Temperature-programmed Deoxygenation of Acetic Acid on Molybdenum Carbide Catalysts

Summary

Presented here is a protocol for the operation of a micro-scale temperature-programmed reactor for evaluating the catalytic performance of molybdenum carbide during acetic acid deoxygenation.

Abstract

Temperature programmed reaction (TPRxn) is a simple yet powerful tool for screening solid catalyst performance at a variety of conditions. A TPRxn system includes a reactor, furnace, gas and vapor sources, flow control, instrumentation to quantify reaction products (e.g., gas chromatograph), and instrumentation to monitor the reaction in real time (e.g., mass spectrometer). Here, we apply the TPRxn methodology to study molybdenum carbide catalysts for the deoxygenation of acetic acid, an important reaction among many in the upgrading/stabilization of biomass pyrolysis vapors. TPRxn is used to evaluate catalyst activity and selectivity and to test hypothetical reaction pathways (e.g., decarbonylation, ketonization, and hydrogenation). The results of the TPRxn study of acetic acid deoxygenation show that molybdenum carbide is an active catalyst for this reaction at temperatures above ca. 300 °C and that the reaction favors deoxygenation (i.e., C-O bond-breaking) products at temperatures below ca. 400 °C and decarbonylation (i.e., C-C bond-breaking) products at temperatures above ca. 400 °C.

Introduction

Temperature programmed reaction (TPRxn) is one of many temperature programmed methods, including desorption (TPD), oxidation (TPO), and reduction (TPR), and proceeds via exposure of a catalyst to a reactant concurrent with or followed by a steady increase in temperature.^1,2,3 TPRxn is a transient technique that provides information about catalyst activity and selectivity as a function of reaction temperature.^4,5,6 It is also a popular technique: a search of the keywords '....

Protocol

CAUTION: Consult safety data sheets (SDS) for all chemicals used prior to operation. Flammable gases may present explosion hazards if combined with air or oxygen and an ignition source. Hydrogen is an extremely flammable gas. Acids are corrosive, and in the case of skin or eye contact, are irritants and may produce burns. Acetic acid is a flammable liquid and vapor and thus may ignite and/or explode in the presence of open flames, sparks and oxidizing agents, in addition to potentially causing severe skin burns and eye damage. When not in a closed system or container, acetic acid should be handled inside of a chemical fume hood. The hazards of nanomaterial catalysts a....

Results

The online MS provides the capability to analyze the gas composition at the reactor outlet in real-time. The online MS is not coupled with any device to separate products prior to analysis, and thus species identification is challenging when differentiating between compounds with overlapping mass fragmentation patterns. As shown in Table 2, many of the common products from acetic acid TPRxn experiments are characterized by multiple common m/z signals. Deconvolution of the.......

Discussion

The TPRxn method is a powerful tool for screening of catalytic materials, providing information about the activity and selectivity of a catalyst as a function of reaction temperature. Other temperature-programmed methods such as TPD, TPO and TPR can provide information on the adsorption strength of reactants, number of adsorption sites, and appropriate catalyst pre-treatment procedures, but do not provide direct catalytic performance data. It is important to note that the TPRxn method detailed in this work does not measu.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
glacial acetic acid	Cole-Parmer	EW-88401-62	alternate supplier acceptable if ACS purity grade. See caution statement in protocol for safety information
UHP He	Airgas	HE R300SS	alternate supplier acceptable if >99.99% purity
UHP Ar	Arigas	AR R200	alternate supplier acceptable if >99.99% purity
acetone	VWR International	BDH1101-4LP	alternate supplier acceptable if >99.5% purity
quartz chips	Powder Technology Inc.	Crushed Quartz	sieved 180-300 µm, calcined in air at 500 °C overnight
mass spectrometer - turbo vacuum pump	Pfeiffer Vacuum	TSU 071	mass spectrometer is controlled with LabVIEW 2010 software package (National Instruments)
mass spectrometer - turbo vacuum pump	Stanford Research Systems	RGA100
micro gas chromatograph	Agilent	CP740388	490 Micro GC; 4-channel system Channel 1: 494001360 Molseive 10m, heated backflush Channel 2: 494001460 PPU 10m, heated backflush Channel 3: 490040 AL2O3/KCL 10+0.2m, heated backflush SPECIAL Channel 4: 492005750 5CB 15m, heated
GC software	Aglient	OpenLAB CDS EZChrom Edition
clean gas filters	Agilent	CP17974	for use on GC carrier gases (He, Ar)
quartz "U-tube" reactor	n/a		hand blown glass, custom built to order
bubbler	n/a		custom built to order
ceramic furnace	Watlow	discontinued	Similar furnace part #: VC401J12A-B000R
heat tape controller	n/a		custom built with Watlow EZ-zone parts
heat tape	Omega	FGH051-060	alternate supplier for extreme temperature heat tape acceptable
heat tape insulation	JEGS	710-80809	alternate supplier acceptable
thermocouple	Omega	e.g., KMQSS-062U-18	K-type thermocouples; alternate sizes may be required
thermocouple O-ring	Swagelok	VT-7-OR-001-1/2	perfluoroelastomer(fluorocarbon FKM) O-ring
2 µm solids filter, VCR gasket	Swagelok	SS-4-VCR-2-2M
1 µm orifice, VCR gasket	Lenox Laser	SS-4-VCR-2	for mass spectrometer orifice
316/316L stainless steel tubing and fittings	Swagelok	Varies	See Swagelok 'VCR Metal Gasket Face Seal Fittings' and 'Stainless Steel Seamless Tubing and Tube Support Systems' catalogs for more information
316/316L stainless steel tubing and fittings	Swagelok	Varies	See Swagelok 'Integral-Bonnet Needle Valves', 'Bellows-Sealed Valves' and 'One-Piece Instrumentation Ball Valves' catalogs for more information