Determining the Strength of BrÃ¸nsted Acid Sites for Hydrodewaxing

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Determining the Strength of Brønsted Acid Sites for Hydrodewaxing over Shape Selective Catalysts Stephen J Miller, Howard S. Lacheen, and Cong-Yan Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01081 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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Industrial & Engineering Chemistry Research

Determining the Strength of Brønsted Acid Sites for Hydrodewaxing over Shape Selective Catalysts Stephen J. Miller, Howard S. Lacheen*, and Cong-Yan Chen Chevron Energy Technology Company, 100 Chevron Way, Richmond, CA 94802 *Corresponding author: Fax: 1-510-242-2823 E-mail address: [email protected].

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Keywords

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Zeolite; Brønsted acidity; Hydrodewaxing; Wax isomerization; Ammonia TPD;

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Pyridine FTIR; Reaction kinetics

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Abstract

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This paper reports a method for directly determining the effective strength of those acid

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sites on a molecular sieve catalyst, which are actually involved in reactions such as

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hydrodewaxing. The method is here applied to the conversion of a waxy paraffin over a

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catalyst containing platinum and a SAPO-11 sieve of high isomerization selectivity, as

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well as a catalyst containing platinum and a H-ZSM-5 zeolite (SiO2/Al2O3 = 150) of

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high cracking selectivity. Results are compared against the measured Brønsted acid site

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strength on these molecular sieves determined from TPD of ammonia and FTIR of

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adsorbed pyridine. Characterization of acid sites showed both higher concentration and

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apparent acid strength for H-ZSM-5 than for SAPO-11, with the latter having a finite

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but smaller amount of strong acid sites that adsorbed pyridine at 573 and 673 K.

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Brønsted Acidity

Chevron Energy Technology Company

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Reactions with n-hexadecane gave significantly higher conversion rates on H-ZSM-5,

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and it is shown that a strongly adsorbing molecule like ammonia can be used in situ to

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estimate the strength of acid sites involved in hydrocarbon conversion steps. Using the

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method, the “effective acid site strength” for the SAPO-11 catalyst is determined to be

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about 30 kJ/mol less than on the H-ZSM-5 catalyst. The reactivity of these catalysts is

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discussed in light of the acid sites and other factors including crystal structure and size.

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1.0 Introduction

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There has been considerable interest in achieving a better understanding of the

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conversion of paraffinic hydrocarbons over acidic molecular sieve catalysts, in

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particular the hydroconversion of long-chain paraffins, important for the production of

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modern transportation fuels and lubricants. Of special interest has been the

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hydroconversion (hydrodewaxing) of those paraffins over intermediate pore shape

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selective zeolite catalysts, such as ZSM-5, to selectively remove waxy molecules

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allowing those fuels and lubricants to operate at low temperature. Included in that field

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is the selective skeletal isomerization of long chain paraffins over a hydrogenation

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component such as Pt on a 1-D intermediate pore molecular sieve such as SAPO-11,

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first reported over 20 years ago.1-4 Since that time, a number of commercial processes

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and catalysts have been introduced, including Chevron’s ISODEWAXING process for

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reducing the pour point of lubricating oils and middle distillate fuels. This technology

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made it economically possible to meet the growing demand in the market place for

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lubricating oils and fuels with advanced performance, environmental, and safety

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benefits, from a wide range of waxy feedstocks, including those of high wax content.

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Bronstead Acidity

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There have been a number of studies to better understand the catalysis and the

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relationship between the activity and selectivity of those shape selective molecular

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sieves and their physical properties. These studies have focused on two main aspects, 1)

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the influence of the shape selectivity on the carbenium ion chemistry of the catalysis,

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and 2) the influence of the acid sites, particularly Brønsted acid site number and

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strength. For those catalysts of high isomerization selectivity, such as SAPO-11,5–16

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researchers have pointed to the constraint of the pores in inhibiting the formation of

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highly branched reaction intermediates that readily crack, as well as reduced acid site

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strength as compared to the acid sites in zeolites such as ZSM-5,17-19 where acid site

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density and strength are typically determined by techniques such as microcalorimetry

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or temperature programmed desorption (TPD) of ammonia or pyridine. To date, this

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has involved measuring site density and strength, and then, when there is a distribution

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of strengths, inferring or guessing which sites take part in the catalysis. In many cases,

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sites responsible for the catalysis have been labeled as “strong” or “medium” in terms

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of their acid strengths. One particular drawback to this approach is that there is no

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general agreement on which sites are “strong” and which are “medium”, where the

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dividing line between the two is often drawn arbitrarily, leading to uncertainty as to

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which sites are actually involved in the catalysis. Consequently, the current definition

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of “medium” is arbitrary and of limited value to anyone developing an industrial

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catalyst. Further, for catalysts such as SAPO-11, where there is a broad distribution of

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acid site strengths, there has been, to date, no direct measurement of the strength of the

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acid sites actually involved in the catalysis.

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Being able to determine the strength of the acid sites on a catalyst responsible for a

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particular acid-catalyzed reaction would be of benefit in allowing us to: Confidentiality Classification: Public

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Brønsted Acidity

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1. More efficiently screen new materials and new catalyst preparations for specific reactions.

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2. Design and optimize molecular sieves and other acid catalysts for specific reactions.

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This paper reports a method for determining the effective strength of acid sites on a

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catalyst for reactions such as hydrodewaxing. It uses the impact, in a continuous feed

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reactor, of different dosage levels of ammonia on the reaction acid sites, where all other

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conditions are constant, including selectivity, such that the only thing that changes upon

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changing the ammonia level is the catalyst temperature required at that level to

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maintain a fixed condition. The method is here applied to the conversion of a waxy

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paraffin over a catalyst containing platinum and a SAPO-11 sieve of high isomerization

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selectivity and a broad distribution of acid site strengths, as well as a catalyst

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containing platinum and a H-ZSM-5 zeolite of high cracking selectivity and a narrow

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distribution of acid site strengths. The paper also compares those results against acid

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site distribution and strength of these two molecular sieves, prior to Pt addition, using

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analytical methods (pyridine titration and ammonia temperature programmed

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desorption (TPD)) taught in the literature.

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2.0 Theoretical

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The simple rate equation for a unimolecular surface reaction in the presence of an

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inhibitor can be expressed as

Rate = kKp/ (1+ Kp +Kppp),

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(1)



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Bronstead Acidity

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where k is the rate constant, K and Kp the adsorption equilibrium constants for the

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reactant and inhibitor, respectively, and p and pp the partial pressures of reactant and

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inhibitor, respectively.

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In the case where the inhibitor is more strongly adsorbed than the reactant and

Kppp >> 1+ Kp,

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(2)

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then the rate equation simplifies to

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Rate = kKp/ Kppp.

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(3)

This equation can then be rewritten as

Rate = Ae-E/RTA1eλ/RTp/A2eλp/RTpp,

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(4)

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where E is the activation energy for conversion of the hydrocarbon, λ and λp are the

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heats of adsorption of hydrocarbon and inhibitor, respectively, and A, A1, and A2 are

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constants.

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In this case any impact of the presence of an inhibitor on the rate may be compensated

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by adjusting temperature. If p is held constant, and the rate is held constant by

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operating at constant conversion, then

pp = C e-(E-λ+λp)/RT , where C = AA1p/(Rate x A2) ,

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(5)

or


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Brønsted Acidity


pp = C e-(Ea+λp)/RT,

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(6)

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where Ea is the apparent activation energy, equal to E-λ. If we then vary pp at constant

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hydrocarbon conversion, and plot ln pp versus 1/T, we should get a straight line with a

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slope equal to –(Ea +λp)/R, as long as the assumption holds that Kppp >> 1+ Kp. If we

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determine Ea separately, we can then solve for λp, the heat of adsorption of the

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inhibitor, or that required to desorb the inhibitor so that the hydrocarbon can react. If

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we choose NH3 as the inhibitor, this will give us a measure of the “effective acid

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strength” of the active site for the hydrocarbon conversion reaction, assuming

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adsorption and reaction occur on the same site. We call this the “effective acid

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strength” since there is likely a distribution of strengths, rather than a single strength,

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which can catalyze the reaction in question.

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It should also be pointed out, as noted in a previous publication,2 that the selectivity for

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hydroisomerizing n-hexadecane over Pt-SAPO-11 was unaffected by adding up to 200

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ppm nitrogen as n-butylamine to the feed, which then converts to NH3 at the top of the

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catalyst bed.

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3.0 Materials and Methods

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3.1 Infrared Spectroscopy

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Vibrational spectra were measured using transmission FTIR. Molecular sieves in

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proton form were ground and pressed into wafers (5-10 mg/cm2) and dehydrated in

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vacuum at 723 K for 1 h in a steel cell with CaF windows. Spectra were recorded at

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423 K with a MCT detector with 128 scans from 400 cm-1 to 4000 cm-1. Each

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Bronstead Acidity

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dehydrated sample spectrum was used as a baseline, and subtracted from the absorption

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spectrum after pyridine exposure, to calculate the difference spectra in the 1400-1800

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cm-1 range.

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Pyridine was adsorbed on dehydrated samples at 423 K. Excess pyridine was removed

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by evacuating at 423 K for 1 h; this also allowed for the removal of the weakly

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adsorbed pyridine on the sample and infrared cell. Then samples were further

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evacuated at 573 K and 673 K to progressively desorb pyridine. A vibrational mode in

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the 1540-1550 cm-1 region was attributed to pyridinium ions, providing evidence of

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Brønsted acidity and used to calculate the acid site concentration. H-ZSM-5

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(SiO2/Al2O3 = 54) was used to calibrate the 1540 cm-1 infrared band.

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3.2 Ammonia Temperature Programmed Desorption

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For evaluation of acid strength, ammonia TPD was measured in a quartz flow through

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cell with approximately 300 mg of pelletized catalyst (Micromeritics Autochem II

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2920). Samples consisted of zeolites in proton form without Pt since the metal could

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interact with ammonia and also with acid sites of the catalyst in the unreduced state.

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Samples were dried at 773 K in argon for 1 h, then cooled to 393 K and exposed to a

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flow of 5% ammonia/argon for 0.5 h. Then the flow was switched to argon to sweep

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residual ammonia out for 1 h. The sample temperature was then increased at a constant

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heating rate of 0.17 K/s to a final temperature of 773 K. Flow to weight ratio was

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varied from 8 x 10-4 to 8 x 10-3 m3/(kg s) in separate tests while heating the sample in

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argon. Ammonia concentration was monitored by a thermal conductivity detector. A

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deconvolution procedure was used to separate overlapping peaks using Origin 8.5


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Brønsted Acidity


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software. The baseline was subtracted with a linear function spanning the temperature

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axis and the components of the TPD profile were modeled with Gaussian functions.

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3.3 Catalysts

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The SAPO-11 molecular sieve used in this study was synthesized in our laboratory. It

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was identified by X-ray diffraction analysis and found to contain no impurity

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crystalline phases. The sieve was dried and then calcined in air at 863 K for six hours.

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The molar ratios of oxides were as follows:

0.24 SiO2:Al2O3:1.02 P2O5

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The ZSM-5 zeolite was CBV1502, obtained from PQ. Its H-form obtained via ion

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exchange with a SiO2/Al2O3 molar ratio of about 150 based on ICP. Neutron activation

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analysis determined a SiO2/Al2O3 molar ratio of 130. External surface areas (ESA)

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were measured using N2 adsorption at 77 K and calculated from the slope of the

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straight line obtained by plotting the adsorbed volume versus the de Boer thickness

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between 0.5-0.8 nm. ESA for H-ZSM-5 and SAPO-11 were 65 m2/g and 47 m2/g,

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respectively.

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3.4 Catalyst Testing

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For both Pt-SAPO-11 and Pt/H-ZSM-5 catalysts, 1.0 wt% Pt was loaded to the

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corresponding sieve via incipient wetness impregnation technique. For example, to

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prepare the Pt/H-ZSM-5 catalyst, an aqueous solution consisting of 0.1019 g

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Pt(NH3)4(NO3)2 and 3.4588 g H2O was added to 5.0772 g H-ZSM-5. The resulting

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mixture was stirred rigorously with a spatula for 10 min, then kept in a sealed glass

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Bronstead Acidity

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bottle at room temperature for 48 hours and subsequently dried in a vacuum oven at

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393 K for 2 h. The dried sample was then calcined in a calcination oven with flowing

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air using the following temperature program: from room temperature to 393 K at 0.017

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K/s and at 393 K for 0.5 h; then from 393 K to 450 K at 0.017 K/s and at 450 K for 2 h;

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subsequently from 450 K to 505 K at 0.017 K/s and at 505 K for 2 h; next from 505 K

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to 561 K at 0.017 K/s and at 561 K for 5 h; finally cooling down to room temperature.

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Pt-SAPO-11 was prepared similarly. The calcined Pt/H-ZSM-5 or Pt-SAPO-11 catalyst

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was then pelletized, crushed and sieved to 20-40 mesh.

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For the catalytic testing, 1.0 g of the meshed catalyst was centered in a down-flow

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stainless steel tube reactor in a split tube furnace. The catalyst was reduced at

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atmospheric pressure in a H2 flow of 60 standard cm3/min in-situ from room

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temperature to 589 K in 3 h and subsequently kept at 589 K for 5 h. Next, the catalyst

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was pre-sulfided with 3.5 cm3 H2S injected in a H2 flow of 60 standard cm3/min over a

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time of 5 min at 589 K and atmospheric pressure prior to introduction of hydrocarbon

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feed to minimize hydrogenolysis. Then the reactor was brought to the reaction pressure

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of 6996 kPa and the preselected reaction temperature (e.g., 561 K). The reaction was

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then started with pure n-hexadecane (> 99%, Aldrich) feed (containing various amounts

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of nitrogen and delivered by an Isco pump) at the preselected LHSV and molar H2 to

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hydrocarbon ratio. The inlet H2 to n-hexadecane molar ratio was kept at 14 although

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LHSV varied. To make a feed containing a defined level of nitrogen in n-hexadecane

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(e.g., 20 ppm N by weight), a stoichiometric amount of n-butylamine was doped into

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the feed bottle of n-hexadecane. n-Butylamine was then converted to NH3 in-situ in the

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reactor under the hydroprocessing conditions applied in this work.


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Brønsted Acidity


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Reaction products from the product flow coming from the reactor outlet were injected

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automatically into an on-line Agilent 5890 gas chromatograph equipped with a 50

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meter long HP-1 capillary column and an FID detector. The on-line GC analysis was

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carried out every 1.5 h. The following GC oven temperature program was used: 5 min

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at 323 K and then from 323 for 5 min to 393 K at 0.05 K/s; from 393 to 523 K at 0.1

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K/s and then at 523 K for 20 min.

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Percent conversion is defined as 100 - wt% n-C16 in the product. Isomerization

192

selectivity is;

100 x (wt% i-C16)product/(percent conversion)

193

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The apparent activation energy, Ea, for hydroconversion of n-hexadecane over each

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catalyst was determined by holding n-C16 conversion constant at 90%, and measuring

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the required reactor temperature for that conversion over the range of liquid hourly

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space velocities (LHSV) between 1 and 8. Since hydrodewaxing normally requires well

198

over 90% conversion of waxy paraffins, we ran at that high conversion to more closely

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match what would be expected for that process. Based on the Arrhenius relationship, Ea

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was determined from the slope of the plot of the natural logarithm of the liquid hourly

201

space velocity against the reciprocal reactor temperature.

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4.0 Results and Discussion

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4.1 FTIR Acid Site Measurements

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Infrared absorption was used to monitor the concentration of hydroxyl species in H-

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ZSM-5 and SAPO-11 (Figs. 1-2). Both materials have absorption bands near 3740 cm-1

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Bronstead Acidity

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arising from silanols. Bridging hydroxyls that give rise to acid sites are observed at

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3605 cm-1 in H-ZSM-5. In SAPO-11, two vibrations at 3620 cm-1 and 3520 cm-1 have

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been assigned to bridging hydroxyls, based on their absence in silicon free ALPO-11

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and their disappearance upon ammonia exposure as reported by Parlitz, et al.20 The

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latter was assigned to bridging hydroxyls distorted by interactions with lattice oxygen.

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An additional sharp, intense band 3675 cm-1, not found in aluminosilicates, arises from

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P-OH groups in SAPO-11.

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Acid sites were further investigated by adsorption of a base, pyridine, to selectively

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titrate acid sites. Pyridine that is protonated at Brønsted acid sites or coordinated to

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Lewis acid sites can be identified by vibrational modes at 1540 and 1450 cm-1,

216

respectively. Adsorption on Brønsted sites was also apparent by the attenuation of the

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hydroxyl band at 3605 cm-1 for H-ZSM-5 (Fig. 1) and 3620 cm-1 in SAPO-11 (Fig. 2)

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after pyridine exposure. The intensity of 3520 cm-1 band was also decreased after

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pyridine adsorption. At 423 K, evidence of minor pyridine adsorption is found at

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silanols (3740 cm-1) in both zeolites and other framework hydroxyls at 3675 cm-1 in

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SAPO-11.

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Desorption at 573 K resulted in pyridine removal on certain sites, restoring the 3675

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cm-1 and 3740 cm-1 bands. In contrast, the 3620 cm-1 band in SAPO-11 retained a

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significant amount of pyridine at 573 K. Acid sites remaining at this temperature are

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sometimes designated as strong acid sites. The 3605 cm-1 hydroxyl peak in H-ZSM-5

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was unchanged when heating from 423 to 573 K, further indicating that pyridine

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remained adsorbed on Brønsted sites. Adsorbed pyridine decreased further after

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evacuation at 673 K on SAPO-11 and also was lowered on H-ZSM-5. This was also


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Brønsted Acidity


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apparent from the pyridine adsorption spectra in Figs. 3-4, which indicated pyridine on

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Brønsted sites at 1540 cm-1 was decreased in intensity on both molecular sieves at 673

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K.

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Both structures possessed sites of varying strength, as indicated from the residual

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surface concentrations with desorption temperature, given in Table 1, including some

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degree of strong acid sites with pyridine remaining at 673 K. Pyridine is adsorbed at

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423 K on H-ZSM-5, 0.23 mmol/g, and SAPO-11, 0.13 mmol/g, and progressively

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desorbed with increasing temperature, as it was removed from weaker sites. Pyridine on

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H-ZSM-5 and SAPO-11 at 573 K was 0.21 mmol/g and 0.07 mmol/g, respectively;

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these are indicative of the quantity of strong acid sites.

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4.2 Ammonia TPD

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Acid sites were further investigated by adsorbing ammonia and then measuring

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desorption profiles (Fig. 5). H-ZSM-5 gave two peaks, a low temperature maxima from

242

weak or non-acidic sites near 473 K, and a strong acid site peak centered near 660 K.

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The deconvoluted peak centered at 660 K in Fig. 5 represented 0.20 mmol/g. This is

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similar to the amount expected from elemental analysis (0.215 mmol/g), implying that a

245

small amount of aluminum, less than 10%, in the sample may not be in the zeolite

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framework.

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The NH3 TPD profiles for SAPO-11 contained a large peak near 450 K and a shoulder

248

near 550 K. The high temperature peak occurred at lower temperature than on the H-

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ZSM-5. It has been attributed to acid sites ranging from medium to strong by other

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investigators, but there is general agreement that SAPO-11 has weaker acid sites on the

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Bronstead Acidity

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average than in zeolites.21-23 Deconvolution of these peaks was not trivial due to the

252

highly overlapping nature of the peaks. The best fit was obtained using gaussian

253

functions for the low temperature and high temperature contributions. SAPO-11 gave

254

an acid site concentration from the high temperature peak of 0.19 ± 0.02 mmol/g.

255

It has been demonstrated that the deprotanation energy gives a rigorous description of

256

acid strength.24 The utility of ammonia adsorption enthalpies, however, has been

257

recently shown as effective for relating the acid site binding enthalpy to reaction rates

258

in the methanol-propene reaction.25 The adsorption enthalpy of acid sites is

259

measureable by changing the conditions of the ammonia desorption experiment, such as

260

testing at variable flowrate, in order to decouple the experimental conditions from the

261

desorption profile.26 Here, we used the method of Sawa, et al., to measure acid strength

262

from ammonia desorption profiles by varying flow to sample mass ratio.27 The

263

corresponding heat of desorption is then calculated by Equation 7, which was derived

264

under the assumption of equilibrium between adsorbed and gas phase ammonia. A plot

265

of ln (Tmax) – ln (A0W/F) and inverse temperature (Fig. 6) gives a straight line, and the

266

slope is equal to the desorption enthalpy.

2

ln Tmax − ln 267

A0W β (1 − θ max ) (∆H − RTmax ) ∆H = + ln F RTmax P 0 exp(∆S / R )

(7)

268

The measured parameters in the TPD experiment are the maximum temperature, Tmax,

269

corresponding to the high temperature peak, and acid site density, A0. The ratio of

270

sample mass, W, to volumetric flow rate, F, is a constant set during the experiment. The


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Brønsted Acidity


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rate of temperature increase is β, θ is the NH3 coverage at peak maximum, and ∆H and

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∆S are the enthalpy and entropy of NH3 desorption.

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H-ZSM-5 gave ammonia adsorption enthalpies that were nearly independent of heating

274

rate, β, of 150 kJ/mol at 0.08 K/s and 146 kJ/mol at 0.17 K/s. These values are within

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the reported range of 137-150 kJ/mol for H-ZSM-5 determined by ammonia desorption

276

and calorimetry.28, 29

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The ammonia heat of desorption for SAPO-11 measured according to Equation 7 was

278

100-109 kJ/mol. The calculated values should be dependent on the distribution of all

279

acid sites in the material. The SAPO-11 may possess sites of higher or lower adsorption

280

enthalpy, as was demonstrated by pyridine adsorption, but the calculated value from

281

Equation 7 determined by ammonia TPD appeared to be of lower strength than found in

282

aluminosilicate zeolites, despite the fact that it contains sites with demonstrated alkane

283

isomerization activity.

284

The ammonia TPD profiles contained overlapping contributions from non-acidic or

285

weakly acidic sites that desorbed below 473 K. Peak profile fitting was done by a

286

gaussian fitting algorithm. It was investigated whether ammonia on the non-acidic sites

287

from the first peak could be preferentially desorbed before the TPD. This was done by

288

desorbing at 463 K in argon after ammonia saturation. This treatment in SAPO-11 gave

289

only one peak, as shown in Fig. 7, confirming that the argon purge removed ammonia

290

from non-acidic sites, but may have led to the desorption from acid sites also since the

291

concentration was lower in comparison to the 393 K purge, with an ammonia

292

desorption of 0.12 ± 0.01 mmol/g. The desorption enthalpy was similar as determined

Page 14



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Bronstead Acidity

293

by desorption starting at 393 K, about 113 kJ/mol, and this confirmed that the

294

deconvolution procedure used above gave a reasonable model of the aggregate of

295

surface adsorption sites.

296

The SAPO-11 acid site densities detected by ammonia TPD were higher than the 0.07

297

mmol/g detected by adsorption of pyridine at 573 K. This may be a consequence of the

298

more strongly bonding sites probed by pyridine at 573 K, since pyridine may have

299

already been removed from the surface at lower temperature. It may also be related to

300

contributions of physisorbed ammonia that were not completely excluded under the

301

peak fitting procedure. The acid site density we find for ammonia TPD, after a 463 K

302

purge, is about the same as for pyridine titration at 423 K (Table 1), therefore we

303

speculate that pyridine adsorption on SAPO-11 at 423 K reflects both medium and

304

strong sites as corroborated by its NH3 desorption profile. A second possibility for the

305

difference is the size of probe molecules; ammonia can interact with acid sites not

306

approachable by the larger pyridine. The pores in SAPO-11, a medium pore zeolite,

307

have dimensions of 6.5 Å by 4.0 Å and could allow for better access of ammonia than

308

pyridine. Restricted diffusion of pyridine in certain zeolites has been reported

309

previously, such as in ferrierite30 and erionite or offretite with stacking fault.31

310

Probing acid sites with NH3 or pyridine can be used to characterize zeolites using well

311

established methods as has been described here. One drawback to this approach is they

312

do not unequivocally distinguish between sites that participate in hydroisomerization

313

reactions and those that do not. In the next section, zeolites will be used in catalytic

314

reactions and the kinetics will be determined with ammonia addition to elucidate the

315

strengths of active sites during realistic conversion conditions.


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Brønsted Acidity

316

Page 16 of 27


Table 1. Residual pyridine on molecular sieves calculated from infrared spectra Sample

Pyr. adsorbed, 423 K

Pyr. adsorbed, 573 K

Pyr. Adsorbed, 673 K

mmol/g

mmol/g

mmol/g

H-ZSM-5

0.23

0.21

0.14

SAPO-11

0.14

0.07

0.02

317 318

Fig. 1. Infrared spectra in the hydroxyl region before and after pyridine adsorption on

319

H-ZSM-5 at 423 K (___) and after pyridine saturation and desorption at 423 K (…..), 573

320

K (----), and 673 K (

).

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Bronstead Acidity

321 322

Fig. 2. Infrared spectra in the hydroxyl region before and after pyridine adsorption on

323

SAPO-11 at 423 K (___) and after pyridine saturation and desorption at 423 K (…..), 573

324

K (----), and 673 K (

).

325 326

Fig. 3. Infrared difference spectra on H-ZSM-5 after pyridine saturation and desorption

327

at 423 K (___), 573 K (----), and 673 K (

).


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Brønsted Acidity

Page 18 of 27


328 329

Fig. 4. Infrared difference spectra on SAPO-11 after pyridine saturation and desorption

330

at 423 K (___), 573 K (----), and 673 K (

).

331 332

Fig. 5. Ammonia TPD profiles during ammonia desorption on SAPO-11 (

333

ZSM-5 (

) and H-

). Heating rate for desorption test was 0.17 K/s.

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Bronstead Acidity

334 335

Fig. 6. Dependence of high temperature maximum on inverse temperature according to

336

Equation 7. Plot shows H-ZSM5 at 0.08 K/s (□) and 0.17 K/s (■) and SAPO-11 at 0.08

337

K/s (○) and 0.17 K/s (●).

338 339

Fig. 7. Ammonia desorption profiles of SAPO-11 with different purge temperatures

340

(393 K,

341

argon flow of 25 standard cm3/min.

342

4.3 Activation Energy Determination

343

As mentioned in Section 3.4, the apparent activation energy, Ea, for hydroconversion of

344

n-hexadecane over each catalyst was determined by holding nC16 conversion constant

) (463 K,

). The desorption step used a heating rate of 0.17 K/s and an


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Brønsted Acidity


345

at 90%, and measuring the required reactor temperature for that conversion over the

346

range of liquid hourly space velocities (LHSV) between 1 and 8. Based on the

347

Arrhenius relationship, the natural logarithm of the liquid hourly space velocity was

348

plotted against the reciprocal reactor temperature (Fig. 8 for Pt-ZSM-5 and Fig. 9 for

349

Pt-SAPO-11), giving in each case a straight line with a slope, or Ea, of 180-185 kJ/mol.

350

This value is similar to literature values reported for skeletal hydroisomerization of

351

long-chain n-paraffins.32 The selectivity to branched isomers of n-hexadecane varied

352

little over that range for Pt-SAPO-11, being 89-92%, with the remainder being

353

hydrocracking. Pt-ZSM-5 had much lower isomerization selectivity, also fairly constant

354

over that range at 15-17%.

355 356

Fig. 8. Arrhenius plot for n-hexadecane conversion over Pt/H-ZSM-5 at 6996 kPa and

357

90 wt% conversion

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Bronstead Acidity

358 359

Fig. 9. Arrhenius plot for n-hexadecane conversion over Pt-SAPO-11 at 6996 kPa and

360

90% conversion

361

4.4 Determination of Acid Site Strength for Reaction

362

The feed was spiked with different levels of organic nitrogen (as n-butylamine),

363

holding LHSV constant at 2.0, and measuring the required temperature to again obtain

364

90% conversion, where the catalyst temperature was held constant for at least 24 hours

365

at each nitrogen level to ensure equilibration. The last nitrogen level tested was the

366

same as the first level tested, to ensure there was no significant deactivation during the

367

testing period. Based on Equation 6, the natural logarithm of the nitrogen content in

368

the feed (in ppm by weight) was plotted versus the reciprocal reactor temperature (Figs.

369

10-11). The level of nitrogen added ranged from 200 ppm down to 2 ppm. Again, a

370

straight line was obtained at least down to around 10 ppm nitrogen, much below which

371

the assumption that Kppp >> 1+ Kp apparently no longer holds. The slope of this line,

372

equivalent according to Equation 6 to Ea + λNH3, assuming all the butylamine was

373

converted to NH3 at the top of the catalyst bed, was about 345 kJ/mol for Pt-ZSM-5

374

(Fig. 10) and 315 kJ/mol for Pt-SAPO-11 (Fig. 11). Subtracting out the 180 kJ/mol

375

found for Ea in each case gave an effective acid site strength, λNH3 of about 165 kJ/mol

376

for Pt-ZSM-5 and 135 kJ/mol for Pt-SAPO-11. The somewhat higher number for


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Brønsted Acidity


377

SAPO-11 by this method than the 113 kJ/mol we found via NH3-TPD could be

378

explained if some of the acid sites accounted for in the NH3-TPD determination are not

379

substantially involved in the catalysis. It could also be that sites of stronger acid

380

strength have a greater impact on the effective acid strength than sites of lesser

381

strength.

382 383

Fig. 10. Effect of nitrogen on Pt/H-ZSM-5 at 90% n-hexadecane conversion

384 385

Fig. 11. Effect of nitrogen on Pt-SAPO-11 at 90% n-hexadecane conversion

386

It is worth noting that the Pt-SAPO-11 catalyst, prior to amine addition, was around 80

387

K less active than the Pt-ZSM-5 catalyst. This may be somewhat surprising in light of

388

the results for pyridine titration at 573 K, which shows the Pt-SAPO-11 to have one-

389

third the strong acid site density (adsorbed at 573 K) as the Pt-ZSM-5. If those sites on

390

the SAPO-11 were equivalent to those on the ZSM-5, then, based on the observed Ea,

Page 22



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Bronstead Acidity

391

the activity difference should only be around 15 K. Besides the lower number of acid

392

sites, there are other factors which could contribute to the observed activity difference.

393

One would be a larger crystallite size for the SAPO-11, and lower external surface area.

394

Meriaudeau et al. have already reported a crystallite size effect on the activity of Pt-

395

SAPO-11 for paraffin isomerization, from which the authors suggest that the reaction is

396

mainly occurring at the pore mouths, whose number should be roughly proportional to

397

the external surface area.5 In any event, since both the SAPO-11 and ZSM-5 in our

398

study had about the same external surface area, we do not see this factor as significantly

399

affecting the catalytic results. Second, as reported by Suzuki et al., acid sites with a

400

lower Brønsted acid strength are found to have a lower turnover frequency (TOF) for

401

the catalytic cracking of n-octane, suggesting the same relation might hold for similar

402

acid catalyzed reactions.29 More recently, Wang et al. have shown a good correlation

403

between the TOF for propene methylation and the calculated adsorption enthalpy for

404

NH3 over a range of metal-substituted ALPO-34 catalysts.25 These findings would also

405

suggest that the somewhat lower effective acid strength for SAPO-11, along with fewer

406

acid sites, would also contribute to its lower activity compared to the ZSM-5. Both the

407

Suzuki and Wang references show that a 20 kJ/mol lower acid site strength can reduce

408

TOF by an order of magnitude. Consequently, a somewhat lower acid strength, coupled

409

with fewer acid sites could account for the activity difference observed for SAPO-11

410

versus the H-ZSM-5.

411

4.5 Conclusions

412

A catalytic test was be used to determine an “effective acid strength” for acid-catalyzed

413

hydrodewaxing over both Pt-SAPO-11 and Pt/H-ZSM-5 catalysts. This test showed the


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Brønsted Acidity


414

effective acid strength for hydroisomerization dewaxing over SAPO-11 to be about 135

415

kJ/mol, around 30 kJ/mole less than that determined by the same test for

416

hydrodewaxing over Pt-H-ZSM-5. While pyridine titration indicated a portion of the

417

sites on SAPO-11 were of similar strength to those found on H-ZSM-5, the lower

418

hydroconversion activity of Pt-SAPO-11 compared to Pt-H-ZSM-5 is consistent with a

419

lower effective acid strength as well as the lower apparent ammonia desorption energy

420

derived from NH3-TPD. The effective acid strength test could serve as a tool to help

421

guide catalyst design.

5.0 References 422 423 424

(1)

Miller, S.J. Catalytic Isomerization Process using a Silicoaluminophosphate Molecular Sieve Containing an Occluded Group VIII Metal therein. U.S. Patent 4,689,138, Aug. 25, 1987.

(2)

Miller, S.J. New Molecular Sieve Process for Lube Dewaxing by Wax Isomerization. Microporous Mater. 1994, 2, 439-449.

(3)

Miller, S.J. Studies on Wax Isomerization for Lubes and Fuels. Stud. Surf. Sci. Catal. 1994, 84, 2319-2326.

(4)

Miller, S.J. Wax Isomerization using Catalyst of Specific Pore Geometry. U.S. Patent 5,135,638, Aug. 4, 1992.

(5)

Meriaudeau, P.; Tuan, Vu.A.; Sapaly, G.; Nghiem, Vu.T.; Naccache, C. Pore Size and Crystal Size Effects on the Selective Hydroisomerization of C8 Paraffins over Pt/Pd-SAPO-11, Pt/Pd-SAPO-41 Bifunctional Catalysts. Catal. Today 1999, 49, 285-292.

(6)

Meriaudeau, P.; Tuan, Vu.A.; Lefebvre, F.; Nghiem, Vu.T; Naccache, C. Isomorphous Substitution of Silicon in the AlPO4 Framework with AEL Structure: n-Octane Hydroconversion. Microporous Mater. 1998, 22, 435-449.

(7)

Campelo, J.M.; Lafont, F.; Marinas, J.M. Hydroconversion of n-Dodecane over Pt/SAPO-11 Catalyst. Applied Catal. A: General 1998, 170, 139-144.

(8)

Campelo, J.M.; Lafont, F.; Marinas, J.M. Pt/SAPO-5 and Pt/SAPO-11 as Catalysts for the Hydroisomerization and Hydrocracking of n-Octane. J. Chem. Soc., Faraday Trans. 1995, 91, 1551-1555.

(9)

Sinha, A.K.; Sivasanker, S. Hydroisomerization of n-Hexane over Pt–SAPO-11 and Pt–SAPO-31 Molecular Sieves. Catal. Today 1999, 49, 293-302.

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Bronstead Acidity

(10) Sinha, A.K.; Sivasanker, S.; Ratnasamy, P. Hydroisomerization of n-Alkanes over Pt−SAPO-11 and Pt−SAPO-31 Synthesized from Aqueous and Nonaqueous Media. Ind. Eng. Chem. Res. 1998, 37, 2208-2214. (11) Akolekar, D.B. Acidity and Catalytic Properties of AIPO4-11, SAPO-11, MAPO11, NiAPO-11, MnAPO-11 and MnAPSO-11 Molecular Sieves. J. Mol. Catal. A: Chemical 1995, 104, 95-102. (12) Tian, H.; Li, C. Structure, Acidity and Catalytic Properties of Dealuminated SAPO-11 Molecular Sieves. J. Mol. Catal. A: Chemical 1999, 149, 205-213. (13) Yang, L.; Aizhen, Y.; Qinhua, X. Acidity, Diffusion and Catalytic Properties of the Silicoaluminophosphate SAPO-11. Applied Catal. 1991, 67, 169-177. 425 426 427 428

(14) Singh, A.K.; Kondamudi, K.; Yadav, R; Upadhyayula, S.; Sakthivel, A. Uniform Mesoporous Silicoaluminophosphate Derived by Vapor Phase Treatment: Its Catalytic and Kinetic Studies in Hydroisomerization of 1-Octene. J. Phys. Chem. C 2014, 118, 27961-27972.

429 430 431

(15) Yadav, R.; Sakthivel, A. Silicoaluminophosphat Molecular Sieves as Potential Catalysts for Hydroisomerization of Alkanes and Alkenes. Applied Catal. A General 2014, 481, 143-160. (16) Singh, A.K.; Yadav, R.; Sudarsan, V.; Kishore, K.; Upadhyayula, S.; Sakthivel, A. Mesoporous SAPO-5 (MESO-SAPO-5); a Potential Catalyst for Hydroisomerization of 1-Octene. RSC Advances 2014, 4, 8727-8734. (17) Parrillo, D.J.; Gorte, R.J. Characterization of Acidity in H-ZSM-5, H-ZSM-12, HMordenite, and H-Y using Microcalorimetry. J. Phys. Chem. 1993, 97, 87868792. (18) Parrillo, D.J.; Biaglow, A.; Gorte, R.J.; White, D. Quantification of Acidity in HZSM-5. Stud. Surf. Sci. Catal. 1994, 84B, 701-708. (19) Gonzalez, M.R.; Sharma, S.B.; Chen, D.T.; Dumesic, J.A. Thermogravimetric and Microcalorimetric Studies of ZSM-5 Acidity. Catal. Lett. 1993, 18, 183-192. (20) Parlitz, B.; Schreier, E.; Lischke, G.; Zubowa, H.L.; Eckelt, R.; Lieske, E.; Fricke, R. Isomerization of n-Heptane over Pd-Loaded Silico-Alumino-Phosphate Molecular Sieves. J. Catal. 1995, 155, 1-11. (21) Wang, Z.; Tian, Z.; Wen, G.; Xu, Y.; Xu, Z.; Lin, L. Hydroisomerization of Long-Chain Alkane over Pt/SAPO-11 Catalysts Synthesized from Nonaqueous Media. Catal. Lett. 2005, 103, 109-116. (22) Zhang, S.; Chen, S.-L.; Dong, P.; Yuan, G.; Xu, K. Characterization and Hydroisomerization Performance of SAPO-11 Molecular Sieves Synthesized in Different Media. Appl. Catal. A General 2007, 332, 46-55. (23) Prakash, A.M.; Satyrnarayana, V.V.; Chilukuri, R.P.; Bagwe, R.P.; Ashtekar, S.; Chakrabarty, D.K. Silicoaluminophosphate Molecular Sieves SAPO-11, SAPO31 and SAPO-41: Synthesis, Characterization and Alkylation of Toluene with Methanol. Micro. Mater. 1996, 6, 89-97.


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Brønsted Acidity


(24) Jones, A.J.; Carr, R.T.; Zones, S.I.; Iglesia, E. Acid Strength and Solvation in Catalysis by MFI Zeolites and Effects of the Identity Concentration and Location of Framework Heteroatoms. J. Catal. 2014, 312, 58-68. (25) Wang, C.-M.; Brogaard, R.Y.; Weckhuysen, B.M.; Norskov, J.K.; Studt, F. Reactivity Descriptor in Solid Acid Catalysts: Predicting Turnover Frequencies for Propene Methylation in Zeotypes. J. Phys. Chem. Lett. 2014, 5, 1516-1521. (26) Cvetanović, R.J.; Amenomiya, Y. Application of a Temperature-Programmed Desorption Technique to Catalyst Studies. Adv. Catal. 1967, 17, 103-149. (27) Sawa, M.; Niwa, M.; Murakami, Y. Derivation of New Theoretical Equation for Temperature-Programmed Desorption of Ammonia with Freely Occurring Readsorption. Zeolites 1990, 10, 307-309. (28) Sharma, S.B.; Meyers, B.L.; Chen, D.T.; Miller, J.; Dumesic, J.A. Characterization of Catalyst Acidity by Microcalorimetry and TemperatureProgrammed Desorption. Appl. Catal. A General 1993, 102, 253-265. (29) Suzuki, K.; Noda, T.; Katada, N.; Niwa, M. IRMS-TPD of Ammonia: Direct and Individual Measurement of Brønsted Acidity in Zeolites and its Relationship with the Catalytic Cracking Activity. J. Catal. 2007, 250, 151-160. (30) Pieterse, A.J.Z.; Veefkind-Reyes, S.; Seshan, K.; Domokos, L.; Lercher, J.A. On the Accessibility of Acid Sites in Ferrierite for Pyridine. J. Catal. 1999, 187, 518520. (31) Auroux, A. Acidity Characterization by Microcalormetry and Relationship with Acitivty. Topics in Catalysis 1997, 4, 71-89. (32) Weitkamp, J.; Jacobs, P.A.; Martens, J.A. Isomerization and Hydrocracking of C9 through C16 n-Alkanes on Pt/HZSM-5 Zeolite. Applied Catal. 1983, 8, 123141. 432

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Bronstead Acidity

Table of Contents Graphic


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Determining the Strength of BrÃ¸nsted Acid Sites for Hydrodewaxing

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