The oxidative degradation rates of a CO 2 sorbent composed of a mesoporous alumina impregnated with poly-(ethylenimine) (PEI) are measured under systematically varied conditions and a reaction rate law is created. Good agreement is shown between the rate of oxidation obtained via in situ calorimetric heat measurement during oxidative degradation reactions and the loss of CO 2 capture performance presented as amine efficiency (mol CO 2 /mol amine). PEI mass loss and elemental composition are tracked over the course of the reaction and used in conjunction with the oxidation rate measurements to shed insight into the oxidation reaction(s). These data, in combination with measurements of the heat of reaction, suggest a common reaction set across the range of temperatures, oxygen concentrations, and sorbent compositions tested. The data are consistent with the basic autoxidation scheme (BAS), the accepted mechanism of autoxidation of aliphatic polymers. We propose a lumped kinetic model to describe the oxidation reaction set and estimate an activation energy of 105 kJ/mol and an oxygen reaction order of 0.5−0.7 from the data accordingly. These parameters can be incorporated into process cycle models to estimate the material lifetime, a critical uncertainty in the deployment of DAC technologies.
Zeolitic nanotubes
Nanotubes generally have solid walls, but a low-dimensional version of zeolites now introduces porosity into such structures. Korde
et al
. used a structure-directing agent with a hydrophobic biphenyl group center connecting two long alkyl chains bearing hydrophilic bulky quaternary ammonium head groups to direct hydrothermal synthesis with silicon-rich precursors (see the Perspective by Fan and Dong). The nanotubes have a mesoporous central channel of approximately 3 nanometers and zeolitic walls with micropores less than 0.6 nanometers. Electron microscopy and modeling showed that the outer surface is a projection of a large-pore zeolite and the inner surface is a projection of a medium-pore zeolite. —PDS
The
catalytic condensation of ethanol to n-butanol
and higher alcohols, known collectively as Guerbet reactions, has
attracted attention in recent years as ethanol becomes increasingly
available as a biorenewable feedstock. Results are presented here
for the continuous, condensed-phase conversion of ethanol to higher
alcohols using Ni/La2O3/γ-Al2O3 catalysts and for catalysts containing a second metal
(Cu, Co, Pd, Pt, Fe, Mo) in addition to nickel. Detailed characterization
of the catalyst surface and bulk properties has been carried out and
is correlated to catalyst activity and selectivity. The best results
obtained for nickel catalysts are a selectivity to higher alcohols
of 75–80% and a turnover frequency of 200 mol ethanol/mol Ni
site/h at 230 °C. The addition of cobalt nearly doubles the ethanol
conversion rate relative to Ni alone, with only a slight reduction
in higher alcohol selectivity. Results of catalyst characterization,
a simple kinetic model, and experiments with reaction intermediates
support the initial dehydrogenation of ethanol as the rate-limiting
step of the condensed-phase reaction.
Previous research has demonstrated that amine polymers
rich in
primary and secondary amines supported on mesoporous substrates are
effective, selective sorbent materials for removal of CO
2
from simulated flue gas and air. Common substrates used include
mesoporous alumina and silica (such as SBA-15 and MCM-41). Conventional
microporous materials are generally less effective, since the pores
are too small to support low volatility amines. Here, we deploy our
newly discovered zeolite nanotubes, a first-of-their-kind quasi-1D
hierarchical zeolite, as a substrate for poly(ethylenimine) (PEI)
for CO
2
capture from dilute feeds. PEI is impregnated into
the zeolite at specific organic loadings. Thermogravimetric analysis
and porosity measurements are obtained to determine organic loading,
pore filling, and surface area of the supported PEI prior to CO
2
capture studies. MCM-41 with comparable pore size and surface
area is also impregnated with PEI to provide a benchmark material
that allows for insight into the role of the zeolite nanotube intrawall
micropores on CO
2
uptake rates and capacities. Over a range
of PEI loadings, from 20 to 70 w/w%, the zeolite allows for increased
CO
2
capture capacity over the mesoporous silica by ∼25%.
Additionally, uptake kinetics for nanotube-supported PEI are roughly
4 times faster than that of a comparable PEI impregnated in SBA-15.
It is anticipated that this new zeolite will offer numerous opportunities
for engineering additional advantaged reaction and separation processes.
Reaction conditions and reactor geometry for producing acrylates in high yield from lactic acid-derived 2acetoxypropanoic acid (APA) esters are presented. An acrylate ester yield of 75% is achieved from methyl and benzyl APA esters at 550 °C in a fixed bed reactor filled with nonporous silica particles, carbon dioxide as a diluent gas, and acetic acid as a co-feed with the APA ester. The yield from methyl and benzyl APA esters is remarkably higher than from ethyl or butyl esters of APA, which have hydrogen atoms on the β-carbon of the ester functional group and thus can undergo alkene elimination, leading to reduced acrylate yield. Under optimum conditions, APA conversion to acrylates is stable over 30 h of continuous operation with little carbon deposition on the contact material.
Mn−Cr−O spinel catalysts (MnCr 2 O 4 and Mn 1.5 Cr 1.5 O 4 ) and single oxides of Mn and Cr are coked under ethylene and ethylene−steam mixtures to simulate reaction conditions occurring in steam cracker furnaces. The coking rates are measured using thermogravimetric analysis, and the Mn 1.5 Cr 1.5 O 4 catalyst displayed considerably less coke deposition (an order of magnitude) under ethylene−steam flow. Analysis of the outlet gas flow suggests the Mn 1.5 Cr 1.5 O 4 catalyst gasified the radically formed coke into carbon oxides without significantly affecting ethylene conversion. The anticoking performance of the Mn 1.5 Cr 1.5 O 4 catalyst is attributed to the presence of Mn 3+ species in the spinel phase structure that are active for coke gasification in the presence of steam. No significant changes in structure or performance are observed for the Mn 1.5 Cr 1.5 O 4 catalyst across several coking and decoking cycles. The Mn−Cr−O spinel oxide with higher Mn content is suggested to have desirable activity and stability to limit coke deposition during steam cracking.
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