Reflections on Heterogeneous Catalyst Development - Applied Catalysts

One of our Senior Engineers at Applied Catalysts asked me to reflect on my experiences in Heterogeneous Catalysis in my career at Applied Catalysts and at a major chemical manufacturer in the Midwest, so here we go.

 

First, as an experimentalist, it is a great field due to the many complexities.  There are many tools for understanding the catalysis of a given catalyst and reaction, and none of these tools, on their own or combined, will enable the direct identification of a commercially viable catalyst for a given process. There are three critical properties in Industrial Catalysis: Activity, Selectivity, and Lifetime.  Each of these properties has challenges that may need catalyst discovery, catalyst development, process development, scale-up and manufacturing engineering. 

The Core Challenges of Industrial Catalysis

In industrial catalysis in specialty chemicals, we get around the lifetime problem by doing batch chemistry, where the catalyst only needs to work for one conversion of the batch to the desired chemical.  So-called slurry catalysts are used in ~1% concentrations and filtered from the product and discarded or reprocessed.  The active metal in Platinum Group Metal (PGM) catalysts offer advantages in activity and lifetime over less expensive base metal catalysts.  The PGM is often recovered at 90+% and reused or credited towards the next batch of fresh material, making the process surprisingly efficient. Alternatively, base metal catalysts, such as Ni Raney™ or Ni sponge catalysts, may be used.  To have a continuous process, the catalyst must allow the chemistry to occur without being chemically transformed, poisoned, or fouled for years of operation.  Deactivation is the most prominent issue in fixed bed catalysis; however, some modes of deactivation are permanent, and others are reversible. Solutions may include developing a regeneration strategy, making structural-functional changes to the catalyst that prevent deactivation, or making process changes to the manufacturing system that minimize deactivation.  

 

The second problem, selectivity, is how to have the catalyst direct the chemistry to the desired product.  Selectivity is where catalysts show the most value.  High selectivity is critical for limiting by-products and, when not remediated, pollutants.  High selectivity can be achieved by the choice of catalyst, additives to the reaction, and process conditions, and can be compensated for by post reactor separations.  For continuous processes at large volumes, the selectivities to the desired product is usually greater than 90% and for older commodity processes, the selectivity may be greater than 99+%.  The selectivity is also paramount in fine chemical, agricultural, and pharmaceutical processes, which employ multi-step processes where losses in each reaction propagate to lower final product yield, and high solvent costs, which have a significant impact on process economics. 

 

The first problem, catalyst activity, is usually a starting point and may also be challenging.  Often, a combination of historical precedent, chemist intuition, and high through-put experimentation may be used to achieve the activity needed.  In some cases, heat management is also a consideration, so it isn’t just the activity that may limit success.  

 

Lessons from the Field

One of my early experiences with industrial catalysis (1990) was for the hydrogenation of acetylene in ethylene streams destined for making polyethylene.  The ethylene stream comes from a steam cracker where ethane is converted to ethylene, and ~3000 ppm of acetylene by-product is formed.  This level of acetylene would poison the catalysts used for making polyethylene, so it must be removed.  It is done in a hydrogenation reactor by selective hydrogenation of the acetylene to ethylene, without hydrogenating any of the ethylene back to ethane.  For this process, the goal is retaining 100% of the ethylene and adding 3000 ppm of ethylene from the hydrogenation of the ethylene, so you end up with up to 100.3% selectivity in the process.  The state of the art in the 1990s was such that an acceptable catalyst made 100.1% selectivity to ethylene, and the best catalysts approached 100.3% selectivity.  Since the process is used to make billions of pounds of polyethylene per year, such selectivities are required.  

 

The first catalysts used for this process in the late 1950s relied on Palladium (Pd) particles on diatomaceous earth, with silver (Ag) added to modify the Pd particles so small ensembles of Pd were present.  These small ensembles hydrogenate the acetylene to ethylene but are not as active for hydrogenating the ethylene to ethane.   At the same time, a process additive of dilute carbon monoxide is used to achieve high selectivity.  The CO is strongly absorbed on Pd particles, preventing absorption of ethylene, so it isn’t hydrogenated.  Acetylene binds more strongly to the Pd, so it can be hydrogenated.  Consequently, the process uses both a bi-metallic catalyst and process additives to be successful.  

 

Some of the process knowledge was developed by using in-situ IR spectroscopy of CO for Pd-Ag catalysts compared to Pd only catalysts.  In addition, DFT modeling of the metal particles and model Pd slabs, kinetic studies (including microkinetic modeling), and SEM and TEM microscopy studies have been used.  To this day, bi-metallic and additives are still important tools of the trade.  

 

One other aspect of the problem is that the number of Pd atoms participating in the chemistry is small.  Initial surface science investigations assigned an ethylidene intermediate to the process.  It was determined that while a surface species of ethylidene is observed, it is not involved in the rate limiting steps for the acetylene hydrogenation reaction.  An analogy is a sports stadium with 10-20 players and 100,000 fans, where the players are dispersed among the fans, it would be difficult to discern the players.  Many of the characterization techniques are not sensitive enough to distinguish active sites versus non-active sites, and sometimes operando (in-situ) spectroscopy is used.  A complicating factor is that the catalyst surface can re-structure during the reaction, so the active sites may only be present under reaction conditions.  The field then often relies on demonstrating the chemistry to assure a high level of performance has been achieved.  

 

One additional note is the potential danger in these processes.  In one facility operating an acetylene hydrogenation reactor in the early days, an improvement was made to the ethylene production process whereby the amount of by-product CO coming in with the feed was reduced, this resulted in an exotherm in the reactor as ethylene was hydrogenated, generating excessive heat (ΔH: -137 kJ/mol), shutting down the process.  Astute engineers identified the origin of the problem and added additional CO to bring the process back into controlled operation.  

 

Applied Catalysts’ Approach — From Lab to Commercial Scale Catalyst Development

At Applied Catalysts, we have a team of engineers and chemists who manufacture catalysts at the lab, pilot, and commercial scale.  Our customers come to us for catalysts that meet their process needs. Customers come to us when a commercial catalyst is not yet available, and where there may be limited data for catalysts for the desired process.  They also come to us with a recipe they have or with a need to produce a discontinued catalyst.  We use our experience, patent, and published art to propose catalysts for the process, process additives, and process modifications to propose a viable commercial catalyst.  We run proof of concept tests in our pilot labs or at the customer lab to develop the catalyst and process.  From there, we consider manufacturing routes for the catalyst at the pilot and commercial scale.  Applied Catalysts may have the assets in place for the catalyst, add assets, or leverage third-party assets as needed to deliver what the customer needs.

 

In addition to catalyst development and manufacturing, and process engineering services, we deliver pilot and commercial reactor systems with partnering companies.  We have a new partnership with Amar Equipment in India, combining 25 years of mechanical reactor design with 25 years of catalyst and process knowledge to offer clients a clear pathway from concept to commercialization.  In addition, we have a diverse partner network of companies in the United States and abroad that can offer solutions to nearly any challenge one may face in developing a catalytic process. Reach out to Applied Catalysts with any inquiry you may have.