-supported Cu catalysts for the synthesis of dimethyl or diethyl carbonates.

The aim of the work on Cu⁺-exchanged zeolites was to establish the effects of zeolite structure/chemical composition on the activity and selectivity of Cu-exchanged Y (Si/Al = 2.5), ZSM-5 (Si/Al = 12), and Mordenite (Si/Al = 10) for the oxidative carbonylation of methanol to DMC. Catalysts were prepared by solid-state ion-exchange of the H-form of each zeolite with CuCl, and were then characterized by FTIR and X-ray absorption spectroscopy (XAS). The XANES portion of the XAS data showed that all of the copper is present as Cu⁺ cations, and analysis of the EXAFS portion of the data shows the Cu⁺ cations have a Cu-O coordination number of ~2.1 on Cu-Y and ~2.7 on Cu-ZSM-5 and Cu-MOR. Dimethyl carbonate (DMC) was observed as the primary product when a mixture of CH₃OH/CO/O₂ was passed over Cu-Y, whereas dimethoxy methane was the primary product over Cu-ZSM-5 and Cu-MOR. The higher activity and selectivity of Cu-Y for the oxidative carbonylation of CO is attributed to the weaker adsorption of CO on the Cu⁺ cations exchanged into Y zeolite. In situ infrared observations reveal that under reaction conditions, adsorbed CO is displaced by methoxide groups bound to the Cu⁺ cations. The kinetics of DMC synthesis suggests that the rate-limiting step in the formation of this product is the insertion of CO into Cu-OCH₃ bonds. The yield of DMC is observed to decline with methanol conversion due very likely to the hydrolysis of DMC to methanol and carbon dioxide.

Next, the investigation turned to the synthesis of diethyl carbonate (DEC) by oxidative carbonylation of ethanol, using catalysts prepared by the dispersion of CuCl₂ and PdCl₂ on amorphous carbon. Catalysts were characterized extensively by XRD, XAFS, SEM and TEM with the aim of establishing their composition and structure after preparation, pretreatment, and use. It was observed that after preparation and pretreatment in He at 423 K, copper is present almost exclusively as Cu(I), most likely in the form of [CuCl₂]^- anions, whereas palladium is present as large PdCl₂ particles. Catalysts prepared exclusively with copper or palladium chloride are inactive for DEC synthesis, indicating that both components must be present together. Evidence from XANES and EXAFS suggests that the DEC synthesis may occur on [PdCl_2-x][CuCl₂]^x species deposited on the surface of the PdCl₂ particles. As-prepared catalysts exhibited an increase in DEC synthesis activity and selectivity with time on stream, but then reached a maximum activity and selectivity, followed by a slow decrease in DEC activity. The loss of DEC activity was accompanied by a loss in Cl from the catalyst and the appearance of paratacamite.

Further work was undertaken on carbon-supported catalysts, building on insights regarding the active species, this time with activated carbon or carbon nanofibers as the support. The objectives of this last study were to establish the effects of carbon support structure and pretreatment on the dispersion of the catalytically active components and, in turn, on the activity and selectivity of the catalyst for DEC synthesis. At the same surface loading of CuCl₂ and PdCl₂, partially oxidized carbon nanofibers resulted in a higher dispersion of the active components and a higher DEC activity than could be achieved on activated carbon. Catalyst characterization revealed that nearly atomic dispersion of CuCl₂ and PdCl₂ could be achieved on the edges of the graphene sheets comprising the carbon nanofibers. Over oxidation of the edges or their removal by heat treatment of the nanofibers resulted in a loss of catalyst activity. The loss of catalyst activity with time on stream could be overcome by the addition of ppm levels of CCl₄ to the feed. While catalysts prepared with CuCl₂ alone were active, a five-fold increase in activity was realized by using a PdCl₂/CuCl₂ ratio of 1/20. It was proposed that the Pd²⁺ cations interact with [CuCl₂]^- anions to form Pd[CuCl₂]₂ complexes that are stabilized through dative bonds formed with oxygen groups present at the edges of the graphene sheets of the support. A mechanism for DEC synthesis was outlined and a role for the Pd²⁺ cations as part of this mechanism was proposed.